SPDX-License-Identifier: GPL-2.0+

#

(C) Copyright 2000 - 2013

Wolfgang Denk, DENX Software Engineering, wd@denx.de.

Summary:

This directory contains the source code for U-Boot, a boot loader for Embedded boards based on PowerPC, ARM, MIPS and several other processors, which can be installed in a boot ROM and used to initialize and test the hardware or to download and run application code.

The development of U-Boot is closely related to Linux: some parts of the source code originate in the Linux source tree, we have some header files in common, and special provision has been made to support booting of Linux images.

Some attention has been paid to make this software easily configurable and extendable. For instance, all monitor commands are implemented with the same call interface, so that it's very easy to add new commands. Also, instead of permanently adding rarely used code (for instance hardware test utilities) to the monitor, you can load and run it dynamically.

Status:

In general, all boards for which a configuration option exists in the Makefile have been tested to some extent and can be considered "working". In fact, many of them are used in production systems.

In case of problems see the CHANGELOG file to find out who contributed the specific port. In addition, there are various MAINTAINERS files scattered throughout the U-Boot source identifying the people or companies responsible for various boards and subsystems.

Note: As of August, 2010, there is no longer a CHANGELOG file in the actual U-Boot source tree; however, it can be created dynamically from the Git log using:

make CHANGELOG

Where to get help:

In case you have questions about, problems with or contributions for U-Boot, you should send a message to the U-Boot mailing list at u-boot@lists.denx.de. There is also an archive of previous traffic on the mailing list - please search the archive before asking FAQ's. Please see http://lists.denx.de/pipermail/u-boot and http://dir.gmane.org/gmane.comp.boot-loaders.u-boot

Where to get source code:

The U-Boot source code is maintained in the Git repository at git://www.denx.de/git/u-boot.git ; you can browse it online at http://www.denx.de/cgi-bin/gitweb.cgi?p=u-boot.git;a=summary

The "snapshot" links on this page allow you to download tarballs of any version you might be interested in. Official releases are also available for FTP download from the ftp://ftp.denx.de/pub/u-boot/ directory.

Pre-built (and tested) images are available from ftp://ftp.denx.de/pub/u-boot/images/

Where we come from:

• start from 8xxrom sources
• create PPCBoot project (http://sourceforge.net/projects/ppcboot)
• clean up code
• make it easier to add custom boards
• make it possible to add other [PowerPC] CPUs
• extend functions, especially:
  • Provide extended interface to Linux boot loader
  • S-Record download
  • network boot
  • PCMCIA / CompactFlash / ATA disk / SCSI ... boot
• create ARMBoot project (http://sourceforge.net/projects/armboot)
• add other CPU families (starting with ARM)
• create U-Boot project (http://sourceforge.net/projects/u-boot)
• current project page: see http://www.denx.de/wiki/U-Boot

Names and Spelling:

The "official" name of this project is "Das U-Boot". The spelling "U-Boot" shall be used in all written text (documentation, comments in source files etc.). Example:

This is the README file for the U-Boot project.

File names etc. shall be based on the string "u-boot". Examples:

include/asm-ppc/u-boot.h

#include <asm/u-boot.h>

Variable names, preprocessor constants etc. shall be either based on the string "u_boot" or on "U_BOOT". Example:

U_BOOT_VERSION      u_boot_logo
IH_OS_U_BOOT        u_boot_hush_start

Versioning:

Starting with the release in October 2008, the names of the releases were changed from numerical release numbers without deeper meaning into a time stamp based numbering. Regular releases are identified by names consisting of the calendar year and month of the release date. Additional fields (if present) indicate release candidates or bug fix releases in "stable" maintenance trees.

Examples: U-Boot v2009.11 - Release November 2009 U-Boot v2009.11.1 - Release 1 in version November 2009 stable tree U-Boot v2010.09-rc1 - Release candidate 1 for September 2010 release

Directory Hierarchy:

/arch Architecture specific files /arc Files generic to ARC architecture /arm Files generic to ARM architecture /m68k Files generic to m68k architecture /microblaze Files generic to microblaze architecture /mips Files generic to MIPS architecture /nds32 Files generic to NDS32 architecture /nios2 Files generic to Altera NIOS2 architecture /openrisc Files generic to OpenRISC architecture /powerpc Files generic to PowerPC architecture /riscv Files generic to RISC-V architecture /sandbox Files generic to HW-independent "sandbox" /sh Files generic to SH architecture /x86 Files generic to x86 architecture /api Machine/arch independent API for external apps /board Board dependent files /cmd U-Boot commands functions /common Misc architecture independent functions /configs Board default configuration files /disk Code for disk drive partition handling /doc Documentation (don't expect too much) /drivers Commonly used device drivers /dts Contains Makefile for building internal U-Boot fdt. /examples Example code for standalone applications, etc. /fs Filesystem code (cramfs, ext2, jffs2, etc.) /include Header Files /lib Library routines generic to all architectures /Licenses Various license files /net Networking code /post Power On Self Test /scripts Various build scripts and Makefiles /test Various unit test files /tools Tools to build S-Record or U-Boot images, etc.

Software Configuration:

Configuration is usually done using C preprocessor defines; the rationale behind that is to avoid dead code whenever possible.

There are two classes of configuration variables:

• Configuration OPTIONS: These are selectable by the user and have names beginning with "CONFIG_".

• Configuration SETTINGS: These depend on the hardware etc. and should not be meddled with if you don't know what you're doing; they have names beginning with "CONFIGSYS".

Previously, all configuration was done by hand, which involved creating symbolic links and editing configuration files manually. More recently, U-Boot has added the Kbuild infrastructure used by the Linux kernel, allowing you to use the "make menuconfig" command to configure your build.

Selection of Processor Architecture and Board Type:

For all supported boards there are ready-to-use default configurations available; just type "make _defconfig".

Example: For a TQM823L module type:

cd u-boot
make TQM823L_defconfig

Note: If you're looking for the default configuration file for a board you're sure used to be there but is now missing, check the file doc/README.scrapyard for a list of no longer supported boards.

Sandbox Environment:

U-Boot can be built natively to run on a Linux host using the 'sandbox' board. This allows feature development which is not board- or architecture- specific to be undertaken on a native platform. The sandbox is also used to run some of U-Boot's tests.

See board/sandbox/README.sandbox for more details.

Board Initialisation Flow:

This is the intended start-up flow for boards. This should apply for both SPL and U-Boot proper (i.e. they both follow the same rules).

Note: "SPL" stands for "Secondary Program Loader," which is explained in more detail later in this file.

At present, SPL mostly uses a separate code path, but the function names and roles of each function are the same. Some boards or architectures may not conform to this. At least most ARM boards which use CONFIG_SPL_FRAMEWORK conform to this.

Execution typically starts with an architecture-specific (and possibly CPU-specific) start.S file, such as:

- arch/arm/cpu/armv7/start.S
- arch/powerpc/cpu/mpc83xx/start.S
- arch/mips/cpu/start.S

and so on. From there, three functions are called; the purpose and limitations of each of these functions are described below.

lowlevel_init():

• purpose: essential init to permit execution to reach board_init_f()
• no global_data or BSS
• there is no stack (ARMv7 may have one but it will soon be removed)
• must not set up SDRAM or use console
• must only do the bare minimum to allow execution to continue to board_init_f()
• this is almost never needed
• return normally from this function

board_init_f():

• purpose: set up the machine ready for running board_init_r(): i.e. SDRAM and serial UART
• global_data is available
• stack is in SRAM

• BSS is not available, so you cannot use global/static variables, only stack variables and global_data

  Non-SPL-specific notes:

• dram_init() is called to set up DRAM. If already done in SPL this can do nothing

  SPL-specific notes:

• you can override the entire board_init_f() function with your own version as needed.
• preloader_console_init() can be called here in extremis
• should set up SDRAM, and anything needed to make the UART work
• these is no need to clear BSS, it will be done by crt0.S
• must return normally from this function (don't call board_init_r() directly)

Here the BSS is cleared. For SPL, if CONFIG_SPL_STACK_R is defined, then at this point the stack and global_data are relocated to below CONFIG_SPL_STACK_R_ADDR. For non-SPL, U-Boot is relocated to run at the top of memory.

board_init_r():

• purpose: main execution, common code
• global_data is available
• SDRAM is available
• BSS is available, all static/global variables can be used

• execution eventually continues to main_loop()

  Non-SPL-specific notes:

• U-Boot is relocated to the top of memory and is now running from there.

  SPL-specific notes:

• stack is optionally in SDRAM, if CONFIG_SPL_STACK_R is defined and CONFIG_SPL_STACK_R_ADDR points into SDRAM
• preloader_console_init() can be called here - typically this is done by selecting CONFIG_SPL_BOARD_INIT and then supplying a spl_board_init() function containing this call
• loads U-Boot or (in falcon mode) Linux

Configuration Options:

Configuration depends on the combination of board and CPU type; all such information is kept in a configuration file "include/configs/.h".

Example: For a TQM823L module, all configuration settings are in "include/configs/TQM823L.h".

Many of the options are named exactly as the corresponding Linux kernel configuration options. The intention is to make it easier to build a config tool - later.

• ARM Platform Bus Type(CCI): CoreLink Cache Coherent Interconnect (CCI) is ARM BUS which provides full cache coherency between two clusters of multi-core CPUs and I/O coherency for devices and I/O masters

  CONFIG_SYS_FSL_HAS_CCI400
  
  Defined For SoC that has cache coherent interconnect
  CCN-400
  
  CONFIG_SYS_FSL_HAS_CCN504
  
  Defined for SoC that has cache coherent interconnect CCN-504

The following options need to be configured:

• CPU Type: Define exactly one, e.g. CONFIG_MPC85XX.

• Board Type: Define exactly one, e.g. CONFIG_MPC8540ADS.

• 85xx CPU Options: CONFIG_SYS_PPC64

  Specifies that the core is a 64-bit PowerPC implementation (implements
  the "64" category of the Power ISA). This is necessary for ePAPR
  compliance, among other possible reasons.
  
  CONFIG_SYS_FSL_TBCLK_DIV
  
  Defines the core time base clock divider ratio compared to the
  system clock.  On most PQ3 devices this is 8, on newer QorIQ
  devices it can be 16 or 32.  The ratio varies from SoC to Soc.
  
  CONFIG_SYS_FSL_PCIE_COMPAT
  
  Defines the string to utilize when trying to match PCIe device
  tree nodes for the given platform.
  
  CONFIG_SYS_FSL_ERRATUM_A004510
  
  Enables a workaround for erratum A004510.  If set,
  then CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV and
  CONFIG_SYS_FSL_CORENET_SNOOPVEC_COREONLY must be set.
  
  CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV
  CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV2 (optional)
  
  Defines one or two SoC revisions (low 8 bits of SVR)
  for which the A004510 workaround should be applied.
  
  The rest of SVR is either not relevant to the decision
  of whether the erratum is present (e.g. p2040 versus
  p2041) or is implied by the build target, which controls
  whether CONFIG_SYS_FSL_ERRATUM_A004510 is set.
  
  See Freescale App Note 4493 for more information about
  this erratum.
  
  CONFIG_A003399_NOR_WORKAROUND
  Enables a workaround for IFC erratum A003399. It is only
  required during NOR boot.
  
  CONFIG_A008044_WORKAROUND
  Enables a workaround for T1040/T1042 erratum A008044. It is only
  required during NAND boot and valid for Rev 1.0 SoC revision
  
  CONFIG_SYS_FSL_CORENET_SNOOPVEC_COREONLY
  
  This is the value to write into CCSR offset 0x18600
  according to the A004510 workaround.
  
  CONFIG_SYS_FSL_DSP_DDR_ADDR
  This value denotes start offset of DDR memory which is
  connected exclusively to the DSP cores.
  
  CONFIG_SYS_FSL_DSP_M2_RAM_ADDR
  This value denotes start offset of M2 memory
  which is directly connected to the DSP core.
  
  CONFIG_SYS_FSL_DSP_M3_RAM_ADDR
  This value denotes start offset of M3 memory which is directly
  connected to the DSP core.
  
  CONFIG_SYS_FSL_DSP_CCSRBAR_DEFAULT
  This value denotes start offset of DSP CCSR space.
  
  CONFIG_SYS_FSL_SINGLE_SOURCE_CLK
  Single Source Clock is clocking mode present in some of FSL SoC's.
  In this mode, a single differential clock is used to supply
  clocks to the sysclock, ddrclock and usbclock.
  
  CONFIG_SYS_CPC_REINIT_F
  This CONFIG is defined when the CPC is configured as SRAM at the
  time of U-Boot entry and is required to be re-initialized.
  
  CONFIG_DEEP_SLEEP
  Indicates this SoC supports deep sleep feature. If deep sleep is
  supported, core will start to execute uboot when wakes up.

• Generic CPU options: CONFIG_SYS_BIG_ENDIAN, CONFIG_SYS_LITTLE_ENDIAN

  Defines the endianess of the CPU. Implementation of those
  values is arch specific.
  
  CONFIG_SYS_FSL_DDR
  Freescale DDR driver in use. This type of DDR controller is
  found in mpc83xx, mpc85xx, mpc86xx as well as some ARM core
  SoCs.
  
  CONFIG_SYS_FSL_DDR_ADDR
  Freescale DDR memory-mapped register base.
  
  CONFIG_SYS_FSL_DDR_EMU
  Specify emulator support for DDR. Some DDR features such as
  deskew training are not available.
  
  CONFIG_SYS_FSL_DDRC_GEN1
  Freescale DDR1 controller.
  
  CONFIG_SYS_FSL_DDRC_GEN2
  Freescale DDR2 controller.
  
  CONFIG_SYS_FSL_DDRC_GEN3
  Freescale DDR3 controller.
  
  CONFIG_SYS_FSL_DDRC_GEN4
  Freescale DDR4 controller.
  
  CONFIG_SYS_FSL_DDRC_ARM_GEN3
  Freescale DDR3 controller for ARM-based SoCs.
  
  CONFIG_SYS_FSL_DDR1
  Board config to use DDR1. It can be enabled for SoCs with
  Freescale DDR1 or DDR2 controllers, depending on the board
  implemetation.
  
  CONFIG_SYS_FSL_DDR2
  Board config to use DDR2. It can be enabled for SoCs with
  Freescale DDR2 or DDR3 controllers, depending on the board
  implementation.
  
  CONFIG_SYS_FSL_DDR3
  Board config to use DDR3. It can be enabled for SoCs with
  Freescale DDR3 or DDR3L controllers.
  
  CONFIG_SYS_FSL_DDR3L
  Board config to use DDR3L. It can be enabled for SoCs with
  DDR3L controllers.
  
  CONFIG_SYS_FSL_DDR4
  Board config to use DDR4. It can be enabled for SoCs with
  DDR4 controllers.
  
  CONFIG_SYS_FSL_IFC_BE
  Defines the IFC controller register space as Big Endian
  
  CONFIG_SYS_FSL_IFC_LE
  Defines the IFC controller register space as Little Endian
  
  CONFIG_SYS_FSL_IFC_CLK_DIV
  Defines divider of platform clock(clock input to IFC controller).
  
  CONFIG_SYS_FSL_LBC_CLK_DIV
  Defines divider of platform clock(clock input to eLBC controller).
  
  CONFIG_SYS_FSL_PBL_PBI
  It enables addition of RCW (Power on reset configuration) in built image.
  Please refer doc/README.pblimage for more details
  
  CONFIG_SYS_FSL_PBL_RCW
  It adds PBI(pre-boot instructions) commands in u-boot build image.
  PBI commands can be used to configure SoC before it starts the execution.
  Please refer doc/README.pblimage for more details
  
  CONFIG_SPL_FSL_PBL
  It adds a target to create boot binary having SPL binary in PBI format
  concatenated with u-boot binary.
  
  CONFIG_SYS_FSL_DDR_BE
  Defines the DDR controller register space as Big Endian
  
  CONFIG_SYS_FSL_DDR_LE
  Defines the DDR controller register space as Little Endian
  
  CONFIG_SYS_FSL_DDR_SDRAM_BASE_PHY
  Physical address from the view of DDR controllers. It is the
  same as CONFIG_SYS_DDR_SDRAM_BASE for  all Power SoCs. But
  it could be different for ARM SoCs.
  
  CONFIG_SYS_FSL_DDR_INTLV_256B
  DDR controller interleaving on 256-byte. This is a special
  interleaving mode, handled by Dickens for Freescale layerscape
  SoCs with ARM core.
  
  CONFIG_SYS_FSL_DDR_MAIN_NUM_CTRLS
  Number of controllers used as main memory.
  
  CONFIG_SYS_FSL_OTHER_DDR_NUM_CTRLS
  Number of controllers used for other than main memory.
  
  CONFIG_SYS_FSL_HAS_DP_DDR
  Defines the SoC has DP-DDR used for DPAA.
  
  CONFIG_SYS_FSL_SEC_BE
  Defines the SEC controller register space as Big Endian
  
  CONFIG_SYS_FSL_SEC_LE
  Defines the SEC controller register space as Little Endian

• MIPS CPU options: CONFIG_SYS_INIT_SP_OFFSET

  Offset relative to CONFIG_SYS_SDRAM_BASE for initial stack
  pointer. This is needed for the temporary stack before
  relocation.
  
  CONFIG_XWAY_SWAP_BYTES
  
  Enable compilation of tools/xway-swap-bytes needed for Lantiq
  XWAY SoCs for booting from NOR flash. The U-Boot image needs to
  be swapped if a flash programmer is used.

• ARM options: CONFIG_SYS_EXCEPTION_VECTORS_HIGH

  Select high exception vectors of the ARM core, e.g., do not
  clear the V bit of the c1 register of CP15.
  
  COUNTER_FREQUENCY
  Generic timer clock source frequency.
  
  COUNTER_FREQUENCY_REAL
  Generic timer clock source frequency if the real clock is
  different from COUNTER_FREQUENCY, and can only be determined
  at run time.

• Tegra SoC options: CONFIG_TEGRA_SUPPORT_NON_SECURE

  Support executing U-Boot in non-secure (NS) mode. Certain
  impossible actions will be skipped if the CPU is in NS mode,
  such as ARM architectural timer initialization.

• Linux Kernel Interface: CONFIG_CLOCKS_IN_MHZ

  U-Boot stores all clock information in Hz
  internally. For binary compatibility with older Linux
  kernels (which expect the clocks passed in the
  bd_info data to be in MHz) the environment variable
  "clocks_in_mhz" can be defined so that U-Boot
  converts clock data to MHZ before passing it to the
  Linux kernel.
  When CONFIG_CLOCKS_IN_MHZ is defined, a definition of
  "clocks_in_mhz=1" is automatically included in the
  default environment.
  
  CONFIG_MEMSIZE_IN_BYTES     [relevant for MIPS only]
  
  When transferring memsize parameter to Linux, some versions
  expect it to be in bytes, others in MB.
  Define CONFIG_MEMSIZE_IN_BYTES to make it in bytes.
  
  CONFIG_OF_LIBFDT
  
  New kernel versions are expecting firmware settings to be
  passed using flattened device trees (based on open firmware
  concepts).
  
  CONFIG_OF_LIBFDT
   * New libfdt-based support
   * Adds the "fdt" command
   * The bootm command automatically updates the fdt
  
  OF_TBCLK - The timebase frequency.
  OF_STDOUT_PATH - The path to the console device
  
  boards with QUICC Engines require OF_QE to set UCC MAC
  addresses
  
  CONFIG_OF_BOARD_SETUP
  
  Board code has addition modification that it wants to make
  to the flat device tree before handing it off to the kernel
  
  CONFIG_OF_SYSTEM_SETUP
  
  Other code has addition modification that it wants to make
  to the flat device tree before handing it off to the kernel.
  This causes ft_system_setup() to be called before booting
  the kernel.
  
  CONFIG_OF_IDE_FIXUP
  
  U-Boot can detect if an IDE device is present or not.
  If not, and this new config option is activated, U-Boot
  removes the ATA node from the DTS before booting Linux,
  so the Linux IDE driver does not probe the device and
  crash. This is needed for buggy hardware (uc101) where
  no pull down resistor is connected to the signal IDE5V_DD7.
  
  CONFIG_MACH_TYPE    [relevant for ARM only][mandatory]
  
  This setting is mandatory for all boards that have only one
  machine type and must be used to specify the machine type
  number as it appears in the ARM machine registry
  (see http://www.arm.linux.org.uk/developer/machines/).
  Only boards that have multiple machine types supported
  in a single configuration file and the machine type is
  runtime discoverable, do not have to use this setting.

• vxWorks boot parameters:

  bootvx constructs a valid bootline using the following
  environments variables: bootdev, bootfile, ipaddr, netmask,
  serverip, gatewayip, hostname, othbootargs.
  It loads the vxWorks image pointed bootfile.
  
  Note: If a "bootargs" environment is defined, it will overwride
  the defaults discussed just above.

• Cache Configuration: CONFIG_SYS_ICACHE_OFF - Do not enable instruction cache in U-Boot CONFIG_SYS_DCACHE_OFF - Do not enable data cache in U-Boot CONFIG_SYS_L2CACHE_OFF- Do not enable L2 cache in U-Boot

• Cache Configuration for ARM: CONFIG_SYS_L2_PL310 - Enable support for ARM PL310 L2 cache controller CONFIG_SYS_PL310_BASE - Physical base address of PL310 controller register space

• Serial Ports: CONFIG_PL010_SERIAL

  Define this if you want support for Amba PrimeCell PL010 UARTs.
  
  CONFIG_PL011_SERIAL
  
  Define this if you want support for Amba PrimeCell PL011 UARTs.
  
  CONFIG_PL011_CLOCK
  
  If you have Amba PrimeCell PL011 UARTs, set this variable to
  the clock speed of the UARTs.
  
  CONFIG_PL01x_PORTS
  
  If you have Amba PrimeCell PL010 or PL011 UARTs on your board,
  define this to a list of base addresses for each (supported)
  port. See e.g. include/configs/versatile.h
  
  CONFIG_SERIAL_HW_FLOW_CONTROL
  
  Define this variable to enable hw flow control in serial driver.
  Current user of this option is drivers/serial/nsl16550.c driver

• Console Baudrate: CONFIG_BAUDRATE - in bps Select one of the baudrates listed in CONFIG_SYS_BAUDRATE_TABLE, see below.

• Autoboot Command: CONFIG_BOOTCOMMAND Only needed when CONFIG_BOOTDELAY is enabled; define a command string that is automatically executed when no character is read on the console interface within "Boot Delay" after reset.

  CONFIG_RAMBOOT and CONFIG_NFSBOOT
  The value of these goes into the environment as
  "ramboot" and "nfsboot" respectively, and can be used
  as a convenience, when switching between booting from
  RAM and NFS.

• Pre-Boot Commands: CONFIG_PREBOOT

  When this option is #defined, the existence of the
  environment variable "preboot" will be checked
  immediately before starting the CONFIG_BOOTDELAY
  countdown and/or running the auto-boot command resp.
  entering interactive mode.
  
  This feature is especially useful when "preboot" is
  automatically generated or modified. For an example
  see the LWMON board specific code: here "preboot" is
  modified when the user holds down a certain
  combination of keys on the (special) keyboard when
  booting the systems

• Serial Download Echo Mode: CONFIG_LOADS_ECHO If defined to 1, all characters received during a serial download (using the "loads" command) are echoed back. This might be needed by some terminal emulations (like "cu"), but may as well just take time on others. This setting #define's the initial value of the "loads_echo" environment variable.

• Kgdb Serial Baudrate: (if CONFIG_CMD_KGDB is defined) CONFIG_KGDB_BAUDRATE Select one of the baudrates listed in CONFIG_SYS_BAUDRATE_TABLE, see below.

• Removal of commands If no commands are needed to boot, you can disable CONFIG_CMDLINE to remove them. In this case, the command line will not be available, and when U-Boot wants to execute the boot command (on start-up) it will call board_run_command() instead. This can reduce image size significantly for very simple boot procedures.

• Regular expression support: CONFIG_REGEX If this variable is defined, U-Boot is linked against the SLRE (Super Light Regular Expression) library, which adds regex support to some commands, as for example "env grep" and "setexpr".

• Device tree: CONFIG_OF_CONTROL If this variable is defined, U-Boot will use a device tree to configure its devices, instead of relying on statically compiled #defines in the board file. This option is experimental and only available on a few boards. The device tree is available in the global data as gd->fdt_blob.

  U-Boot needs to get its device tree from somewhere. This can
  be done using one of the three options below:
  
  CONFIG_OF_EMBED
  If this variable is defined, U-Boot will embed a device tree
  binary in its image. This device tree file should be in the
  board directory and called <soc>-<board>.dts. The binary file
  is then picked up in board_init_f() and made available through
  the global data structure as gd->fdt_blob.
  
  CONFIG_OF_SEPARATE
  If this variable is defined, U-Boot will build a device tree
  binary. It will be called u-boot.dtb. Architecture-specific
  code will locate it at run-time. Generally this works by:
  
      cat u-boot.bin u-boot.dtb >image.bin
  
  and in fact, U-Boot does this for you, creating a file called
  u-boot-dtb.bin which is useful in the common case. You can
  still use the individual files if you need something more
  exotic.
  
  CONFIG_OF_BOARD
  If this variable is defined, U-Boot will use the device tree
  provided by the board at runtime instead of embedding one with
  the image. Only boards defining board_fdt_blob_setup() support
  this option (see include/fdtdec.h file).

• Watchdog: CONFIG_WATCHDOG If this variable is defined, it enables watchdog support for the SoC. There must be support in the SoC specific code for a watchdog. For the 8xx CPUs, the SIU Watchdog feature is enabled in the SYPCR register. When supported for a specific SoC is available, then no further board specific code should be needed to use it.

  CONFIG_HW_WATCHDOG
  When using a watchdog circuitry external to the used
  SoC, then define this variable and provide board
  specific code for the "hw_watchdog_reset" function.
  
  CONFIG_AT91_HW_WDT_TIMEOUT
  specify the timeout in seconds. default 2 seconds.

• Real-Time Clock:

  When CONFIG_CMD_DATE is selected, the type of the RTC
  has to be selected, too. Define exactly one of the
  following options:
  
  CONFIG_RTC_PCF8563  - use Philips PCF8563 RTC
  CONFIG_RTC_MC13XXX  - use MC13783 or MC13892 RTC
  CONFIG_RTC_MC146818 - use MC146818 RTC
  CONFIG_RTC_DS1307   - use Maxim, Inc. DS1307 RTC
  CONFIG_RTC_DS1337   - use Maxim, Inc. DS1337 RTC
  CONFIG_RTC_DS1338   - use Maxim, Inc. DS1338 RTC
  CONFIG_RTC_DS1339   - use Maxim, Inc. DS1339 RTC
  CONFIG_RTC_DS164x   - use Dallas DS164x RTC
  CONFIG_RTC_ISL1208  - use Intersil ISL1208 RTC
  CONFIG_RTC_MAX6900  - use Maxim, Inc. MAX6900 RTC
  CONFIG_RTC_DS1337_NOOSC - Turn off the OSC output for DS1337
  CONFIG_SYS_RV3029_TCR   - enable trickle charger on
                RV3029 RTC.
  
  Note that if the RTC uses I2C, then the I2C interface
  must also be configured. See I2C Support, below.

• GPIO Support: CONFIG_PCA953X - use NXP's PCA953X series I2C GPIO

  The CONFIG_SYS_I2C_PCA953X_WIDTH option specifies a list of
  chip-ngpio pairs that tell the PCA953X driver the number of
  pins supported by a particular chip.
  
  Note that if the GPIO device uses I2C, then the I2C interface
  must also be configured. See I2C Support, below.

• I/O tracing: When CONFIG_IO_TRACE is selected, U-Boot intercepts all I/O accesses and can checksum them or write a list of them out to memory. See the 'iotrace' command for details. This is useful for testing device drivers since it can confirm that the driver behaves the same way before and after a code change. Currently this is supported on sandbox and arm. To add support for your architecture, add '#include ' to the bottom of arch//include/asm/io.h and test.

  Example output from the 'iotrace stats' command is below.
  Note that if the trace buffer is exhausted, the checksum will
  still continue to operate.
  
      iotrace is enabled
      Start:  10000000    (buffer start address)
      Size:   00010000    (buffer size)
      Offset: 00000120    (current buffer offset)
      Output: 10000120    (start + offset)
      Count:  00000018    (number of trace records)
      CRC32:  9526fb66    (CRC32 of all trace records)

• Timestamp Support:

  When CONFIG_TIMESTAMP is selected, the timestamp
  (date and time) of an image is printed by image
  commands like bootm or iminfo. This option is
  automatically enabled when you select CONFIG_CMD_DATE .

• Partition Labels (disklabels) Supported: Zero or more of the following: CONFIG_MAC_PARTITION Apple's MacOS partition table. CONFIG_ISO_PARTITION ISO partition table, used on CDROM etc. CONFIG_EFI_PARTITION GPT partition table, common when EFI is the bootloader. Note 2TB partition limit; see disk/part_efi.c CONFIG_SCSI) you must configure support for at least one non-MTD partition type as well.

• IDE Reset method: CONFIG_IDE_RESET_ROUTINE - this is defined in several board configurations files but used nowhere!

  CONFIG_IDE_RESET - is this is defined, IDE Reset will
  be performed by calling the function
      ide_set_reset(int reset)
  which has to be defined in a board specific file

• ATAPI Support: CONFIG_ATAPI

  Set this to enable ATAPI support.

• LBA48 Support CONFIG_LBA48

  Set this to enable support for disks larger than 137GB
  Also look at CONFIG_SYS_64BIT_LBA.
  Whithout these , LBA48 support uses 32bit variables and will 'only'
  support disks up to 2.1TB.
  
  CONFIG_SYS_64BIT_LBA:
      When enabled, makes the IDE subsystem use 64bit sector addresses.
      Default is 32bit.

• SCSI Support: CONFIG_SYS_SCSI_MAX_LUN [8], CONFIG_SYS_SCSI_MAX_SCSI_ID [7] and CONFIG_SYS_SCSI_MAX_DEVICE [CONFIG_SYS_SCSI_MAX_SCSI_ID * CONFIG_SYS_SCSI_MAX_LUN] can be adjusted to define the maximum numbers of LUNs, SCSI ID's and target devices.

  The environment variable 'scsidevs' is set to the number of
  SCSI devices found during the last scan.

• NETWORK Support (PCI): CONFIG_E1000 Support for Intel 8254x/8257x gigabit chips.

  CONFIG_E1000_SPI
  Utility code for direct access to the SPI bus on Intel 8257x.
  This does not do anything useful unless you set at least one
  of CONFIG_CMD_E1000 or CONFIG_E1000_SPI_GENERIC.
  
  CONFIG_E1000_SPI_GENERIC
  Allow generic access to the SPI bus on the Intel 8257x, for
  example with the "sspi" command.
  
  CONFIG_EEPRO100
  Support for Intel 82557/82559/82559ER chips.
  Optional CONFIG_EEPRO100_SROM_WRITE enables EEPROM
  write routine for first time initialisation.
  
  CONFIG_TULIP
  Support for Digital 2114x chips.
  Optional CONFIG_TULIP_SELECT_MEDIA for board specific
  modem chip initialisation (KS8761/QS6611).
  
  CONFIG_NATSEMI
  Support for National dp83815 chips.
  
  CONFIG_NS8382X
  Support for National dp8382[01] gigabit chips.

• NETWORK Support (other):

  CONFIG_DRIVER_AT91EMAC
  Support for AT91RM9200 EMAC.
  
      CONFIG_RMII
      Define this to use reduced MII inteface
  
      CONFIG_DRIVER_AT91EMAC_QUIET
      If this defined, the driver is quiet.
      The driver doen't show link status messages.
  
  CONFIG_CALXEDA_XGMAC
  Support for the Calxeda XGMAC device
  
  CONFIG_LAN91C96
  Support for SMSC's LAN91C96 chips.
  
      CONFIG_LAN91C96_USE_32_BIT
      Define this to enable 32 bit addressing
  
  CONFIG_SMC91111
  Support for SMSC's LAN91C111 chip
  
      CONFIG_SMC91111_BASE
      Define this to hold the physical address
      of the device (I/O space)
  
      CONFIG_SMC_USE_32_BIT
      Define this if data bus is 32 bits
  
      CONFIG_SMC_USE_IOFUNCS
      Define this to use i/o functions instead of macros
      (some hardware wont work with macros)
  
      CONFIG_SYS_DAVINCI_EMAC_PHY_COUNT
      Define this if you have more then 3 PHYs.
  
  CONFIG_FTGMAC100
  Support for Faraday's FTGMAC100 Gigabit SoC Ethernet
  
      CONFIG_FTGMAC100_EGIGA
      Define this to use GE link update with gigabit PHY.
      Define this if FTGMAC100 is connected to gigabit PHY.
      If your system has 10/100 PHY only, it might not occur
      wrong behavior. Because PHY usually return timeout or
      useless data when polling gigabit status and gigabit
      control registers. This behavior won't affect the
      correctnessof 10/100 link speed update.
  
  CONFIG_SH_ETHER
  Support for Renesas on-chip Ethernet controller
  
      CONFIG_SH_ETHER_USE_PORT
      Define the number of ports to be used
  
      CONFIG_SH_ETHER_PHY_ADDR
      Define the ETH PHY's address
  
      CONFIG_SH_ETHER_CACHE_WRITEBACK
      If this option is set, the driver enables cache flush.

• PWM Support: CONFIG_PWM_IMX Support for PWM module on the imx6.

• TPM Support: CONFIG_TPM Support TPM devices.

  CONFIG_TPM_TIS_INFINEON
  Support for Infineon i2c bus TPM devices. Only one device
  per system is supported at this time.
  
      CONFIG_TPM_TIS_I2C_BURST_LIMITATION
      Define the burst count bytes upper limit
  
  CONFIG_TPM_ST33ZP24
  Support for STMicroelectronics TPM devices. Requires DM_TPM support.
  
      CONFIG_TPM_ST33ZP24_I2C
      Support for STMicroelectronics ST33ZP24 I2C devices.
      Requires TPM_ST33ZP24 and I2C.
  
      CONFIG_TPM_ST33ZP24_SPI
      Support for STMicroelectronics ST33ZP24 SPI devices.
      Requires TPM_ST33ZP24 and SPI.
  
  CONFIG_TPM_ATMEL_TWI
  Support for Atmel TWI TPM device. Requires I2C support.
  
  CONFIG_TPM_TIS_LPC
  Support for generic parallel port TPM devices. Only one device
  per system is supported at this time.
  
      CONFIG_TPM_TIS_BASE_ADDRESS
      Base address where the generic TPM device is mapped
      to. Contemporary x86 systems usually map it at
      0xfed40000.
  
  CONFIG_TPM
  Define this to enable the TPM support library which provides
  functional interfaces to some TPM commands.
  Requires support for a TPM device.
  
  CONFIG_TPM_AUTH_SESSIONS
  Define this to enable authorized functions in the TPM library.
  Requires CONFIG_TPM and CONFIG_SHA1.

• USB Support: At the moment only the UHCI host controller is supported (PIP405, MIP405); define CONFIG_USB_UHCI to enable it. define CONFIG_USB_KEYBOARD to enable the USB Keyboard and define CONFIG_USB_STORAGE to enable the USB storage devices. Note: Supported are USB Keyboards and USB Floppy drives (TEAC FD-05PUB).

  CONFIG_USB_EHCI_TXFIFO_THRESH enables setting of the
  txfilltuning field in the EHCI controller on reset.
  
  CONFIG_USB_DWC2_REG_ADDR the physical CPU address of the DWC2
  HW module registers.

• USB Device: Define the below if you wish to use the USB console. Once firmware is rebuilt from a serial console issue the command "setenv stdin usbtty; setenv stdout usbtty" and attach your USB cable. The Unix command "dmesg" should print it has found a new device. The environment variable usbtty can be set to gserial or cdc_acm to enable your device to appear to a USB host as a Linux gserial device or a Common Device Class Abstract Control Model serial device. If you select usbtty = gserial you should be able to enumerate a Linux host by

  modprobe usbserial vendor=0xVendorID product=0xProductID

  else if using cdc_acm, simply setting the environment
  variable usbtty to be cdc_acm should suffice. The following
  might be defined in YourBoardName.h
  
      CONFIG_USB_DEVICE
      Define this to build a UDC device
  
      CONFIG_USB_TTY
      Define this to have a tty type of device available to
      talk to the UDC device
  
      CONFIG_USBD_HS
      Define this to enable the high speed support for usb
      device and usbtty. If this feature is enabled, a routine
      int is_usbd_high_speed(void)
      also needs to be defined by the driver to dynamically poll
      whether the enumeration has succeded at high speed or full
      speed.
  
      CONFIG_SYS_CONSOLE_IS_IN_ENV
      Define this if you want stdin, stdout &/or stderr to
      be set to usbtty.
  
  If you have a USB-IF assigned VendorID then you may wish to
  define your own vendor specific values either in BoardName.h
  or directly in usbd_vendor_info.h. If you don't define
  CONFIG_USBD_MANUFACTURER, CONFIG_USBD_PRODUCT_NAME,
  CONFIG_USBD_VENDORID and CONFIG_USBD_PRODUCTID, then U-Boot
  should pretend to be a Linux device to it's target host.
  
      CONFIG_USBD_MANUFACTURER
      Define this string as the name of your company for
      - CONFIG_USBD_MANUFACTURER "my company"
  
      CONFIG_USBD_PRODUCT_NAME
      Define this string as the name of your product
      - CONFIG_USBD_PRODUCT_NAME "acme usb device"
  
      CONFIG_USBD_VENDORID
      Define this as your assigned Vendor ID from the USB
      Implementors Forum. This *must* be a genuine Vendor ID
      to avoid polluting the USB namespace.
      - CONFIG_USBD_VENDORID 0xFFFF
  
      CONFIG_USBD_PRODUCTID
      Define this as the unique Product ID
      for your device
      - CONFIG_USBD_PRODUCTID 0xFFFF

• ULPI Layer Support: The ULPI (UTMI Low Pin (count) Interface) PHYs are supported via the generic ULPI layer. The generic layer accesses the ULPI PHY via the platform viewport, so you need both the genric layer and the viewport enabled. Currently only Chipidea/ARC based viewport is supported. To enable the ULPI layer support, define CONFIG_USB_ULPI and CONFIG_USB_ULPI_VIEWPORT in your board configuration file. If your ULPI phy needs a different reference clock than the standard 24 MHz then you have to define CONFIG_ULPI_REF_CLK to the appropriate value in Hz.

• MMC Support: The MMC controller on the Intel PXA is supported. To enable this define CONFIG_MMC. The MMC can be accessed from the boot prompt by mapping the device to physical memory similar to flash. Command line is enabled with CONFIG_CMD_MMC. The MMC driver also works with the FAT fs. This is enabled with CONFIG_CMD_FAT.

  CONFIG_SH_MMCIF
  Support for Renesas on-chip MMCIF controller
  
      CONFIG_SH_MMCIF_ADDR
      Define the base address of MMCIF registers
  
      CONFIG_SH_MMCIF_CLK
      Define the clock frequency for MMCIF
  
  CONFIG_SUPPORT_EMMC_BOOT
  Enable some additional features of the eMMC boot partitions.

• USB Device Firmware Update (DFU) class support: CONFIG_DFU_OVER_USB This enables the USB portion of the DFU USB class

  CONFIG_DFU_MMC
  This enables support for exposing (e)MMC devices via DFU.
  
  CONFIG_DFU_NAND
  This enables support for exposing NAND devices via DFU.
  
  CONFIG_DFU_RAM
  This enables support for exposing RAM via DFU.
  Note: DFU spec refer to non-volatile memory usage, but
  allow usages beyond the scope of spec - here RAM usage,
  one that would help mostly the developer.
  
  CONFIG_SYS_DFU_DATA_BUF_SIZE
  Dfu transfer uses a buffer before writing data to the
  raw storage device. Make the size (in bytes) of this buffer
  configurable. The size of this buffer is also configurable
  through the "dfu_bufsiz" environment variable.
  
  CONFIG_SYS_DFU_MAX_FILE_SIZE
  When updating files rather than the raw storage device,
  we use a static buffer to copy the file into and then write
  the buffer once we've been given the whole file.  Define
  this to the maximum filesize (in bytes) for the buffer.
  Default is 4 MiB if undefined.
  
  DFU_DEFAULT_POLL_TIMEOUT
  Poll timeout [ms], is the timeout a device can send to the
  host. The host must wait for this timeout before sending
  a subsequent DFU_GET_STATUS request to the device.
  
  DFU_MANIFEST_POLL_TIMEOUT
  Poll timeout [ms], which the device sends to the host when
  entering dfuMANIFEST state. Host waits this timeout, before
  sending again an USB request to the device.

• Journaling Flash filesystem support: CONFIG_JFFS2_NAND Define these for a default partition on a NAND device

  CONFIG_SYS_JFFS2_FIRST_SECTOR,
  CONFIG_SYS_JFFS2_FIRST_BANK, CONFIG_SYS_JFFS2_NUM_BANKS
  Define these for a default partition on a NOR device

• Keyboard Support: See Kconfig help for available keyboard drivers.

  CONFIG_KEYBOARD
  
  Define this to enable a custom keyboard support.
  This simply calls drv_keyboard_init() which must be
  defined in your board-specific files. This option is deprecated
  and is only used by novena. For new boards, use driver model
  instead.

• Video support: CONFIG_FSL_DIU_FB Enable the Freescale DIU video driver. Reference boards for SOCs that have a DIU should define this macro to enable DIU support, and should also define these other macros:

      CONFIG_SYS_DIU_ADDR
      CONFIG_VIDEO
      CONFIG_CFB_CONSOLE
      CONFIG_VIDEO_SW_CURSOR
      CONFIG_VGA_AS_SINGLE_DEVICE
      CONFIG_VIDEO_LOGO
      CONFIG_VIDEO_BMP_LOGO
  
  The DIU driver will look for the 'video-mode' environment
  variable, and if defined, enable the DIU as a console during
  boot.  See the documentation file doc/README.video for a
  description of this variable.

• LCD Support: CONFIG_LCD

  Define this to enable LCD support (for output to LCD
  display); also select one of the supported displays
  by defining one of these:
  
  CONFIG_ATMEL_LCD:
  
      HITACHI TX09D70VM1CCA, 3.5", 240x320.
  
  CONFIG_NEC_NL6448AC33:
  
      NEC NL6448AC33-18. Active, color, single scan.
  
  CONFIG_NEC_NL6448BC20
  
      NEC NL6448BC20-08. 6.5", 640x480.
      Active, color, single scan.
  
  CONFIG_NEC_NL6448BC33_54
  
      NEC NL6448BC33-54. 10.4", 640x480.
      Active, color, single scan.
  
  CONFIG_SHARP_16x9
  
      Sharp 320x240. Active, color, single scan.
      It isn't 16x9, and I am not sure what it is.
  
  CONFIG_SHARP_LQ64D341
  
      Sharp LQ64D341 display, 640x480.
      Active, color, single scan.
  
  CONFIG_HLD1045
  
      HLD1045 display, 640x480.
      Active, color, single scan.
  
  CONFIG_OPTREX_BW
  
      Optrex   CBL50840-2 NF-FW 99 22 M5
      or
      Hitachi  LMG6912RPFC-00T
      or
      Hitachi  SP14Q002
  
      320x240. Black & white.
  
  CONFIG_LCD_ALIGNMENT
  
  Normally the LCD is page-aligned (typically 4KB). If this is
  defined then the LCD will be aligned to this value instead.
  For ARM it is sometimes useful to use MMU_SECTION_SIZE
  here, since it is cheaper to change data cache settings on
  a per-section basis.
  
  CONFIG_LCD_ROTATION
  
  Sometimes, for example if the display is mounted in portrait
  mode or even if it's mounted landscape but rotated by 180degree,
  we need to rotate our content of the display relative to the
  framebuffer, so that user can read the messages which are
  printed out.
  Once CONFIG_LCD_ROTATION is defined, the lcd_console will be
  initialized with a given rotation from "vl_rot" out of
  "vidinfo_t" which is provided by the board specific code.
  The value for vl_rot is coded as following (matching to
  fbcon=rotate:<n> linux-kernel commandline):
  0 = no rotation respectively 0 degree
  1 = 90 degree rotation
  2 = 180 degree rotation
  3 = 270 degree rotation
  
  If CONFIG_LCD_ROTATION is not defined, the console will be
  initialized with 0degree rotation.
  
  CONFIG_LCD_BMP_RLE8
  
  Support drawing of RLE8-compressed bitmaps on the LCD.
  
  CONFIG_I2C_EDID
  
  Enables an 'i2c edid' command which can read EDID
  information over I2C from an attached LCD display.

• Splash Screen Support: CONFIG_SPLASH_SCREEN

  If this option is set, the environment is checked for
  a variable "splashimage". If found, the usual display
  of logo, copyright and system information on the LCD
  is suppressed and the BMP image at the address
  specified in "splashimage" is loaded instead. The
  console is redirected to the "nulldev", too. This
  allows for a "silent" boot where a splash screen is
  loaded very quickly after power-on.
  
  CONFIG_SPLASHIMAGE_GUARD
  
  If this option is set, then U-Boot will prevent the environment
  variable "splashimage" from being set to a problematic address
  (see doc/README.displaying-bmps).
  This option is useful for targets where, due to alignment
  restrictions, an improperly aligned BMP image will cause a data
  abort. If you think you will not have problems with unaligned
  accesses (for example because your toolchain prevents them)
  there is no need to set this option.
  
  CONFIG_SPLASH_SCREEN_ALIGN
  
  If this option is set the splash image can be freely positioned
  on the screen. Environment variable "splashpos" specifies the
  position as "x,y". If a positive number is given it is used as
  number of pixel from left/top. If a negative number is given it
  is used as number of pixel from right/bottom. You can also
  specify 'm' for centering the image.
  
  Example:
  setenv splashpos m,m
      => image at center of screen
  
  setenv splashpos 30,20
      => image at x = 30 and y = 20
  
  setenv splashpos -10,m
      => vertically centered image
         at x = dspWidth - bmpWidth - 9

• Gzip compressed BMP image support: CONFIG_VIDEO_BMP_GZIP

  If this option is set, additionally to standard BMP
  images, gzipped BMP images can be displayed via the
  splashscreen support or the bmp command.

• Run length encoded BMP image (RLE8) support: CONFIG_VIDEO_BMP_RLE8

  If this option is set, 8-bit RLE compressed BMP images
  can be displayed via the splashscreen support or the
  bmp command.

• Compression support: CONFIG_GZIP

  Enabled by default to support gzip compressed images.
  
  CONFIG_BZIP2
  
  If this option is set, support for bzip2 compressed
  images is included. If not, only uncompressed and gzip
  compressed images are supported.
  
  NOTE: the bzip2 algorithm requires a lot of RAM, so
  the malloc area (as defined by CONFIG_SYS_MALLOC_LEN) should
  be at least 4MB.

• MII/PHY support: CONFIG_PHY_CLOCK_FREQ (ppc4xx)

  The clock frequency of the MII bus
  
  CONFIG_PHY_RESET_DELAY
  
  Some PHY like Intel LXT971A need extra delay after
  reset before any MII register access is possible.
  For such PHY, set this option to the usec delay
  required. (minimum 300usec for LXT971A)
  
  CONFIG_PHY_CMD_DELAY (ppc4xx)
  
  Some PHY like Intel LXT971A need extra delay after
  command issued before MII status register can be read

• IP address: CONFIG_IPADDR

  Define a default value for the IP address to use for
  the default Ethernet interface, in case this is not
  determined through e.g. bootp.
  (Environment variable "ipaddr")

• Server IP address: CONFIG_SERVERIP

  Defines a default value for the IP address of a TFTP
  server to contact when using the "tftboot" command.
  (Environment variable "serverip")
  
  CONFIG_KEEP_SERVERADDR
  
  Keeps the server's MAC address, in the env 'serveraddr'
  for passing to bootargs (like Linux's netconsole option)

• Gateway IP address: CONFIG_GATEWAYIP

  Defines a default value for the IP address of the
  default router where packets to other networks are
  sent to.
  (Environment variable "gatewayip")

• Subnet mask: CONFIG_NETMASK

  Defines a default value for the subnet mask (or
  routing prefix) which is used to determine if an IP
  address belongs to the local subnet or needs to be
  forwarded through a router.
  (Environment variable "netmask")

• Multicast TFTP Mode: CONFIG_MCAST_TFTP

  Defines whether you want to support multicast TFTP as per
  rfc-2090; for example to work with atftp.  Lets lots of targets
  tftp down the same boot image concurrently.  Note: the Ethernet
  driver in use must provide a function: mcast() to join/leave a
  multicast group.

• BOOTP Recovery Mode: CONFIG_BOOTP_RANDOM_DELAY

  If you have many targets in a network that try to
  boot using BOOTP, you may want to avoid that all
  systems send out BOOTP requests at precisely the same
  moment (which would happen for instance at recovery
  from a power failure, when all systems will try to
  boot, thus flooding the BOOTP server. Defining
  CONFIG_BOOTP_RANDOM_DELAY causes a random delay to be
  inserted before sending out BOOTP requests. The
  following delays are inserted then:
  
  1st BOOTP request:  delay 0 ... 1 sec
  2nd BOOTP request:  delay 0 ... 2 sec
  3rd BOOTP request:  delay 0 ... 4 sec
  4th and following
  BOOTP requests:     delay 0 ... 8 sec
  
  CONFIG_BOOTP_ID_CACHE_SIZE
  
  BOOTP packets are uniquely identified using a 32-bit ID. The
  server will copy the ID from client requests to responses and
  U-Boot will use this to determine if it is the destination of
  an incoming response. Some servers will check that addresses
  aren't in use before handing them out (usually using an ARP
  ping) and therefore take up to a few hundred milliseconds to
  respond. Network congestion may also influence the time it
  takes for a response to make it back to the client. If that
  time is too long, U-Boot will retransmit requests. In order
  to allow earlier responses to still be accepted after these
  retransmissions, U-Boot's BOOTP client keeps a small cache of
  IDs. The CONFIG_BOOTP_ID_CACHE_SIZE controls the size of this
  cache. The default is to keep IDs for up to four outstanding
  requests. Increasing this will allow U-Boot to accept offers
  from a BOOTP client in networks with unusually high latency.

• DHCP Advanced Options: You can fine tune the DHCP functionality by defining CONFIGBOOTP* symbols:

  CONFIG_BOOTP_NISDOMAIN
  CONFIG_BOOTP_BOOTFILESIZE
  CONFIG_BOOTP_SEND_HOSTNAME
  CONFIG_BOOTP_NTPSERVER
  CONFIG_BOOTP_TIMEOFFSET
  CONFIG_BOOTP_VENDOREX
  CONFIG_BOOTP_MAY_FAIL
  
  CONFIG_BOOTP_SERVERIP - TFTP server will be the serverip
  environment variable, not the BOOTP server.
  
  CONFIG_BOOTP_MAY_FAIL - If the DHCP server is not found
  after the configured retry count, the call will fail
  instead of starting over.  This can be used to fail over
  to Link-local IP address configuration if the DHCP server
  is not available.
  
  CONFIG_BOOTP_SEND_HOSTNAME - Some DHCP servers are capable
  to do a dynamic update of a DNS server. To do this, they
  need the hostname of the DHCP requester.
  If CONFIG_BOOTP_SEND_HOSTNAME is defined, the content
  of the "hostname" environment variable is passed as
  option 12 to the DHCP server.
  
  CONFIG_BOOTP_DHCP_REQUEST_DELAY
  
  A 32bit value in microseconds for a delay between
  receiving a "DHCP Offer" and sending the "DHCP Request".
  This fixes a problem with certain DHCP servers that don't
  respond 100% of the time to a "DHCP request". E.g. On an
  AT91RM9200 processor running at 180MHz, this delay needed
  to be *at least* 15,000 usec before a Windows Server 2003
  DHCP server would reply 100% of the time. I recommend at
  least 50,000 usec to be safe. The alternative is to hope
  that one of the retries will be successful but note that
  the DHCP timeout and retry process takes a longer than
  this delay.

  • Link-local IP address negotiation: Negotiate with other link-local clients on the local network for an address that doesn't require explicit configuration. This is especially useful if a DHCP server cannot be guaranteed to exist in all environments that the device must operate.

    See doc/README.link-local for more information.

  • MAC address from environment variables

    FDT_SEQ_MACADDR_FROM_ENV

    Fix-up device tree with MAC addresses fetched sequentially from environment variables. This config work on assumption that non-usable ethernet node of device-tree are either not present or their status has been marked as "disabled".

  • CDP Options: CONFIG_CDP_DEVICE_ID

    The device id used in CDP trigger frames.

    CONFIG_CDP_DEVICE_ID_PREFIX

    A two character string which is prefixed to the MAC address of the device.

    CONFIG_CDP_PORT_ID

    A printf format string which contains the ascii name of the port. Normally is set to "eth%d" which sets eth0 for the first Ethernet, eth1 for the second etc.

    CONFIG_CDP_CAPABILITIES

    A 32bit integer which indicates the device capabilities; 0x00000010 for a normal host which does not forwards.

    CONFIG_CDP_VERSION

    An ascii string containing the version of the software.

    CONFIG_CDP_PLATFORM

    An ascii string containing the name of the platform.

    CONFIG_CDP_TRIGGER

    A 32bit integer sent on the trigger.

    CONFIG_CDP_POWER_CONSUMPTION

    A 16bit integer containing the power consumption of the device in .1 of milliwatts.

    CONFIG_CDP_APPLIANCE_VLAN_TYPE

    A byte containing the id of the VLAN.

• Status LED: CONFIG_LED_STATUS

  Several configurations allow to display the current
  status using a LED. For instance, the LED will blink
  fast while running U-Boot code, stop blinking as
  soon as a reply to a BOOTP request was received, and
  start blinking slow once the Linux kernel is running
  (supported by a status LED driver in the Linux
  kernel). Defining CONFIG_LED_STATUS enables this
  feature in U-Boot.
  
  Additional options:
  
  CONFIG_LED_STATUS_GPIO
  The status LED can be connected to a GPIO pin.
  In such cases, the gpio_led driver can be used as a
  status LED backend implementation. Define CONFIG_LED_STATUS_GPIO
  to include the gpio_led driver in the U-Boot binary.
  
  CONFIG_GPIO_LED_INVERTED_TABLE
  Some GPIO connected LEDs may have inverted polarity in which
  case the GPIO high value corresponds to LED off state and
  GPIO low value corresponds to LED on state.
  In such cases CONFIG_GPIO_LED_INVERTED_TABLE may be defined
  with a list of GPIO LEDs that have inverted polarity.

• I2C Support: CONFIG_SYS_I2C

  This enable the NEW i2c subsystem, and will allow you to use
  i2c commands at the u-boot command line (as long as you set
  CONFIG_CMD_I2C in CONFIG_COMMANDS) and communicate with i2c
  based realtime clock chips or other i2c devices. See
  common/cmd_i2c.c for a description of the command line
  interface.
  
  ported i2c driver to the new framework:
  - drivers/i2c/soft_i2c.c:
    - activate first bus with CONFIG_SYS_I2C_SOFT define
      CONFIG_SYS_I2C_SOFT_SPEED and CONFIG_SYS_I2C_SOFT_SLAVE
      for defining speed and slave address
    - activate second bus with I2C_SOFT_DECLARATIONS2 define
      CONFIG_SYS_I2C_SOFT_SPEED_2 and CONFIG_SYS_I2C_SOFT_SLAVE_2
      for defining speed and slave address
    - activate third bus with I2C_SOFT_DECLARATIONS3 define
      CONFIG_SYS_I2C_SOFT_SPEED_3 and CONFIG_SYS_I2C_SOFT_SLAVE_3
      for defining speed and slave address
    - activate fourth bus with I2C_SOFT_DECLARATIONS4 define
      CONFIG_SYS_I2C_SOFT_SPEED_4 and CONFIG_SYS_I2C_SOFT_SLAVE_4
      for defining speed and slave address
  
  - drivers/i2c/fsl_i2c.c:
    - activate i2c driver with CONFIG_SYS_I2C_FSL
      define CONFIG_SYS_FSL_I2C_OFFSET for setting the register
      offset CONFIG_SYS_FSL_I2C_SPEED for the i2c speed and
      CONFIG_SYS_FSL_I2C_SLAVE for the slave addr of the first
      bus.
    - If your board supports a second fsl i2c bus, define
      CONFIG_SYS_FSL_I2C2_OFFSET for the register offset
      CONFIG_SYS_FSL_I2C2_SPEED for the speed and
      CONFIG_SYS_FSL_I2C2_SLAVE for the slave address of the
      second bus.
  
  - drivers/i2c/tegra_i2c.c:
    - activate this driver with CONFIG_SYS_I2C_TEGRA
    - This driver adds 4 i2c buses with a fix speed from
      100000 and the slave addr 0!
  
  - drivers/i2c/ppc4xx_i2c.c
    - activate this driver with CONFIG_SYS_I2C_PPC4XX
    - CONFIG_SYS_I2C_PPC4XX_CH0 activate hardware channel 0
    - CONFIG_SYS_I2C_PPC4XX_CH1 activate hardware channel 1
  
  - drivers/i2c/i2c_mxc.c
    - activate this driver with CONFIG_SYS_I2C_MXC
    - enable bus 1 with CONFIG_SYS_I2C_MXC_I2C1
    - enable bus 2 with CONFIG_SYS_I2C_MXC_I2C2
    - enable bus 3 with CONFIG_SYS_I2C_MXC_I2C3
    - enable bus 4 with CONFIG_SYS_I2C_MXC_I2C4
    - define speed for bus 1 with CONFIG_SYS_MXC_I2C1_SPEED
    - define slave for bus 1 with CONFIG_SYS_MXC_I2C1_SLAVE
    - define speed for bus 2 with CONFIG_SYS_MXC_I2C2_SPEED
    - define slave for bus 2 with CONFIG_SYS_MXC_I2C2_SLAVE
    - define speed for bus 3 with CONFIG_SYS_MXC_I2C3_SPEED
    - define slave for bus 3 with CONFIG_SYS_MXC_I2C3_SLAVE
    - define speed for bus 4 with CONFIG_SYS_MXC_I2C4_SPEED
    - define slave for bus 4 with CONFIG_SYS_MXC_I2C4_SLAVE
  If those defines are not set, default value is 100000
  for speed, and 0 for slave.
  
  - drivers/i2c/rcar_i2c.c:
    - activate this driver with CONFIG_SYS_I2C_RCAR
    - This driver adds 4 i2c buses
  
    - CONFIG_SYS_RCAR_I2C0_BASE for setting the register channel 0
    - CONFIG_SYS_RCAR_I2C0_SPEED for for the speed channel 0
    - CONFIG_SYS_RCAR_I2C1_BASE for setting the register channel 1
    - CONFIG_SYS_RCAR_I2C1_SPEED for for the speed channel 1
    - CONFIG_SYS_RCAR_I2C2_BASE for setting the register channel 2
    - CONFIG_SYS_RCAR_I2C2_SPEED for for the speed channel 2
    - CONFIG_SYS_RCAR_I2C3_BASE for setting the register channel 3
    - CONFIG_SYS_RCAR_I2C3_SPEED for for the speed channel 3
    - CONFIF_SYS_RCAR_I2C_NUM_CONTROLLERS for number of i2c buses
  
  - drivers/i2c/sh_i2c.c:
    - activate this driver with CONFIG_SYS_I2C_SH
    - This driver adds from 2 to 5 i2c buses
  
    - CONFIG_SYS_I2C_SH_BASE0 for setting the register channel 0
    - CONFIG_SYS_I2C_SH_SPEED0 for for the speed channel 0
    - CONFIG_SYS_I2C_SH_BASE1 for setting the register channel 1
    - CONFIG_SYS_I2C_SH_SPEED1 for for the speed channel 1
    - CONFIG_SYS_I2C_SH_BASE2 for setting the register channel 2
    - CONFIG_SYS_I2C_SH_SPEED2 for for the speed channel 2
    - CONFIG_SYS_I2C_SH_BASE3 for setting the register channel 3
    - CONFIG_SYS_I2C_SH_SPEED3 for for the speed channel 3
    - CONFIG_SYS_I2C_SH_BASE4 for setting the register channel 4
    - CONFIG_SYS_I2C_SH_SPEED4 for for the speed channel 4
    - CONFIG_SYS_I2C_SH_NUM_CONTROLLERS for number of i2c buses
  
  - drivers/i2c/omap24xx_i2c.c
    - activate this driver with CONFIG_SYS_I2C_OMAP24XX
    - CONFIG_SYS_OMAP24_I2C_SPEED speed channel 0
    - CONFIG_SYS_OMAP24_I2C_SLAVE slave addr channel 0
    - CONFIG_SYS_OMAP24_I2C_SPEED1 speed channel 1
    - CONFIG_SYS_OMAP24_I2C_SLAVE1 slave addr channel 1
    - CONFIG_SYS_OMAP24_I2C_SPEED2 speed channel 2
    - CONFIG_SYS_OMAP24_I2C_SLAVE2 slave addr channel 2
    - CONFIG_SYS_OMAP24_I2C_SPEED3 speed channel 3
    - CONFIG_SYS_OMAP24_I2C_SLAVE3 slave addr channel 3
    - CONFIG_SYS_OMAP24_I2C_SPEED4 speed channel 4
    - CONFIG_SYS_OMAP24_I2C_SLAVE4 slave addr channel 4
  
  - drivers/i2c/zynq_i2c.c
    - activate this driver with CONFIG_SYS_I2C_ZYNQ
    - set CONFIG_SYS_I2C_ZYNQ_SPEED for speed setting
    - set CONFIG_SYS_I2C_ZYNQ_SLAVE for slave addr
  
  - drivers/i2c/s3c24x0_i2c.c:
    - activate this driver with CONFIG_SYS_I2C_S3C24X0
    - This driver adds i2c buses (11 for Exynos5250, Exynos5420
      9 i2c buses for Exynos4 and 1 for S3C24X0 SoCs from Samsung)
      with a fix speed from 100000 and the slave addr 0!
  
  - drivers/i2c/ihs_i2c.c
    - activate this driver with CONFIG_SYS_I2C_IHS
    - CONFIG_SYS_I2C_IHS_CH0 activate hardware channel 0
    - CONFIG_SYS_I2C_IHS_SPEED_0 speed channel 0
    - CONFIG_SYS_I2C_IHS_SLAVE_0 slave addr channel 0
    - CONFIG_SYS_I2C_IHS_CH1 activate hardware channel 1
    - CONFIG_SYS_I2C_IHS_SPEED_1 speed channel 1
    - CONFIG_SYS_I2C_IHS_SLAVE_1 slave addr channel 1
    - CONFIG_SYS_I2C_IHS_CH2 activate hardware channel 2
    - CONFIG_SYS_I2C_IHS_SPEED_2 speed channel 2
    - CONFIG_SYS_I2C_IHS_SLAVE_2 slave addr channel 2
    - CONFIG_SYS_I2C_IHS_CH3 activate hardware channel 3
    - CONFIG_SYS_I2C_IHS_SPEED_3 speed channel 3
    - CONFIG_SYS_I2C_IHS_SLAVE_3 slave addr channel 3
    - activate dual channel with CONFIG_SYS_I2C_IHS_DUAL
    - CONFIG_SYS_I2C_IHS_SPEED_0_1 speed channel 0_1
    - CONFIG_SYS_I2C_IHS_SLAVE_0_1 slave addr channel 0_1
    - CONFIG_SYS_I2C_IHS_SPEED_1_1 speed channel 1_1
    - CONFIG_SYS_I2C_IHS_SLAVE_1_1 slave addr channel 1_1
    - CONFIG_SYS_I2C_IHS_SPEED_2_1 speed channel 2_1
    - CONFIG_SYS_I2C_IHS_SLAVE_2_1 slave addr channel 2_1
    - CONFIG_SYS_I2C_IHS_SPEED_3_1 speed channel 3_1
    - CONFIG_SYS_I2C_IHS_SLAVE_3_1 slave addr channel 3_1
  
  additional defines:
  
  CONFIG_SYS_NUM_I2C_BUSES
  Hold the number of i2c buses you want to use.
  
  CONFIG_SYS_I2C_DIRECT_BUS
  define this, if you don't use i2c muxes on your hardware.
  if CONFIG_SYS_I2C_MAX_HOPS is not defined or == 0 you can
  omit this define.
  
  CONFIG_SYS_I2C_MAX_HOPS
  define how many muxes are maximal consecutively connected
  on one i2c bus. If you not use i2c muxes, omit this
  define.
  
  CONFIG_SYS_I2C_BUSES
  hold a list of buses you want to use, only used if
  CONFIG_SYS_I2C_DIRECT_BUS is not defined, for example
  a board with CONFIG_SYS_I2C_MAX_HOPS = 1 and
  CONFIG_SYS_NUM_I2C_BUSES = 9:
  
   CONFIG_SYS_I2C_BUSES   {{0, {I2C_NULL_HOP}}, \
              {0, {{I2C_MUX_PCA9547, 0x70, 1}}}, \
              {0, {{I2C_MUX_PCA9547, 0x70, 2}}}, \
              {0, {{I2C_MUX_PCA9547, 0x70, 3}}}, \
              {0, {{I2C_MUX_PCA9547, 0x70, 4}}}, \
              {0, {{I2C_MUX_PCA9547, 0x70, 5}}}, \
              {1, {I2C_NULL_HOP}}, \
              {1, {{I2C_MUX_PCA9544, 0x72, 1}}}, \
              {1, {{I2C_MUX_PCA9544, 0x72, 2}}}, \
              }
  
  which defines
      bus 0 on adapter 0 without a mux
      bus 1 on adapter 0 with a PCA9547 on address 0x70 port 1
      bus 2 on adapter 0 with a PCA9547 on address 0x70 port 2
      bus 3 on adapter 0 with a PCA9547 on address 0x70 port 3
      bus 4 on adapter 0 with a PCA9547 on address 0x70 port 4
      bus 5 on adapter 0 with a PCA9547 on address 0x70 port 5
      bus 6 on adapter 1 without a mux
      bus 7 on adapter 1 with a PCA9544 on address 0x72 port 1
      bus 8 on adapter 1 with a PCA9544 on address 0x72 port 2
  
  If you do not have i2c muxes on your board, omit this define.

• Legacy I2C Support: If you use the software i2c interface (CONFIG_SYS_I2C_SOFT) then the following macros need to be defined (examples are from include/configs/lwmon.h):

  I2C_INIT
  
  (Optional). Any commands necessary to enable the I2C
  controller or configure ports.
  
  eg: #define I2C_INIT (immr->im_cpm.cp_pbdir |=  PB_SCL)
  
  I2C_ACTIVE
  
  The code necessary to make the I2C data line active
  (driven).  If the data line is open collector, this
  define can be null.
  
  eg: #define I2C_ACTIVE (immr->im_cpm.cp_pbdir |=  PB_SDA)
  
  I2C_TRISTATE
  
  The code necessary to make the I2C data line tri-stated
  (inactive).  If the data line is open collector, this
  define can be null.
  
  eg: #define I2C_TRISTATE (immr->im_cpm.cp_pbdir &= ~PB_SDA)
  
  I2C_READ
  
  Code that returns true if the I2C data line is high,
  false if it is low.
  
  eg: #define I2C_READ ((immr->im_cpm.cp_pbdat & PB_SDA) != 0)
  
  I2C_SDA(bit)
  
  If <bit> is true, sets the I2C data line high. If it
  is false, it clears it (low).
  
  eg: #define I2C_SDA(bit) \
      if(bit) immr->im_cpm.cp_pbdat |=  PB_SDA; \
      else    immr->im_cpm.cp_pbdat &= ~PB_SDA
  
  I2C_SCL(bit)
  
  If <bit> is true, sets the I2C clock line high. If it
  is false, it clears it (low).
  
  eg: #define I2C_SCL(bit) \
      if(bit) immr->im_cpm.cp_pbdat |=  PB_SCL; \
      else    immr->im_cpm.cp_pbdat &= ~PB_SCL
  
  I2C_DELAY
  
  This delay is invoked four times per clock cycle so this
  controls the rate of data transfer.  The data rate thus
  is 1 / (I2C_DELAY * 4). Often defined to be something
  like:
  
  #define I2C_DELAY  udelay(2)
  
  CONFIG_SOFT_I2C_GPIO_SCL / CONFIG_SOFT_I2C_GPIO_SDA
  
  If your arch supports the generic GPIO framework (asm/gpio.h),
  then you may alternatively define the two GPIOs that are to be
  used as SCL / SDA.  Any of the previous I2C_xxx macros will
  have GPIO-based defaults assigned to them as appropriate.
  
  You should define these to the GPIO value as given directly to
  the generic GPIO functions.
  
  CONFIG_SYS_I2C_INIT_BOARD
  
  When a board is reset during an i2c bus transfer
  chips might think that the current transfer is still
  in progress. On some boards it is possible to access
  the i2c SCLK line directly, either by using the
  processor pin as a GPIO or by having a second pin
  connected to the bus. If this option is defined a
  custom i2c_init_board() routine in boards/xxx/board.c
  is run early in the boot sequence.
  
  CONFIG_I2C_MULTI_BUS
  
  This option allows the use of multiple I2C buses, each of which
  must have a controller.  At any point in time, only one bus is
  active.  To switch to a different bus, use the 'i2c dev' command.
  Note that bus numbering is zero-based.
  
  CONFIG_SYS_I2C_NOPROBES
  
  This option specifies a list of I2C devices that will be skipped
  when the 'i2c probe' command is issued.  If CONFIG_I2C_MULTI_BUS
  is set, specify a list of bus-device pairs.  Otherwise, specify
  a 1D array of device addresses
  
  e.g.
      #undef  CONFIG_I2C_MULTI_BUS
      #define CONFIG_SYS_I2C_NOPROBES {0x50,0x68}
  
  will skip addresses 0x50 and 0x68 on a board with one I2C bus
  
      #define CONFIG_I2C_MULTI_BUS
      #define CONFIG_SYS_I2C_NOPROBES {{0,0x50},{0,0x68},{1,0x54}}
  
  will skip addresses 0x50 and 0x68 on bus 0 and address 0x54 on bus 1
  
  CONFIG_SYS_SPD_BUS_NUM
  
  If defined, then this indicates the I2C bus number for DDR SPD.
  If not defined, then U-Boot assumes that SPD is on I2C bus 0.
  
  CONFIG_SYS_RTC_BUS_NUM
  
  If defined, then this indicates the I2C bus number for the RTC.
  If not defined, then U-Boot assumes that RTC is on I2C bus 0.
  
  CONFIG_SOFT_I2C_READ_REPEATED_START
  
  defining this will force the i2c_read() function in
  the soft_i2c driver to perform an I2C repeated start
  between writing the address pointer and reading the
  data.  If this define is omitted the default behaviour
  of doing a stop-start sequence will be used.  Most I2C
  devices can use either method, but some require one or
  the other.

• SPI Support: CONFIG_SPI

  Enables SPI driver (so far only tested with
  SPI EEPROM, also an instance works with Crystal A/D and
  D/As on the SACSng board)
  
  CONFIG_SOFT_SPI
  
  Enables a software (bit-bang) SPI driver rather than
  using hardware support. This is a general purpose
  driver that only requires three general I/O port pins
  (two outputs, one input) to function. If this is
  defined, the board configuration must define several
  SPI configuration items (port pins to use, etc). For
  an example, see include/configs/sacsng.h.
  
  CONFIG_SYS_SPI_MXC_WAIT
  Timeout for waiting until spi transfer completed.
  default: (CONFIG_SYS_HZ/100)     /* 10 ms */

• FPGA Support: CONFIG_FPGA

  Enables FPGA subsystem.
  
  CONFIG_FPGA_<vendor>
  
  Enables support for specific chip vendors.
  (ALTERA, XILINX)
  
  CONFIG_FPGA_<family>
  
  Enables support for FPGA family.
  (SPARTAN2, SPARTAN3, VIRTEX2, CYCLONE2, ACEX1K, ACEX)
  
  CONFIG_FPGA_COUNT
  
  Specify the number of FPGA devices to support.
  
  CONFIG_SYS_FPGA_PROG_FEEDBACK
  
  Enable printing of hash marks during FPGA configuration.
  
  CONFIG_SYS_FPGA_CHECK_BUSY
  
  Enable checks on FPGA configuration interface busy
  status by the configuration function. This option
  will require a board or device specific function to
  be written.
  
  CONFIG_FPGA_DELAY
  
  If defined, a function that provides delays in the FPGA
  configuration driver.
  
  CONFIG_SYS_FPGA_CHECK_CTRLC
  Allow Control-C to interrupt FPGA configuration
  
  CONFIG_SYS_FPGA_CHECK_ERROR
  
  Check for configuration errors during FPGA bitfile
  loading. For example, abort during Virtex II
  configuration if the INIT_B line goes low (which
  indicated a CRC error).
  
  CONFIG_SYS_FPGA_WAIT_INIT
  
  Maximum time to wait for the INIT_B line to de-assert
  after PROB_B has been de-asserted during a Virtex II
  FPGA configuration sequence. The default time is 500
  ms.
  
  CONFIG_SYS_FPGA_WAIT_BUSY
  
  Maximum time to wait for BUSY to de-assert during
  Virtex II FPGA configuration. The default is 5 ms.
  
  CONFIG_SYS_FPGA_WAIT_CONFIG
  
  Time to wait after FPGA configuration. The default is
  200 ms.

• Configuration Management: CONFIG_BUILD_TARGET

  Some SoCs need special image types (e.g. U-Boot binary
  with a special header) as build targets. By defining
  CONFIG_BUILD_TARGET in the SoC / board header, this
  special image will be automatically built upon calling
  make / buildman.
  
  CONFIG_IDENT_STRING
  
  If defined, this string will be added to the U-Boot
  version information (U_BOOT_VERSION)

• Vendor Parameter Protection:

  U-Boot considers the values of the environment
  variables "serial#" (Board Serial Number) and
  "ethaddr" (Ethernet Address) to be parameters that
  are set once by the board vendor / manufacturer, and
  protects these variables from casual modification by
  the user. Once set, these variables are read-only,
  and write or delete attempts are rejected. You can
  change this behaviour:
  
  If CONFIG_ENV_OVERWRITE is #defined in your config
  file, the write protection for vendor parameters is
  completely disabled. Anybody can change or delete
  these parameters.
  
  Alternatively, if you define _both_ an ethaddr in the
  default env _and_ CONFIG_OVERWRITE_ETHADDR_ONCE, a default
  Ethernet address is installed in the environment,
  which can be changed exactly ONCE by the user. [The
  serial# is unaffected by this, i. e. it remains
  read-only.]
  
  The same can be accomplished in a more flexible way
  for any variable by configuring the type of access
  to allow for those variables in the ".flags" variable
  or define CONFIG_ENV_FLAGS_LIST_STATIC.

• Protected RAM: CONFIG_PRAM

  Define this variable to enable the reservation of
  "protected RAM", i. e. RAM which is not overwritten
  by U-Boot. Define CONFIG_PRAM to hold the number of
  kB you want to reserve for pRAM. You can overwrite
  this default value by defining an environment
  variable "pram" to the number of kB you want to
  reserve. Note that the board info structure will
  still show the full amount of RAM. If pRAM is
  reserved, a new environment variable "mem" will
  automatically be defined to hold the amount of
  remaining RAM in a form that can be passed as boot
  argument to Linux, for instance like that:
  
      setenv bootargs ... mem=\${mem}
      saveenv
  
  This way you can tell Linux not to use this memory,
  either, which results in a memory region that will
  not be affected by reboots.
  
  *WARNING* If your board configuration uses automatic
  detection of the RAM size, you must make sure that
  this memory test is non-destructive. So far, the
  following board configurations are known to be
  "pRAM-clean":
  
      IVMS8, IVML24, SPD8xx,
      HERMES, IP860, RPXlite, LWMON,
      FLAGADM

• Access to physical memory region (> 4GB) Some basic support is provided for operations on memory not normally accessible to U-Boot - e.g. some architectures support access to more than 4GB of memory on 32-bit machines using physical address extension or similar. Define CONFIG_PHYSMEM to access this basic support, which currently only supports clearing the memory.

• Error Recovery: CONFIG_NET_RETRY_COUNT

  This variable defines the number of retries for
  network operations like ARP, RARP, TFTP, or BOOTP
  before giving up the operation. If not defined, a
  default value of 5 is used.
  
  CONFIG_ARP_TIMEOUT
  
  Timeout waiting for an ARP reply in milliseconds.
  
  CONFIG_NFS_TIMEOUT
  
  Timeout in milliseconds used in NFS protocol.
  If you encounter "ERROR: Cannot umount" in nfs command,
  try longer timeout such as
  #define CONFIG_NFS_TIMEOUT 10000UL

• Command Interpreter: CONFIG_SYS_PROMPT_HUSH_PS2

  This defines the secondary prompt string, which is
  printed when the command interpreter needs more input
  to complete a command. Usually "> ".

  Note:

  In the current implementation, the local variables
  space and global environment variables space are
  separated. Local variables are those you define by
  simply typing `name=value'. To access a local
  variable later on, you have write `$name' or
  `${name}'; to execute the contents of a variable
  directly type `$name' at the command prompt.
  
  Global environment variables are those you use
  setenv/printenv to work with. To run a command stored
  in such a variable, you need to use the run command,
  and you must not use the '$' sign to access them.
  
  To store commands and special characters in a
  variable, please use double quotation marks
  surrounding the whole text of the variable, instead
  of the backslashes before semicolons and special
  symbols.

• Command Line Editing and History: CONFIG_CMDLINE_PS_SUPPORT

  Enable support for changing the command prompt string
  at run-time. Only static string is supported so far.
  The string is obtained from environment variables PS1
  and PS2.

• Default Environment: CONFIG_EXTRA_ENV_SETTINGS

  Define this to contain any number of null terminated
  strings (variable = value pairs) that will be part of
  the default environment compiled into the boot image.
  
  For example, place something like this in your
  board's config file:
  
  #define CONFIG_EXTRA_ENV_SETTINGS \
      "myvar1=value1\0" \
      "myvar2=value2\0"
  
  Warning: This method is based on knowledge about the
  internal format how the environment is stored by the
  U-Boot code. This is NOT an official, exported
  interface! Although it is unlikely that this format
  will change soon, there is no guarantee either.
  You better know what you are doing here.
  
  Note: overly (ab)use of the default environment is
  discouraged. Make sure to check other ways to preset
  the environment like the "source" command or the
  boot command first.
  
  CONFIG_DELAY_ENVIRONMENT
  
  Normally the environment is loaded when the board is
  initialised so that it is available to U-Boot. This inhibits
  that so that the environment is not available until
  explicitly loaded later by U-Boot code. With CONFIG_OF_CONTROL
  this is instead controlled by the value of
  /config/load-environment.

• Serial Flash support Usage requires an initial 'sf probe' to define the serial flash parameters, followed by read/write/erase/update commands.

  The following defaults may be provided by the platform
  to handle the common case when only a single serial
  flash is present on the system.
  
  CONFIG_SF_DEFAULT_BUS       Bus identifier
  CONFIG_SF_DEFAULT_CS        Chip-select
  CONFIG_SF_DEFAULT_MODE      (see include/spi.h)
  CONFIG_SF_DEFAULT_SPEED     in Hz

• TFTP Fixed UDP Port: CONFIG_TFTP_PORT

  If this is defined, the environment variable tftpsrcp
  is used to supply the TFTP UDP source port value.
  If tftpsrcp isn't defined, the normal pseudo-random port
  number generator is used.
  
  Also, the environment variable tftpdstp is used to supply
  the TFTP UDP destination port value.  If tftpdstp isn't
  defined, the normal port 69 is used.
  
  The purpose for tftpsrcp is to allow a TFTP server to
  blindly start the TFTP transfer using the pre-configured
  target IP address and UDP port. This has the effect of
  "punching through" the (Windows XP) firewall, allowing
  the remainder of the TFTP transfer to proceed normally.
  A better solution is to properly configure the firewall,
  but sometimes that is not allowed.

• Show boot progress: CONFIG_SHOW_BOOT_PROGRESS

  Defining this option allows to add some board-
  specific code (calling a user-provided function
  "show_boot_progress(int)") that enables you to show
  the system's boot progress on some display (for
  example, some LED's) on your board. At the moment,
  the following checkpoints are implemented:

Legacy uImage format:

Arg Where When 1 common/cmd_bootm.c before attempting to boot an image -1 common/cmd_bootm.c Image header has bad magic number 2 common/cmd_bootm.c Image header has correct magic number -2 common/cmd_bootm.c Image header has bad checksum 3 common/cmd_bootm.c Image header has correct checksum -3 common/cmd_bootm.c Image data has bad checksum 4 common/cmd_bootm.c Image data has correct checksum -4 common/cmd_bootm.c Image is for unsupported architecture 5 common/cmd_bootm.c Architecture check OK -5 common/cmd_bootm.c Wrong Image Type (not kernel, multi) 6 common/cmd_bootm.c Image Type check OK -6 common/cmd_bootm.c gunzip uncompression error -7 common/cmd_bootm.c Unimplemented compression type 7 common/cmd_bootm.c Uncompression OK 8 common/cmd_bootm.c No uncompress/copy overwrite error -9 common/cmd_bootm.c Unsupported OS (not Linux, BSD, VxWorks, QNX)

9   common/image.c      Start initial ramdisk verification

-10 common/image.c Ramdisk header has bad magic number -11 common/image.c Ramdisk header has bad checksum 10 common/image.c Ramdisk header is OK -12 common/image.c Ramdisk data has bad checksum 11 common/image.c Ramdisk data has correct checksum 12 common/image.c Ramdisk verification complete, start loading -13 common/image.c Wrong Image Type (not PPC Linux ramdisk) 13 common/image.c Start multifile image verification 14 common/image.c No initial ramdisk, no multifile, continue.

15 arch//lib/bootm.c All preparation done, transferring control to OS

-30 arch/powerpc/lib/board.c Fatal error, hang the system -31 post/post.c POST test failed, detected by post_output_backlog() -32 post/post.c POST test failed, detected by post_run_single()

34 common/cmd_doc.c before loading a Image from a DOC device -35 common/cmd_doc.c Bad usage of "doc" command 35 common/cmd_doc.c correct usage of "doc" command -36 common/cmd_doc.c No boot device 36 common/cmd_doc.c correct boot device -37 common/cmd_doc.c Unknown Chip ID on boot device 37 common/cmd_doc.c correct chip ID found, device available -38 common/cmd_doc.c Read Error on boot device 38 common/cmd_doc.c reading Image header from DOC device OK -39 common/cmd_doc.c Image header has bad magic number 39 common/cmd_doc.c Image header has correct magic number -40 common/cmd_doc.c Error reading Image from DOC device 40 common/cmd_doc.c Image header has correct magic number 41 common/cmd_ide.c before loading a Image from a IDE device -42 common/cmd_ide.c Bad usage of "ide" command 42 common/cmd_ide.c correct usage of "ide" command -43 common/cmd_ide.c No boot device 43 common/cmd_ide.c boot device found -44 common/cmd_ide.c Device not available 44 common/cmd_ide.c Device available -45 common/cmd_ide.c wrong partition selected 45 common/cmd_ide.c partition selected -46 common/cmd_ide.c Unknown partition table 46 common/cmd_ide.c valid partition table found -47 common/cmd_ide.c Invalid partition type 47 common/cmd_ide.c correct partition type -48 common/cmd_ide.c Error reading Image Header on boot device 48 common/cmd_ide.c reading Image Header from IDE device OK -49 common/cmd_ide.c Image header has bad magic number 49 common/cmd_ide.c Image header has correct magic number -50 common/cmd_ide.c Image header has bad checksum 50 common/cmd_ide.c Image header has correct checksum -51 common/cmd_ide.c Error reading Image from IDE device 51 common/cmd_ide.c reading Image from IDE device OK 52 common/cmd_nand.c before loading a Image from a NAND device -53 common/cmd_nand.c Bad usage of "nand" command 53 common/cmd_nand.c correct usage of "nand" command -54 common/cmd_nand.c No boot device 54 common/cmd_nand.c boot device found -55 common/cmd_nand.c Unknown Chip ID on boot device 55 common/cmd_nand.c correct chip ID found, device available -56 common/cmd_nand.c Error reading Image Header on boot device 56 common/cmd_nand.c reading Image Header from NAND device OK -57 common/cmd_nand.c Image header has bad magic number 57 common/cmd_nand.c Image header has correct magic number -58 common/cmd_nand.c Error reading Image from NAND device 58 common/cmd_nand.c reading Image from NAND device OK

-60 common/env_common.c Environment has a bad CRC, using default

64 net/eth.c starting with Ethernet configuration. -64 net/eth.c no Ethernet found. 65 net/eth.c Ethernet found.

-80 common/cmd_net.c usage wrong 80 common/cmd_net.c before calling net_loop() -81 common/cmd_net.c some error in net_loop() occurred 81 common/cmd_net.c net_loop() back without error -82 common/cmd_net.c size == 0 (File with size 0 loaded) 82 common/cmd_net.c trying automatic boot 83 common/cmd_net.c running "source" command -83 common/cmd_net.c some error in automatic boot or "source" command 84 common/cmd_net.c end without errors

FIT uImage format:

Arg Where When 100 common/cmd_bootm.c Kernel FIT Image has correct format -100 common/cmd_bootm.c Kernel FIT Image has incorrect format 101 common/cmd_bootm.c No Kernel subimage unit name, using configuration -101 common/cmd_bootm.c Can't get configuration for kernel subimage 102 common/cmd_bootm.c Kernel unit name specified -103 common/cmd_bootm.c Can't get kernel subimage node offset 103 common/cmd_bootm.c Found configuration node 104 common/cmd_bootm.c Got kernel subimage node offset -104 common/cmd_bootm.c Kernel subimage hash verification failed 105 common/cmd_bootm.c Kernel subimage hash verification OK -105 common/cmd_bootm.c Kernel subimage is for unsupported architecture 106 common/cmd_bootm.c Architecture check OK -106 common/cmd_bootm.c Kernel subimage has wrong type 107 common/cmd_bootm.c Kernel subimage type OK -107 common/cmd_bootm.c Can't get kernel subimage data/size 108 common/cmd_bootm.c Got kernel subimage data/size -108 common/cmd_bootm.c Wrong image type (not legacy, FIT) -109 common/cmd_bootm.c Can't get kernel subimage type -110 common/cmd_bootm.c Can't get kernel subimage comp -111 common/cmd_bootm.c Can't get kernel subimage os -112 common/cmd_bootm.c Can't get kernel subimage load address -113 common/cmd_bootm.c Image uncompress/copy overwrite error

120 common/image.c Start initial ramdisk verification -120 common/image.c Ramdisk FIT image has incorrect format 121 common/image.c Ramdisk FIT image has correct format 122 common/image.c No ramdisk subimage unit name, using configuration -122 common/image.c Can't get configuration for ramdisk subimage 123 common/image.c Ramdisk unit name specified -124 common/image.c Can't get ramdisk subimage node offset 125 common/image.c Got ramdisk subimage node offset -125 common/image.c Ramdisk subimage hash verification failed 126 common/image.c Ramdisk subimage hash verification OK -126 common/image.c Ramdisk subimage for unsupported architecture 127 common/image.c Architecture check OK -127 common/image.c Can't get ramdisk subimage data/size 128 common/image.c Got ramdisk subimage data/size 129 common/image.c Can't get ramdisk load address -129 common/image.c Got ramdisk load address

-130 common/cmd_doc.c Incorrect FIT image format 131 common/cmd_doc.c FIT image format OK

-140 common/cmd_ide.c Incorrect FIT image format 141 common/cmd_ide.c FIT image format OK

-150 common/cmd_nand.c Incorrect FIT image format 151 common/cmd_nand.c FIT image format OK

• Standalone program support: CONFIG_STANDALONE_LOAD_ADDR

  This option defines a board specific value for the
  address where standalone program gets loaded, thus
  overwriting the architecture dependent default
  settings.

• Frame Buffer Address: CONFIG_FB_ADDR

  Define CONFIG_FB_ADDR if you want to use specific
  address for frame buffer.  This is typically the case
  when using a graphics controller has separate video
  memory.  U-Boot will then place the frame buffer at
  the given address instead of dynamically reserving it
  in system RAM by calling lcd_setmem(), which grabs
  the memory for the frame buffer depending on the
  configured panel size.
  
  Please see board_init_f function.

• Automatic software updates via TFTP server CONFIG_UPDATE_TFTP CONFIG_UPDATE_TFTP_CNT_MAX CONFIG_UPDATE_TFTP_MSEC_MAX

  These options enable and control the auto-update feature;
  for a more detailed description refer to doc/README.update.

• MTD Support (mtdparts command, UBI support) CONFIG_MTD_UBI_WL_THRESHOLD This parameter defines the maximum difference between the highest erase counter value and the lowest erase counter value of eraseblocks of UBI devices. When this threshold is exceeded, UBI starts performing wear leveling by means of moving data from eraseblock with low erase counter to eraseblocks with high erase counter.

  The default value should be OK for SLC NAND flashes, NOR flashes and
  other flashes which have eraseblock life-cycle 100000 or more.
  However, in case of MLC NAND flashes which typically have eraseblock
  life-cycle less than 10000, the threshold should be lessened (e.g.,
  to 128 or 256, although it does not have to be power of 2).
  
  default: 4096
  
  CONFIG_MTD_UBI_BEB_LIMIT
  This option specifies the maximum bad physical eraseblocks UBI
  expects on the MTD device (per 1024 eraseblocks). If the
  underlying flash does not admit of bad eraseblocks (e.g. NOR
  flash), this value is ignored.
  
  NAND datasheets often specify the minimum and maximum NVM
  (Number of Valid Blocks) for the flashes' endurance lifetime.
  The maximum expected bad eraseblocks per 1024 eraseblocks
  then can be calculated as "1024 * (1 - MinNVB / MaxNVB)",
  which gives 20 for most NANDs (MaxNVB is basically the total
  count of eraseblocks on the chip).
  
  To put it differently, if this value is 20, UBI will try to
  reserve about 1.9% of physical eraseblocks for bad blocks
  handling. And that will be 1.9% of eraseblocks on the entire
  NAND chip, not just the MTD partition UBI attaches. This means
  that if you have, say, a NAND flash chip admits maximum 40 bad
  eraseblocks, and it is split on two MTD partitions of the same
  size, UBI will reserve 40 eraseblocks when attaching a
  partition.
  
  default: 20
  
  CONFIG_MTD_UBI_FASTMAP
  Fastmap is a mechanism which allows attaching an UBI device
  in nearly constant time. Instead of scanning the whole MTD device it
  only has to locate a checkpoint (called fastmap) on the device.
  The on-flash fastmap contains all information needed to attach
  the device. Using fastmap makes only sense on large devices where
  attaching by scanning takes long. UBI will not automatically install
  a fastmap on old images, but you can set the UBI parameter
  CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT to 1 if you want so. Please note
  that fastmap-enabled images are still usable with UBI implementations
  without fastmap support. On typical flash devices the whole fastmap
  fits into one PEB. UBI will reserve PEBs to hold two fastmaps.
  
  CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT
  Set this parameter to enable fastmap automatically on images
  without a fastmap.
  default: 0
  
  CONFIG_MTD_UBI_FM_DEBUG
  Enable UBI fastmap debug
  default: 0

• SPL framework CONFIG_SPL Enable building of SPL globally.

  CONFIG_SPL_LDSCRIPT
  LDSCRIPT for linking the SPL binary.
  
  CONFIG_SPL_MAX_FOOTPRINT
  Maximum size in memory allocated to the SPL, BSS included.
  When defined, the linker checks that the actual memory
  used by SPL from _start to __bss_end does not exceed it.
  CONFIG_SPL_MAX_FOOTPRINT and CONFIG_SPL_BSS_MAX_SIZE
  must not be both defined at the same time.
  
  CONFIG_SPL_MAX_SIZE
  Maximum size of the SPL image (text, data, rodata, and
  linker lists sections), BSS excluded.
  When defined, the linker checks that the actual size does
  not exceed it.
  
  CONFIG_SPL_TEXT_BASE
  TEXT_BASE for linking the SPL binary.
  
  CONFIG_SPL_RELOC_TEXT_BASE
  Address to relocate to.  If unspecified, this is equal to
  CONFIG_SPL_TEXT_BASE (i.e. no relocation is done).
  
  CONFIG_SPL_BSS_START_ADDR
  Link address for the BSS within the SPL binary.
  
  CONFIG_SPL_BSS_MAX_SIZE
  Maximum size in memory allocated to the SPL BSS.
  When defined, the linker checks that the actual memory used
  by SPL from __bss_start to __bss_end does not exceed it.
  CONFIG_SPL_MAX_FOOTPRINT and CONFIG_SPL_BSS_MAX_SIZE
  must not be both defined at the same time.
  
  CONFIG_SPL_STACK
  Adress of the start of the stack SPL will use
  
  CONFIG_SPL_PANIC_ON_RAW_IMAGE
  When defined, SPL will panic() if the image it has
  loaded does not have a signature.
  Defining this is useful when code which loads images
  in SPL cannot guarantee that absolutely all read errors
  will be caught.
  An example is the LPC32XX MLC NAND driver, which will
  consider that a completely unreadable NAND block is bad,
  and thus should be skipped silently.
  
  CONFIG_SPL_RELOC_STACK
  Adress of the start of the stack SPL will use after
  relocation.  If unspecified, this is equal to
  CONFIG_SPL_STACK.
  
  CONFIG_SYS_SPL_MALLOC_START
  Starting address of the malloc pool used in SPL.
  When this option is set the full malloc is used in SPL and
  it is set up by spl_init() and before that, the simple malloc()
  can be used if CONFIG_SYS_MALLOC_F is defined.
  
  CONFIG_SYS_SPL_MALLOC_SIZE
  The size of the malloc pool used in SPL.
  
  CONFIG_SPL_OS_BOOT
  Enable booting directly to an OS from SPL.
  See also: doc/README.falcon
  
  CONFIG_SPL_DISPLAY_PRINT
  For ARM, enable an optional function to print more information
  about the running system.
  
  CONFIG_SPL_INIT_MINIMAL
  Arch init code should be built for a very small image
  
  CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION
  Partition on the MMC to load U-Boot from when the MMC is being
  used in raw mode
  
  CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR
  Sector to load kernel uImage from when MMC is being
  used in raw mode (for Falcon mode)
  
  CONFIG_SYS_MMCSD_RAW_MODE_ARGS_SECTOR,
  CONFIG_SYS_MMCSD_RAW_MODE_ARGS_SECTORS
  Sector and number of sectors to load kernel argument
  parameters from when MMC is being used in raw mode
  (for falcon mode)
  
  CONFIG_SYS_MMCSD_FS_BOOT_PARTITION
  Partition on the MMC to load U-Boot from when the MMC is being
  used in fs mode
  
  CONFIG_SPL_FS_LOAD_PAYLOAD_NAME
  Filename to read to load U-Boot when reading from filesystem
  
  CONFIG_SPL_FS_LOAD_KERNEL_NAME
  Filename to read to load kernel uImage when reading
  from filesystem (for Falcon mode)
  
  CONFIG_SPL_FS_LOAD_ARGS_NAME
  Filename to read to load kernel argument parameters
  when reading from filesystem (for Falcon mode)
  
  CONFIG_SPL_MPC83XX_WAIT_FOR_NAND
  Set this for NAND SPL on PPC mpc83xx targets, so that
  start.S waits for the rest of the SPL to load before
  continuing (the hardware starts execution after just
  loading the first page rather than the full 4K).
  
  CONFIG_SPL_SKIP_RELOCATE
  Avoid SPL relocation
  
  CONFIG_SPL_NAND_BASE
  Include nand_base.c in the SPL.  Requires
  CONFIG_SPL_NAND_DRIVERS.
  
  CONFIG_SPL_NAND_DRIVERS
  SPL uses normal NAND drivers, not minimal drivers.
  
  CONFIG_SPL_NAND_IDENT
  SPL uses the chip ID list to identify the NAND flash.
  Requires CONFIG_SPL_NAND_BASE.
  
  CONFIG_SPL_NAND_ECC
  Include standard software ECC in the SPL
  
  CONFIG_SPL_NAND_SIMPLE
  Support for NAND boot using simple NAND drivers that
  expose the cmd_ctrl() interface.
  
  CONFIG_SPL_UBI
  Support for a lightweight UBI (fastmap) scanner and
  loader
  
  CONFIG_SPL_NAND_RAW_ONLY
  Support to boot only raw u-boot.bin images. Use this only
  if you need to save space.
  
  CONFIG_SPL_COMMON_INIT_DDR
  Set for common ddr init with serial presence detect in
  SPL binary.
  
  CONFIG_SYS_NAND_5_ADDR_CYCLE, CONFIG_SYS_NAND_PAGE_COUNT,
  CONFIG_SYS_NAND_PAGE_SIZE, CONFIG_SYS_NAND_OOBSIZE,
  CONFIG_SYS_NAND_BLOCK_SIZE, CONFIG_SYS_NAND_BAD_BLOCK_POS,
  CONFIG_SYS_NAND_ECCPOS, CONFIG_SYS_NAND_ECCSIZE,
  CONFIG_SYS_NAND_ECCBYTES
  Defines the size and behavior of the NAND that SPL uses
  to read U-Boot
  
  CONFIG_SPL_NAND_BOOT
  Add support NAND boot
  
  CONFIG_SYS_NAND_U_BOOT_OFFS
  Location in NAND to read U-Boot from
  
  CONFIG_SYS_NAND_U_BOOT_DST
  Location in memory to load U-Boot to
  
  CONFIG_SYS_NAND_U_BOOT_SIZE
  Size of image to load
  
  CONFIG_SYS_NAND_U_BOOT_START
  Entry point in loaded image to jump to
  
  CONFIG_SYS_NAND_HW_ECC_OOBFIRST
  Define this if you need to first read the OOB and then the
  data. This is used, for example, on davinci platforms.
  
  CONFIG_SPL_RAM_DEVICE
  Support for running image already present in ram, in SPL binary
  
  CONFIG_SPL_PAD_TO
  Image offset to which the SPL should be padded before appending
  the SPL payload. By default, this is defined as
  CONFIG_SPL_MAX_SIZE, or 0 if CONFIG_SPL_MAX_SIZE is undefined.
  CONFIG_SPL_PAD_TO must be either 0, meaning to append the SPL
  payload without any padding, or >= CONFIG_SPL_MAX_SIZE.
  
  CONFIG_SPL_TARGET
  Final target image containing SPL and payload.  Some SPLs
  use an arch-specific makefile fragment instead, for
  example if more than one image needs to be produced.
  
  CONFIG_SPL_FIT_PRINT
  Printing information about a FIT image adds quite a bit of
  code to SPL. So this is normally disabled in SPL. Use this
  option to re-enable it. This will affect the output of the
  bootm command when booting a FIT image.

• TPL framework CONFIG_TPL Enable building of TPL globally.

  CONFIG_TPL_PAD_TO
  Image offset to which the TPL should be padded before appending
  the TPL payload. By default, this is defined as
  CONFIG_SPL_MAX_SIZE, or 0 if CONFIG_SPL_MAX_SIZE is undefined.
  CONFIG_SPL_PAD_TO must be either 0, meaning to append the SPL
  payload without any padding, or >= CONFIG_SPL_MAX_SIZE.

• Interrupt support (PPC):

  There are common interrupt_init() and timer_interrupt()
  for all PPC archs. interrupt_init() calls interrupt_init_cpu()
  for CPU specific initialization. interrupt_init_cpu()
  should set decrementer_count to appropriate value. If
  CPU resets decrementer automatically after interrupt
  (ppc4xx) it should set decrementer_count to zero.
  timer_interrupt() calls timer_interrupt_cpu() for CPU
  specific handling. If board has watchdog / status_led
  / other_activity_monitor it works automatically from
  general timer_interrupt().

Board initialization settings:

During Initialization u-boot calls a number of board specific functions to allow the preparation of board specific prerequisites, e.g. pin setup before drivers are initialized. To enable these callbacks the following configuration macros have to be defined. Currently this is architecture specific, so please check arch/your_architecture/lib/board.c typically in board_init_f() and board_init_r().

• CONFIG_BOARD_EARLY_INIT_F: Call board_early_init_f()
• CONFIG_BOARD_EARLY_INIT_R: Call board_early_init_r()
• CONFIG_BOARD_LATE_INIT: Call board_late_init()
• CONFIG_BOARD_POSTCLK_INIT: Call board_postclk_init()

Configuration Settings:

• CONFIG_SYS_SUPPORT_64BIT_DATA: Defined automatically if compiled as 64-bit. Optionally it can be defined to support 64-bit memory commands.

• CONFIG_SYS_LONGHELP: Defined when you want long help messages included; undefine this when you're short of memory.

• CONFIG_SYS_HELP_CMD_WIDTH: Defined when you want to override the default width of the commands listed in the 'help' command output.

• CONFIG_SYS_PROMPT: This is what U-Boot prints on the console to prompt for user input.

• CONFIG_SYS_CBSIZE: Buffer size for input from the Console

• CONFIG_SYS_PBSIZE: Buffer size for Console output

• CONFIG_SYS_MAXARGS: max. Number of arguments accepted for monitor commands

• CONFIG_SYS_BARGSIZE: Buffer size for Boot Arguments which are passed to the application (usually a Linux kernel) when it is booted

• CONFIG_SYS_BAUDRATE_TABLE: List of legal baudrate settings for this board.

• CONFIG_SYS_MEMTEST_START, CONFIG_SYS_MEMTEST_END: Begin and End addresses of the area used by the simple memory test.

• CONFIG_SYS_MEMTEST_SCRATCH: Scratch address used by the alternate memory test You only need to set this if address zero isn't writeable

• CONFIG_SYS_MEM_RESERVE_SECURE Only implemented for ARMv8 for now. If defined, the size of CONFIG_SYS_MEM_RESERVE_SECURE memory is substracted from total RAM and won't be reported to OS. This memory can be used as secure memory. A variable gd->arch.secure_ram is used to track the location. In systems the RAM base is not zero, or RAM is divided into banks, this variable needs to be recalcuated to get the address.

• CONFIG_SYS_MEM_TOP_HIDE: If CONFIG_SYS_MEM_TOP_HIDE is defined in the board config header, this specified memory area will get subtracted from the top (end) of RAM and won't get "touched" at all by U-Boot. By fixing up gd->ram_size the Linux kernel should gets passed the now "corrected" memory size and won't touch it either. This should work for arch/ppc and arch/powerpc. Only Linux board ports in arch/powerpc with bootwrapper support that recalculate the memory size from the SDRAM controller setup will have to get fixed in Linux additionally.

  This option can be used as a workaround for the 440EPx/GRx
  CHIP 11 errata where the last 256 bytes in SDRAM shouldn't
  be touched.
  
  WARNING: Please make sure that this value is a multiple of
  the Linux page size (normally 4k). If this is not the case,
  then the end address of the Linux memory will be located at a
  non page size aligned address and this could cause major
  problems.

• CONFIG_SYS_LOADS_BAUD_CHANGE: Enable temporary baudrate change while serial download

• CONFIG_SYS_SDRAM_BASE: Physical start address of SDRAM. Must be 0 here.

• CONFIG_SYS_FLASH_BASE: Physical start address of Flash memory.

• CONFIG_SYS_MONITOR_BASE: Physical start address of boot monitor code (set by make config files to be same as the text base address (CONFIG_SYS_TEXT_BASE) used when linking) - same as CONFIG_SYS_FLASH_BASE when booting from flash.

• CONFIG_SYS_MONITOR_LEN: Size of memory reserved for monitor code, used to determine _at_compiletime (!) if the environment is embedded within the U-Boot image, or in a separate flash sector.

• CONFIG_SYS_MALLOC_LEN: Size of DRAM reserved for malloc() use.

• CONFIG_SYS_MALLOC_F_LEN Size of the malloc() pool for use before relocation. If this is defined, then a very simple malloc() implementation will become available before relocation. The address is just below the global data, and the stack is moved down to make space.

  This feature allocates regions with increasing addresses
  within the region. calloc() is supported, but realloc()
  is not available. free() is supported but does nothing.
  The memory will be freed (or in fact just forgotten) when
  U-Boot relocates itself.

• CONFIG_SYS_MALLOC_SIMPLE Provides a simple and small malloc() and calloc() for those boards which do not use the full malloc in SPL (which is enabled with CONFIG_SYS_SPL_MALLOC_START).

• CONFIG_SYS_NONCACHED_MEMORY: Size of non-cached memory area. This area of memory will be typically located right below the malloc() area and mapped uncached in the MMU. This is useful for drivers that would otherwise require a lot of explicit cache maintenance. For some drivers it's also impossible to properly maintain the cache. For example if the regions that need to be flushed are not a multiple of the cache-line size, and padding cannot be allocated between the regions to align them (i.e. if the HW requires a contiguous array of regions, and the size of each region is not cache-aligned), then a flush of one region may result in overwriting data that hardware has written to another region in the same cache-line. This can happen for example in network drivers where descriptors for buffers are typically smaller than the CPU cache-line (e.g. 16 bytes vs. 32 or 64 bytes).

  Non-cached memory is only supported on 32-bit ARM at present.

• CONFIG_SYS_BOOTM_LEN: Normally compressed uImages are limited to an uncompressed size of 8 MBytes. If this is not enough, you can define CONFIG_SYS_BOOTM_LEN in your board config file to adjust this setting to your needs.

• CONFIG_SYS_BOOTMAPSZ: Maximum size of memory mapped by the startup code of the Linux kernel; all data that must be processed by the Linux kernel (bd_info, boot arguments, FDT blob if used) must be put below this limit, unless "bootm_low" environment variable is defined and non-zero. In such case all data for the Linux kernel must be between "bootm_low" and "bootm_low" + CONFIG_SYS_BOOTMAPSZ. The environment variable "bootm_mapsize" will override the value of CONFIG_SYS_BOOTMAPSZ. If CONFIG_SYS_BOOTMAPSZ is undefined, then the value in "bootm_size" will be used instead.

• CONFIG_SYS_BOOT_RAMDISK_HIGH: Enable initrd_high functionality. If defined then the initrd_high feature is enabled and the bootm ramdisk subcommand is enabled.

• CONFIG_SYS_BOOT_GET_CMDLINE: Enables allocating and saving kernel cmdline in space between "bootm_low" and "bootm_low" + BOOTMAPSZ.

• CONFIG_SYS_BOOT_GET_KBD: Enables allocating and saving a kernel copy of the bd_info in space between "bootm_low" and "bootm_low" + BOOTMAPSZ.

• CONFIG_SYS_MAX_FLASH_BANKS: Max number of Flash memory banks

• CONFIG_SYS_MAX_FLASH_SECT: Max number of sectors on a Flash chip

• CONFIG_SYS_FLASH_ERASE_TOUT: Timeout for Flash erase operations (in ms)

• CONFIG_SYS_FLASH_WRITE_TOUT: Timeout for Flash write operations (in ms)

• CONFIG_SYS_FLASH_LOCK_TOUT Timeout for Flash set sector lock bit operation (in ms)

• CONFIG_SYS_FLASH_UNLOCK_TOUT Timeout for Flash clear lock bits operation (in ms)

• CONFIG_SYS_FLASH_PROTECTION If defined, hardware flash sectors protection is used instead of U-Boot software protection.

• CONFIG_SYS_DIRECT_FLASH_TFTP:

  Enable TFTP transfers directly to flash memory;
  without this option such a download has to be
  performed in two steps: (1) download to RAM, and (2)
  copy from RAM to flash.
  
  The two-step approach is usually more reliable, since
  you can check if the download worked before you erase
  the flash, but in some situations (when system RAM is
  too limited to allow for a temporary copy of the
  downloaded image) this option may be very useful.

• CONFIG_SYS_FLASH_CFI: Define if the flash driver uses extra elements in the common flash structure for storing flash geometry.

• CONFIG_FLASH_CFI_DRIVER This option also enables the building of the cfi_flash driver in the drivers directory

• CONFIG_FLASH_CFI_MTD This option enables the building of the cfi_mtd driver in the drivers directory. The driver exports CFI flash to the MTD layer.

• CONFIG_SYS_FLASH_USE_BUFFER_WRITE Use buffered writes to flash.

• CONFIG_FLASH_SPANSION_S29WS_N s29ws-n MirrorBit flash has non-standard addresses for buffered write commands.

• CONFIG_SYS_FLASH_QUIET_TEST If this option is defined, the common CFI flash doesn't print it's warning upon not recognized FLASH banks. This is useful, if some of the configured banks are only optionally available.

• CONFIG_FLASH_SHOW_PROGRESS If defined (must be an integer), print out countdown digits and dots. Recommended value: 45 (9..1) for 80 column displays, 15 (3..1) for 40 column displays.

• CONFIG_FLASH_VERIFY If defined, the content of the flash (destination) is compared against the source after the write operation. An error message will be printed when the contents are not identical. Please note that this option is useless in nearly all cases, since such flash programming errors usually are detected earlier while unprotecting/erasing/programming. Please only enable this option if you really know what you are doing.

• CONFIG_SYS_RX_ETH_BUFFER: Defines the number of Ethernet receive buffers. On some Ethernet controllers it is recommended to set this value to 8 or even higher (EEPRO100 or 405 EMAC), since all buffers can be full shortly after enabling the interface on high Ethernet traffic. Defaults to 4 if not defined.

• CONFIG_ENV_MAX_ENTRIES

  Maximum number of entries in the hash table that is used internally to store the environment settings. The default setting is supposed to be generous and should work in most cases. This setting can be used to tune behaviour; see lib/hashtable.c for details.

• CONFIG_ENV_FLAGS_LIST_DEFAULT

• CONFIG_ENV_FLAGS_LIST_STATIC Enable validation of the values given to environment variables when calling env set. Variables can be restricted to only decimal, hexadecimal, or boolean. If CONFIG_CMD_NET is also defined, the variables can also be restricted to IP address or MAC address.

  The format of the list is: type_attribute = [s|d|x|b|i|m] access_attribute = [a|r|o|c] attributes = type_attribute[access_attribute] entry = variable_name[:attributes] list = entry[,list]

  The type attributes are: s - String (default) d - Decimal x - Hexadecimal b - Boolean ([1yYtT|0nNfF]) i - IP address m - MAC address

  The access attributes are: a - Any (default) r - Read-only o - Write-once c - Change-default

  • CONFIG_ENV_FLAGS_LIST_DEFAULT Define this to a list (string) to define the ".flags" environment variable in the default or embedded environment.

  • CONFIG_ENV_FLAGS_LIST_STATIC Define this to a list (string) to define validation that should be done if an entry is not found in the ".flags" environment variable. To override a setting in the static list, simply add an entry for the same variable name to the ".flags" variable.

  If CONFIG_REGEX is defined, the variable_name above is evaluated as a regular expression. This allows multiple variables to define the same flags without explicitly listing them for each variable.

• CONFIG_ENV_ACCESS_IGNORE_FORCE If defined, don't allow the -f switch to env set override variable access flags.

The following definitions that deal with the placement and management of environment data (variable area); in general, we support the following configurations:

• CONFIG_BUILD_ENVCRC:

  Builds up envcrc with the target environment so that external utils may easily extract it and embed it in final U-Boot images.

BE CAREFUL! The first access to the environment happens quite early in U-Boot initialization (when we try to get the setting of for the console baudrate). You MUST have mapped your NVRAM area then, or U-Boot will hang.

Please note that even with NVRAM we still use a copy of the environment in RAM: we could work on NVRAM directly, but we want to keep settings there always unmodified except somebody uses "saveenv" to save the current settings.

BE CAREFUL! For some special cases, the local device can not use "saveenv" command. For example, the local device will get the environment stored in a remote NOR flash by SRIO or PCIE link, but it can not erase, write this NOR flash by SRIO or PCIE interface.

• CONFIG_NAND_ENV_DST

  Defines address in RAM to which the nand_spl code should copy the environment. If redundant environment is used, it will be copied to CONFIG_NAND_ENV_DST + CONFIG_ENV_SIZE.

Please note that the environment is read-only until the monitor has been relocated to RAM and a RAM copy of the environment has been created; also, when using EEPROM you will have to use env_get_f() until then to read environment variables.

The environment is protected by a CRC32 checksum. Before the monitor is relocated into RAM, as a result of a bad CRC you will be working with the compiled-in default environment - silently!!! [This is necessary, because the first environment variable we need is the "baudrate" setting for the console - if we have a bad CRC, we don't have any device yet where we could complain.]

Note: once the monitor has been relocated, then it will complain if the default environment is used; a new CRC is computed as soon as you use the "saveenv" command to store a valid environment.

• CONFIG_SYS_FAULT_ECHO_LINK_DOWN: Echo the inverted Ethernet link state to the fault LED.

  Note: If this option is active, then CONFIG_SYS_FAULT_MII_ADDR
        also needs to be defined.

• CONFIG_SYS_FAULT_MII_ADDR: MII address of the PHY to check for the Ethernet link state.

• CONFIG_NS16550_MIN_FUNCTIONS: Define this if you desire to only have use of the NS16550_init and NS16550_putc functions for the serial driver located at drivers/serial/ns16550.c. This option is useful for saving space for already greatly restricted images, including but not limited to NAND_SPL configurations.

• CONFIG_DISPLAY_BOARDINFO Display information about the board that U-Boot is running on when U-Boot starts up. The board function checkboard() is called to do this.

• CONFIG_DISPLAY_BOARDINFO_LATE Similar to the previous option, but display this information later, once stdio is running and output goes to the LCD, if present.

• CONFIG_BOARD_SIZE_LIMIT: Maximum size of the U-Boot image. When defined, the build system checks that the actual size does not exceed it.

Low Level (hardware related) configuration options:

• CONFIG_SYS_CACHELINE_SIZE: Cache Line Size of the CPU.

• CONFIG_SYS_CCSRBAR_DEFAULT: Default (power-on reset) physical address of CCSR on Freescale PowerPC SOCs.

• CONFIG_SYS_CCSRBAR: Virtual address of CCSR. On a 32-bit build, this is typically the same value as CONFIG_SYS_CCSRBAR_DEFAULT.

• CONFIG_SYS_CCSRBAR_PHYS: Physical address of CCSR. CCSR can be relocated to a new physical address, if desired. In this case, this macro should be set to that address. Otherwise, it should be set to the same value as CONFIG_SYS_CCSRBAR_DEFAULT. For example, CCSR is typically relocated on 36-bit builds. It is recommended that this macro be defined via the _HIGH and _LOW macros:

  #define CONFIG_SYS_CCSRBAR_PHYS ((CONFIG_SYS_CCSRBAR_PHYS_HIGH
      * 1ull) << 32 | CONFIG_SYS_CCSRBAR_PHYS_LOW)

• CONFIG_SYS_CCSRBAR_PHYS_HIGH: Bits 33-36 of CONFIG_SYS_CCSRBAR_PHYS. This value is typically either 0 (32-bit build) or 0xF (36-bit build). This macro is used in assembly code, so it must not contain typecasts or integer size suffixes (e.g. "ULL").

• CONFIG_SYS_CCSRBAR_PHYS_LOW: Lower 32-bits of CONFIG_SYS_CCSRBAR_PHYS. This macro is used in assembly code, so it must not contain typecasts or integer size suffixes (e.g. "ULL").

• CONFIG_SYS_CCSR_DO_NOT_RELOCATE: If this macro is defined, then CONFIG_SYS_CCSRBAR_PHYS will be forced to a value that ensures that CCSR is not relocated.

• Floppy Disk Support: CONFIG_SYS_FDC_DRIVE_NUMBER

  the default drive number (default value 0)
  
  CONFIG_SYS_ISA_IO_STRIDE
  
  defines the spacing between FDC chipset registers
  (default value 1)
  
  CONFIG_SYS_ISA_IO_OFFSET
  
  defines the offset of register from address. It
  depends on which part of the data bus is connected to
  the FDC chipset. (default value 0)
  
  If CONFIG_SYS_ISA_IO_STRIDE CONFIG_SYS_ISA_IO_OFFSET and
  CONFIG_SYS_FDC_DRIVE_NUMBER are undefined, they take their
  default value.
  
  if CONFIG_SYS_FDC_HW_INIT is defined, then the function
  fdc_hw_init() is called at the beginning of the FDC
  setup. fdc_hw_init() must be provided by the board
  source code. It is used to make hardware-dependent
  initializations.

• CONFIG_IDE_AHB: Most IDE controllers were designed to be connected with PCI interface. Only few of them were designed for AHB interface. When software is doing ATA command and data transfer to IDE devices through IDE-AHB controller, some additional registers accessing to these kind of IDE-AHB controller is required.

• CONFIG_SYS_IMMR: Physical address of the Internal Memory. DO NOT CHANGE unless you know exactly what you're doing! (11-4) [MPC8xx systems only]

• CONFIG_SYS_INIT_RAM_ADDR:

  Start address of memory area that can be used for
  initial data and stack; please note that this must be
  writable memory that is working WITHOUT special
  initialization, i. e. you CANNOT use normal RAM which
  will become available only after programming the
  memory controller and running certain initialization
  sequences.
  
  U-Boot uses the following memory types:
  - MPC8xx: IMMR (internal memory of the CPU)

• CONFIG_SYS_GBL_DATA_OFFSET:

  Offset of the initial data structure in the memory
  area defined by CONFIG_SYS_INIT_RAM_ADDR. Usually
  CONFIG_SYS_GBL_DATA_OFFSET is chosen such that the initial
  data is located at the end of the available space
  (sometimes written as (CONFIG_SYS_INIT_RAM_SIZE -
  GENERATED_GBL_DATA_SIZE), and the initial stack is just
  below that area (growing from (CONFIG_SYS_INIT_RAM_ADDR +
  CONFIG_SYS_GBL_DATA_OFFSET) downward.

  Note: On the MPC824X (or other systems that use the data cache for initial memory) the address chosen for CONFIG_SYS_INIT_RAM_ADDR is basically arbitrary - it must point to an otherwise UNUSED address space between the top of RAM and the start of the PCI space.

• CONFIG_SYS_SCCR: System Clock and reset Control Register (15-27)

• CONFIG_SYS_OR_TIMING_SDRAM: SDRAM timing

• CONFIG_SYS_MAMR_PTA: periodic timer for refresh

• FLASH_BASE0_PRELIM, FLASH_BASE1_PRELIM, CONFIG_SYS_REMAP_OR_AM, CONFIG_SYS_PRELIM_OR_AM, CONFIG_SYS_OR_TIMING_FLASH, CONFIG_SYS_OR0_REMAP, CONFIG_SYS_OR0_PRELIM, CONFIG_SYS_BR0_PRELIM, CONFIG_SYS_OR1_REMAP, CONFIG_SYS_OR1_PRELIM, CONFIG_SYS_BR1_PRELIM: Memory Controller Definitions: BR0/1 and OR0/1 (FLASH)

• SDRAM_BASE2_PRELIM, SDRAM_BASE3_PRELIM, SDRAM_MAX_SIZE, CONFIG_SYS_OR_TIMING_SDRAM, CONFIG_SYS_OR2_PRELIM, CONFIG_SYS_BR2_PRELIM, CONFIG_SYS_OR3_PRELIM, CONFIG_SYS_BR3_PRELIM: Memory Controller Definitions: BR2/3 and OR2/3 (SDRAM)

• CONFIG_PCI_ENUM_ONLY Only scan through and get the devices on the buses. Don't do any setup work, presumably because someone or something has already done it, and we don't need to do it a second time. Useful for platforms that are pre-booted by coreboot or similar.

• CONFIG_PCI_INDIRECT_BRIDGE: Enable support for indirect PCI bridges.

• CONFIG_SYS_SRIO: Chip has SRIO or not

• CONFIG_SRIO1: Board has SRIO 1 port available

• CONFIG_SRIO2: Board has SRIO 2 port available

• CONFIG_SRIO_PCIE_BOOT_MASTER Board can support master function for Boot from SRIO and PCIE

• CONFIG_SYS_SRIOn_MEM_VIRT: Virtual Address of SRIO port 'n' memory region

• CONFIG_SYS_SRIOn_MEM_PHYS: Physical Address of SRIO port 'n' memory region

• CONFIG_SYS_SRIOn_MEM_SIZE: Size of SRIO port 'n' memory region

• CONFIG_SYS_NAND_BUSWIDTH_16BIT Defined to tell the NAND controller that the NAND chip is using a 16 bit bus. Not all NAND drivers use this symbol. Example of drivers that use it:

  • drivers/mtd/nand/raw/ndfc.c
  • drivers/mtd/nand/raw/mxc_nand.c

• CONFIG_SYS_NDFC_EBC0_CFG Sets the EBC0_CFG register for the NDFC. If not defined a default value will be used.

• CONFIG_SPD_EEPROM Get DDR timing information from an I2C EEPROM. Common with pluggable memory modules such as SODIMMs

  SPD_EEPROM_ADDRESS I2C address of the SPD EEPROM

• CONFIG_SYS_SPD_BUS_NUM If SPD EEPROM is on an I2C bus other than the first one, specify here. Note that the value must resolve to something your driver can deal with.

• CONFIG_SYS_DDR_RAW_TIMING Get DDR timing information from other than SPD. Common with soldered DDR chips onboard without SPD. DDR raw timing parameters are extracted from datasheet and hard-coded into header files or board specific files.

• CONFIG_FSL_DDR_INTERACTIVE Enable interactive DDR debugging. See doc/README.fsl-ddr.

• CONFIG_FSL_DDR_SYNC_REFRESH Enable sync of refresh for multiple controllers.

• CONFIG_FSL_DDR_BIST Enable built-in memory test for Freescale DDR controllers.

• CONFIG_SYS_83XX_DDR_USES_CS0 Only for 83xx systems. If specified, then DDR should be configured using CS0 and CS1 instead of CS2 and CS3.

• CONFIG_RMII Enable RMII mode for all FECs. Note that this is a global option, we can't have one FEC in standard MII mode and another in RMII mode.

• CONFIG_CRC32_VERIFY Add a verify option to the crc32 command. The syntax is:

  => crc32 -v <address> <count> <crc32>
  
  Where address/count indicate a memory area
  and crc32 is the correct crc32 which the
  area should have.

• CONFIG_LOOPW Add the "loopw" memory command. This only takes effect if the memory commands are activated globally (CONFIG_CMD_MEMORY).

• CONFIG_MX_CYCLIC Add the "mdc" and "mwc" memory commands. These are cyclic "md/mw" commands. Examples:

  => mdc.b 10 4 500
  This command will print 4 bytes (10,11,12,13) each 500 ms.
  
  => mwc.l 100 12345678 10
  This command will write 12345678 to address 100 all 10 ms.
  
  This only takes effect if the memory commands are activated
  globally (CONFIG_CMD_MEMORY).

• CONFIG_SKIP_LOWLEVEL_INIT [ARM, NDS32, MIPS, RISC-V only] If this variable is defined, then certain low level initializations (like setting up the memory controller) are omitted and/or U-Boot does not relocate itself into RAM.

  Normally this variable MUST NOT be defined. The only
  exception is when U-Boot is loaded (to RAM) by some
  other boot loader or by a debugger which performs
  these initializations itself.

• CONFIG_SKIP_LOWLEVEL_INIT_ONLY [ARM926EJ-S only] This allows just the call to lowlevel_init() to be skipped. The normal CP15 init (such as enabling the instruction cache) is still performed.

• CONFIG_SPL_BUILD Modifies the behaviour of start.S when compiling a loader that is executed before the actual U-Boot. E.g. when compiling a NAND SPL.

• CONFIG_TPL_BUILD Modifies the behaviour of start.S when compiling a loader that is executed after the SPL and before the actual U-Boot. It is loaded by the SPL.

• CONFIG_SYS_MPC85XX_NO_RESETVEC Only for 85xx systems. If this variable is specified, the section .resetvec is not kept and the section .bootpg is placed in the previous 4k of the .text section.

• CONFIG_ARCH_MAP_SYSMEM Generally U-Boot (and in particular the md command) uses effective address. It is therefore not necessary to regard U-Boot address as virtual addresses that need to be translated to physical addresses. However, sandbox requires this, since it maintains its own little RAM buffer which contains all addressable memory. This option causes some memory accesses to be mapped through map_sysmem() / unmap_sysmem().

• CONFIG_X86_RESET_VECTOR If defined, the x86 reset vector code is included. This is not needed when U-Boot is running from Coreboot.

• CONFIG_SYS_NAND_NO_SUBPAGE_WRITE Option to disable subpage write in NAND driver driver that uses this: drivers/mtd/nand/raw/davinci_nand.c

Freescale QE/FMAN Firmware Support:

The Freescale QUICCEngine (QE) and Frame Manager (FMAN) both support the loading of "firmware", which is encoded in the QE firmware binary format. This firmware often needs to be loaded during U-Boot booting, so macros are used to identify the storage device (NOR flash, SPI, etc) and the address within that device.

• CONFIG_SYS_FMAN_FW_ADDR The address in the storage device where the FMAN microcode is located. The meaning of this address depends on which CONFIG_SYS_QE_FW_IN_xxx macro is also specified.

• CONFIG_SYS_QE_FW_ADDR The address in the storage device where the QE microcode is located. The meaning of this address depends on which CONFIG_SYS_QE_FW_IN_xxx macro is also specified.

• CONFIG_SYS_QE_FMAN_FW_LENGTH The maximum possible size of the firmware. The firmware binary format has a field that specifies the actual size of the firmware, but it might not be possible to read any part of the firmware unless some local storage is allocated to hold the entire firmware first.

• CONFIG_SYS_QE_FMAN_FW_IN_NOR Specifies that QE/FMAN firmware is located in NOR flash, mapped as normal addressable memory via the LBC. CONFIG_SYS_FMAN_FW_ADDR is the virtual address in NOR flash.

• CONFIG_SYS_QE_FMAN_FW_IN_NAND Specifies that QE/FMAN firmware is located in NAND flash. CONFIG_SYS_FMAN_FW_ADDR is the offset within NAND flash.

• CONFIG_SYS_QE_FMAN_FW_IN_MMC Specifies that QE/FMAN firmware is located on the primary SD/MMC device. CONFIG_SYS_FMAN_FW_ADDR is the byte offset on that device.

• CONFIG_SYS_QE_FMAN_FW_IN_REMOTE Specifies that QE/FMAN firmware is located in the remote (master) memory space. CONFIG_SYS_FMAN_FW_ADDR is a virtual address which can be mapped from slave TLB->slave LAW->slave SRIO or PCIE outbound window->master inbound window->master LAW->the ucode address in master's memory space.

Freescale Layerscape Management Complex Firmware Support:

The Freescale Layerscape Management Complex (MC) supports the loading of "firmware". This firmware often needs to be loaded during U-Boot booting, so macros are used to identify the storage device (NOR flash, SPI, etc) and the address within that device.

• CONFIG_FSL_MC_ENET Enable the MC driver for Layerscape SoCs.

Freescale Layerscape Debug Server Support:

The Freescale Layerscape Debug Server Support supports the loading of "Debug Server firmware" and triggering SP boot-rom. This firmware often needs to be loaded during U-Boot booting.

• CONFIG_SYS_MC_RSV_MEM_ALIGN Define alignment of reserved memory MC requires

Reproducible builds

In order to achieve reproducible builds, timestamps used in the U-Boot build process have to be set to a fixed value.

This is done using the SOURCE_DATE_EPOCH environment variable. SOURCE_DATE_EPOCH is to be set on the build host's shell, not as a configuration option for U-Boot or an environment variable in U-Boot.

SOURCE_DATE_EPOCH should be set to a number of seconds since the epoch, in UTC.

Building the Software:

Building U-Boot has been tested in several native build environments and in many different cross environments. Of course we cannot support all possibly existing versions of cross development tools in all (potentially obsolete) versions. In case of tool chain problems we recommend to use the ELDK (see http://www.denx.de/wiki/DULG/ELDK) which is extensively used to build and test U-Boot.

If you are not using a native environment, it is assumed that you have GNU cross compiling tools available in your path. In this case, you must set the environment variable CROSS_COMPILE in your shell. Note that no changes to the Makefile or any other source files are necessary. For example using the ELDK on a 4xx CPU, please enter:

$ CROSS_COMPILE=ppc_4xx-
$ export CROSS_COMPILE

Note: If you wish to generate Windows versions of the utilities in the tools directory you can use the MinGW toolchain (http://www.mingw.org). Set your HOST tools to the MinGW toolchain and execute 'make tools'. For example:

   $ make HOSTCC=i586-mingw32msvc-gcc HOSTSTRIP=i586-mingw32msvc-strip tools

  Binaries such as tools/mkimage.exe will be created which can
  be executed on computers running Windows.

U-Boot is intended to be simple to build. After installing the sources you must configure U-Boot for one specific board type. This is done by typing:

make NAME_defconfig

where "NAME_defconfig" is the name of one of the existing configu- rations; see boards.cfg for supported names.

Note: for some board special configuration names may exist; check if additional information is available from the board vendor; for instance, the TQM823L systems are available without (standard) or with LCD support. You can select such additional "features" when choosing the configuration, i. e.

  make TQM823L_defconfig
- will configure for a plain TQM823L, i. e. no LCD support

  make TQM823L_LCD_defconfig
- will configure for a TQM823L with U-Boot console on LCD

  etc.

Finally, type "make all", and you should get some working U-Boot images ready for download to / installation on your system:

• "u-boot.bin" is a raw binary image
• "u-boot" is an image in ELF binary format
• "u-boot.srec" is in Motorola S-Record format

By default the build is performed locally and the objects are saved in the source directory. One of the two methods can be used to change this behavior and build U-Boot to some external directory:

1. Add O= to the make command line invocations:

   make O=/tmp/build distclean make O=/tmp/build NAME_defconfig make O=/tmp/build all

2. Set environment variable KBUILD_OUTPUT to point to the desired location:

   export KBUILD_OUTPUT=/tmp/build make distclean make NAME_defconfig make all

Note that the command line "O=" setting overrides the KBUILD_OUTPUT environment variable.

User specific CPPFLAGS, AFLAGS and CFLAGS can be passed to the compiler by setting the according environment variables KCPPFLAGS, KAFLAGS and KCFLAGS. For example to treat all compiler warnings as errors:

make KCFLAGS=-Werror

Please be aware that the Makefiles assume you are using GNU make, so for instance on NetBSD you might need to use "gmake" instead of native "make".

If the system board that you have is not listed, then you will need to port U-Boot to your hardware platform. To do this, follow these steps:

1. Create a new directory to hold your board specific code. Add any files you need. In your board directory, you will need at least the "Makefile" and a ".c".
2. Create a new configuration file "include/configs/.h" for your board.
3. If you're porting U-Boot to a new CPU, then also create a new directory to hold your CPU specific code. Add any files you need.
4. Run "make _defconfig" with your new name.
5. Type "make", and you should get a working "u-boot.srec" file to be installed on your target system.
6. Debug and solve any problems that might arise. [Of course, this last step is much harder than it sounds.]

Testing of U-Boot Modifications, Ports to New Hardware, etc.:

If you have modified U-Boot sources (for instance added a new board or support for new devices, a new CPU, etc.) you are expected to provide feedback to the other developers. The feedback normally takes the form of a "patch", i. e. a context diff against a certain (latest official or latest in the git repository) version of U-Boot sources.

But before you submit such a patch, please verify that your modifi- cation did not break existing code. At least make sure that ALL of the supported boards compile WITHOUT ANY compiler warnings. To do so, just run the buildman script (tools/buildman/buildman), which will configure and build U-Boot for ALL supported system. Be warned, this will take a while. Please see the buildman README, or run 'buildman -H' for documentation.

See also "U-Boot Porting Guide" below.

Monitor Commands - Overview:

go - start application at address 'addr' run - run commands in an environment variable bootm - boot application image from memory bootp - boot image via network using BootP/TFTP protocol bootz - boot zImage from memory tftpboot- boot image via network using TFTP protocol and env variables "ipaddr" and "serverip" (and eventually "gatewayip") tftpput - upload a file via network using TFTP protocol rarpboot- boot image via network using RARP/TFTP protocol diskboot- boot from IDE devicebootd - boot default, i.e., run 'bootcmd' loads - load S-Record file over serial line loadb - load binary file over serial line (kermit mode) md - memory display mm - memory modify (auto-incrementing) nm - memory modify (constant address) mw - memory write (fill) cp - memory copy cmp - memory compare crc32 - checksum calculation i2c - I2C sub-system sspi - SPI utility commands base - print or set address offset printenv- print environment variables setenv - set environment variables saveenv - save environment variables to persistent storage protect - enable or disable FLASH write protection erase - erase FLASH memory flinfo - print FLASH memory information nand - NAND memory operations (see doc/README.nand) bdinfo - print Board Info structure iminfo - print header information for application image coninfo - print console devices and informations ide - IDE sub-system loop - infinite loop on address range loopw - infinite write loop on address range mtest - simple RAM test icache - enable or disable instruction cache dcache - enable or disable data cache reset - Perform RESET of the CPU echo - echo args to console version - print monitor version help - print online help ? - alias for 'help'

Monitor Commands - Detailed Description:

TODO.

For now: just type "help ".

Environment Variables:

U-Boot supports user configuration using Environment Variables which can be made persistent by saving to Flash memory.

Environment Variables are set using "setenv", printed using "printenv", and saved to Flash using "saveenv". Using "setenv" without a value can be used to delete a variable from the environment. As long as you don't save the environment you are working with an in-memory copy. In case the Flash area containing the environment is erased by accident, a default environment is provided.

Some configuration options can be set using Environment Variables.

List of environment variables (most likely not complete):

baudrate - see CONFIG_BAUDRATE

bootdelay - see CONFIG_BOOTDELAY

bootcmd - see CONFIG_BOOTCOMMAND

bootargs - Boot arguments when booting an RTOS image

bootfile - Name of the image to load with TFTP

bootm_low - Memory range available for image processing in the bootm command can be restricted. This variable is given as a hexadecimal number and defines lowest address allowed for use by the bootm command. See also "bootm_size" environment variable. Address defined by "bootm_low" is also the base of the initial memory mapping for the Linux kernel -- see the description of CONFIG_SYS_BOOTMAPSZ and bootm_mapsize.

bootm_mapsize - Size of the initial memory mapping for the Linux kernel. This variable is given as a hexadecimal number and it defines the size of the memory region starting at base address bootm_low that is accessible by the Linux kernel during early boot. If unset, CONFIG_SYS_BOOTMAPSZ is used as the default value if it is defined, and bootm_size is used otherwise.

bootm_size - Memory range available for image processing in the bootm command can be restricted. This variable is given as a hexadecimal number and defines the size of the region allowed for use by the bootm command. See also "bootm_low" environment variable.

updatefile - Location of the software update file on a TFTP server, used by the automatic software update feature. Please refer to documentation in doc/README.update for more details.

autoload - if set to "no" (any string beginning with 'n'), "bootp" will just load perform a lookup of the configuration from the BOOTP server, but not try to load any image using TFTP

autostart - if set to "yes", an image loaded using the "bootp", "rarpboot", "tftpboot" or "diskboot" commands will be automatically started (by internally calling "bootm")

      If set to "no", a standalone image passed to the
      "bootm" command will be copied to the load address
      (and eventually uncompressed), but NOT be started.
      This can be used to load and uncompress arbitrary
      data.

fdt_high - if set this restricts the maximum address that the flattened device tree will be copied into upon boot. For example, if you have a system with 1 GB memory at physical address 0x10000000, while Linux kernel only recognizes the first 704 MB as low memory, you may need to set fdt_high as 0x3C000000 to have the device tree blob be copied to the maximum address of the 704 MB low memory, so that Linux kernel can access it during the boot procedure.

      If this is set to the special value 0xFFFFFFFF then
      the fdt will not be copied at all on boot.  For this
      to work it must reside in writable memory, have
      sufficient padding on the end of it for u-boot to
      add the information it needs into it, and the memory
      must be accessible by the kernel.

fdtcontroladdr- if set this is the address of the control flattened device tree used by U-Boot when CONFIG_OF_CONTROL is defined.

i2cfast - (PPC405GP|PPC405EP only) if set to 'y' configures Linux I2C driver for fast mode (400kHZ). This environment variable is used in initialization code. So, for changes to be effective it must be saved and board must be reset.

initrd_high - restrict positioning of initrd images: If this variable is not set, initrd images will be copied to the highest possible address in RAM; this is usually what you want since it allows for maximum initrd size. If for some reason you want to make sure that the initrd image is loaded below the CONFIG_SYS_BOOTMAPSZ limit, you can set this environment variable to a value of "no" or "off" or "0". Alternatively, you can set it to a maximum upper address to use (U-Boot will still check that it does not overwrite the U-Boot stack and data).

      For instance, when you have a system with 16 MB
      RAM, and want to reserve 4 MB from use by Linux,
      you can do this by adding "mem=12M" to the value of
      the "bootargs" variable. However, now you must make
      sure that the initrd image is placed in the first
      12 MB as well - this can be done with

      setenv initrd_high 00c00000

      If you set initrd_high to 0xFFFFFFFF, this is an
      indication to U-Boot that all addresses are legal
      for the Linux kernel, including addresses in flash
      memory. In this case U-Boot will NOT COPY the
      ramdisk at all. This may be useful to reduce the
      boot time on your system, but requires that this
      feature is supported by your Linux kernel.

ipaddr - IP address; needed for tftpboot command

loadaddr - Default load address for commands like "bootp", "rarpboot", "tftpboot", "loadb" or "diskboot"

loads_echo - see CONFIG_LOADS_ECHO

serverip - TFTP server IP address; needed for tftpboot command

bootretry - see CONFIG_BOOT_RETRY_TIME

bootdelaykey - see CONFIG_AUTOBOOT_DELAY_STR

bootstopkey - see CONFIG_AUTOBOOT_STOP_STR

ethprime - controls which interface is used first.

ethact - controls which interface is currently active. For example you can do the following

      => setenv ethact FEC
      => ping 192.168.0.1 # traffic sent on FEC
      => setenv ethact SCC
      => ping 10.0.0.1 # traffic sent on SCC

ethrotate - When set to "no" U-Boot does not go through all available network interfaces. It just stays at the currently selected interface.

netretry - When set to "no" each network operation will either succeed or fail without retrying. When set to "once" the network operation will fail when all the available network interfaces are tried once without success. Useful on scripts which control the retry operation themselves.

npe_ucode - set load address for the NPE microcode

silent_linux - If set then Linux will be told to boot silently, by changing the console to be empty. If "yes" it will be made silent. If "no" it will not be made silent. If unset, then it will be made silent if the U-Boot console is silent.

tftpsrcp - If this is set, the value is used for TFTP's UDP source port.

tftpdstp - If this is set, the value is used for TFTP's UDP destination port instead of the Well Know Port 69.

tftpblocksize - Block size to use for TFTP transfers; if not set, we use the TFTP server's default block size

tftptimeout - Retransmission timeout for TFTP packets (in milli- seconds, minimum value is 1000 = 1 second). Defines when a packet is considered to be lost so it has to be retransmitted. The default is 5000 = 5 seconds. Lowering this value may make downloads succeed faster in networks with high packet loss rates or with unreliable TFTP servers.

tftptimeoutcountmax - maximum count of TFTP timeouts (no unit, minimum value = 0). Defines how many timeouts can happen during a single file transfer before that transfer is aborted. The default is 10, and 0 means 'no timeouts allowed'. Increasing this value may help downloads succeed with high packet loss rates, or with unreliable TFTP servers or client hardware.

vlan - When set to a value < 4095 the traffic over Ethernet is encapsulated/received over 802.1q VLAN tagged frames.

bootpretryperiod - Period during which BOOTP/DHCP sends retries. Unsigned value, in milliseconds. If not set, the period will be either the default (28000), or a value based on CONFIG_NET_RETRY_COUNT, if defined. This value has precedence over the valu based on CONFIG_NET_RETRY_COUNT.

The following image location variables contain the location of images used in booting. The "Image" column gives the role of the image and is not an environment variable name. The other columns are environment variable names. "File Name" gives the name of the file on a TFTP server, "RAM Address" gives the location in RAM the image will be loaded to, and "Flash Location" gives the image's address in NOR flash or offset in NAND flash.

Note - these variables don't have to be defined for all boards, some boards currently use other variables for these purposes, and some boards use these variables for other purposes.

Image File Name RAM Address Flash Location

---

u-boot u-boot u-boot_addr_r u-boot_addr Linux kernel bootfile kernel_addr_r kernel_addr device tree blob fdtfile fdt_addr_r fdt_addr ramdisk ramdiskfile ramdisk_addr_r ramdisk_addr

The following environment variables may be used and automatically updated by the network boot commands ("bootp" and "rarpboot"), depending the information provided by your boot server:

bootfile - see above dnsip - IP address of your Domain Name Server dnsip2 - IP address of your secondary Domain Name Server gatewayip - IP address of the Gateway (Router) to use hostname - Target hostname ipaddr - see above netmask - Subnet Mask rootpath - Pathname of the root filesystem on the NFS server serverip - see above

There are two special Environment Variables:

serial# - contains hardware identification information such as type string and/or serial number ethaddr - Ethernet address

These variables can be set only once (usually during manufacturing of the board). U-Boot refuses to delete or overwrite these variables once they have been set once.

Further special Environment Variables:

ver - Contains the U-Boot version string as printed with the "version" command. This variable is readonly (see CONFIG_VERSION_VARIABLE).

Please note that changes to some configuration parameters may take only effect after the next boot (yes, that's just like Windoze :-).

Callback functions for environment variables:

For some environment variables, the behavior of u-boot needs to change when their values are changed. This functionality allows functions to be associated with arbitrary variables. On creation, overwrite, or deletion, the callback will provide the opportunity for some side effect to happen or for the change to be rejected.

The callbacks are named and associated with a function using the U_BOOT_ENV_CALLBACK macro in your board or driver code.

These callbacks are associated with variables in one of two ways. The static list can be added to by defining CONFIG_ENV_CALLBACK_LIST_STATIC in the board configuration to a string that defines a list of associations. The list must be in the following format:

entry = variable_name[:callback_name]
list = entry[,list]

If the callback name is not specified, then the callback is deleted. Spaces are also allowed anywhere in the list.

Callbacks can also be associated by defining the ".callbacks" variable with the same list format above. Any association in ".callbacks" will override any association in the static list. You can define CONFIG_ENV_CALLBACK_LIST_DEFAULT to a list (string) to define the ".callbacks" environment variable in the default or embedded environment.

If CONFIG_REGEX is defined, the variable_name above is evaluated as a regular expression. This allows multiple variables to be connected to the same callback without explicitly listing them all out.

The signature of the callback functions is:

int callback(const char *name, const char *value, enum env_op op, int flags)

• name - changed environment variable
• value - new value of the environment variable
• op - operation (create, overwrite, or delete)
• flags - attributes of the environment variable change, see flags H_* in include/search.h

The return value is 0 if the variable change is accepted and 1 otherwise.

Command Line Parsing:

There are two different command line parsers available with U-Boot: the old "simple" one, and the much more powerful "hush" shell:

Old, simple command line parser:

• supports environment variables (through setenv / saveenv commands)
• several commands on one line, separated by ';'
• variable substitution using "... ${name} ..." syntax
• special characters ('$', ';') can be escaped by prefixing with '\', for example: setenv bootcmd bootm \${address}
• You can also escape text by enclosing in single apostrophes, for example: setenv addip 'setenv bootargs $bootargs ip=$ipaddr:$serverip:$gatewayip:$netmask:$hostname::off'

Hush shell:

• similar to Bourne shell, with control structures like if...then...else...fi, for...do...done; while...do...done, until...do...done, ...
• supports environment ("global") variables (through setenv / saveenv commands) and local shell variables (through standard shell syntax "name=value"); only environment variables can be used with "run" command

General rules:

(1) If a command line (or an environment variable executed by a "run" command) contains several commands separated by semicolon, and one of these commands fails, then the remaining commands will be executed anyway.

(2) If you execute several variables with one call to run (i. e. calling run with a list of variables as arguments), any failing command will cause "run" to terminate, i. e. the remaining variables are not executed.

Note for Redundant Ethernet Interfaces:

Some boards come with redundant Ethernet interfaces; U-Boot supports such configurations and is capable of automatic selection of a "working" interface when needed. MAC assignment works as follows:

Network interfaces are numbered eth0, eth1, eth2, ... Corresponding MAC addresses can be stored in the environment as "ethaddr" (=>eth0), "eth1addr" (=>eth1), "eth2addr", ...

If the network interface stores some valid MAC address (for instance in SROM), this is used as default address if there is NO correspon- ding setting in the environment; if the corresponding environment variable is set, this overrides the settings in the card; that means:

o If the SROM has a valid MAC address, and there is no address in the environment, the SROM's address is used.

o If there is no valid address in the SROM, and a definition in the environment exists, then the value from the environment variable is used.

o If both the SROM and the environment contain a MAC address, and both addresses are the same, this MAC address is used.

o If both the SROM and the environment contain a MAC address, and the addresses differ, the value from the environment is used and a warning is printed.

o If neither SROM nor the environment contain a MAC address, an error is raised. If CONFIG_NET_RANDOM_ETHADDR is defined, then in this case a random, locally-assigned MAC is used.

If Ethernet drivers implement the 'write_hwaddr' function, valid MAC addresses will be programmed into hardware as part of the initialization process. This may be skipped by setting the appropriate 'ethmacskip' environment variable. The naming convention is as follows: "ethmacskip" (=>eth0), "eth1macskip" (=>eth1) etc.

Image Formats:

U-Boot is capable of booting (and performing other auxiliary operations on) images in two formats:

New uImage format (FIT)

Flexible and powerful format based on Flattened Image Tree -- FIT (similar to Flattened Device Tree). It allows the use of images with multiple components (several kernels, ramdisks, etc.), with contents protected by SHA1, MD5 or CRC32. More details are found in the doc/uImage.FIT directory.

Old uImage format

Old image format is based on binary files which can be basically anything, preceded by a special header; see the definitions in include/image.h for details; basically, the header defines the following image properties:

• Target Operating System (Provisions for OpenBSD, NetBSD, FreeBSD, 4.4BSD, Linux, SVR4, Esix, Solaris, Irix, SCO, Dell, NCR, VxWorks, LynxOS, pSOS, QNX, RTEMS, INTEGRITY; Currently supported: Linux, NetBSD, VxWorks, QNX, RTEMS, LynxOS, INTEGRITY).
• Target CPU Architecture (Provisions for Alpha, ARM, Intel x86, IA64, MIPS, NDS32, Nios II, PowerPC, IBM S390, SuperH, Sparc, Sparc 64 Bit; Currently supported: ARM, Intel x86, MIPS, NDS32, Nios II, PowerPC).
• Compression Type (uncompressed, gzip, bzip2)
• Load Address
• Entry Point
• Image Name
• Image Timestamp

The header is marked by a special Magic Number, and both the header and the data portions of the image are secured against corruption by CRC32 checksums.

Linux Support:

Although U-Boot should support any OS or standalone application easily, the main focus has always been on Linux during the design of U-Boot.

U-Boot includes many features that so far have been part of some special "boot loader" code within the Linux kernel. Also, any "initrd" images to be used are no longer part of one big Linux image; instead, kernel and "initrd" are separate images. This implementation serves several purposes:

• the same features can be used for other OS or standalone applications (for instance: using compressed images to reduce the Flash memory footprint)

• it becomes much easier to port new Linux kernel versions because lots of low-level, hardware dependent stuff are done by U-Boot

• the same Linux kernel image can now be used with different "initrd" images; of course this also means that different kernel images can be run with the same "initrd". This makes testing easier (you don't have to build a new "zImage.initrd" Linux image when you just change a file in your "initrd"). Also, a field-upgrade of the software is easier now.

Linux HOWTO:

Porting Linux to U-Boot based systems:

U-Boot cannot save you from doing all the necessary modifications to configure the Linux device drivers for use with your target hardware (no, we don't intend to provide a full virtual machine interface to Linux :-).

But now you can ignore ALL boot loader code (in arch/powerpc/mbxboot).

Just make sure your machine specific header file (for instance include/asm-ppc/tqm8xx.h) includes the same definition of the Board Information structure as we define in include/asm-/u-boot.h, and make sure that your definition of IMAP_ADDR uses the same value as your U-Boot configuration in CONFIG_SYS_IMMR.

Note that U-Boot now has a driver model, a unified model for drivers. If you are adding a new driver, plumb it into driver model. If there is no uclass available, you are encouraged to create one. See doc/driver-model.

Configuring the Linux kernel:

No specific requirements for U-Boot. Make sure you have some root device (initial ramdisk, NFS) for your target system.

Building a Linux Image:

With U-Boot, "normal" build targets like "zImage" or "bzImage" are not used. If you use recent kernel source, a new build target "uImage" will exist which automatically builds an image usable by U-Boot. Most older kernels also have support for a "pImage" target, which was introduced for our predecessor project PPCBoot and uses a 100% compatible format.

Example:

make TQM850L_defconfig
make oldconfig
make dep
make uImage

The "uImage" build target uses a special tool (in 'tools/mkimage') to encapsulate a compressed Linux kernel image with header information, CRC32 checksum etc. for use with U-Boot. This is what we are doing:

• build a standard "vmlinux" kernel image (in ELF binary format):

• convert the kernel into a raw binary image:

  ${CROSS_COMPILE}-objcopy -O binary \ -R .note -R .comment \ -S vmlinux linux.bin

• compress the binary image:

  gzip -9 linux.bin

• package compressed binary image for U-Boot:

  mkimage -A ppc -O linux -T kernel -C gzip \ -a 0 -e 0 -n "Linux Kernel Image" \ -d linux.bin.gz uImage

The "mkimage" tool can also be used to create ramdisk images for use with U-Boot, either separated from the Linux kernel image, or combined into one file. "mkimage" encapsulates the images with a 64 byte header containing information about target architecture, operating system, image type, compression method, entry points, time stamp, CRC32 checksums, etc.

"mkimage" can be called in two ways: to verify existing images and print the header information, or to build new images.

In the first form (with "-l" option) mkimage lists the information contained in the header of an existing U-Boot image; this includes checksum verification:

tools/mkimage -l image
  -l ==> list image header information

The second form (with "-d" option) is used to build a U-Boot image from a "data file" which is used as image payload:

tools/mkimage -A arch -O os -T type -C comp -a addr -e ep \
          -n name -d data_file image
  -A ==> set architecture to 'arch'
  -O ==> set operating system to 'os'
  -T ==> set image type to 'type'
  -C ==> set compression type 'comp'
  -a ==> set load address to 'addr' (hex)
  -e ==> set entry point to 'ep' (hex)
  -n ==> set image name to 'name'
  -d ==> use image data from 'datafile'

Right now, all Linux kernels for PowerPC systems use the same load address (0x00000000), but the entry point address depends on the kernel version:

• 2.2.x kernels have the entry point at 0x0000000C,
• 2.3.x and later kernels have the entry point at 0x00000000.

So a typical call to build a U-Boot image would read:

-> tools/mkimage -n '2.4.4 kernel for TQM850L' \
> -A ppc -O linux -T kernel -C gzip -a 0 -e 0 \
> -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux.gz \
> examples/uImage.TQM850L
Image Name:   2.4.4 kernel for TQM850L
Created:      Wed Jul 19 02:34:59 2000
Image Type:   PowerPC Linux Kernel Image (gzip compressed)
Data Size:    335725 Bytes = 327.86 kB = 0.32 MB
Load Address: 0x00000000
Entry Point:  0x00000000

To verify the contents of the image (or check for corruption):

-> tools/mkimage -l examples/uImage.TQM850L
Image Name:   2.4.4 kernel for TQM850L
Created:      Wed Jul 19 02:34:59 2000
Image Type:   PowerPC Linux Kernel Image (gzip compressed)
Data Size:    335725 Bytes = 327.86 kB = 0.32 MB
Load Address: 0x00000000
Entry Point:  0x00000000

NOTE: for embedded systems where boot time is critical you can trade speed for memory and install an UNCOMPRESSED image instead: this needs more space in Flash, but boots much faster since it does not need to be uncompressed:

-> gunzip /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux.gz
-> tools/mkimage -n '2.4.4 kernel for TQM850L' \
> -A ppc -O linux -T kernel -C none -a 0 -e 0 \
> -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux \
> examples/uImage.TQM850L-uncompressed
Image Name:   2.4.4 kernel for TQM850L
Created:      Wed Jul 19 02:34:59 2000
Image Type:   PowerPC Linux Kernel Image (uncompressed)
Data Size:    792160 Bytes = 773.59 kB = 0.76 MB
Load Address: 0x00000000
Entry Point:  0x00000000

Similar you can build U-Boot images from a 'ramdisk.image.gz' file when your kernel is intended to use an initial ramdisk:

-> tools/mkimage -n 'Simple Ramdisk Image' \
> -A ppc -O linux -T ramdisk -C gzip \
> -d /LinuxPPC/images/SIMPLE-ramdisk.image.gz examples/simple-initrd
Image Name:   Simple Ramdisk Image
Created:      Wed Jan 12 14:01:50 2000
Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
Data Size:    566530 Bytes = 553.25 kB = 0.54 MB
Load Address: 0x00000000
Entry Point:  0x00000000

The "dumpimage" is a tool to disassemble images built by mkimage. Its "-i" option performs the converse operation of the mkimage's second form (the "-d" option). Given an image built by mkimage, the dumpimage extracts a "data file" from the image:

tools/dumpimage -i image -T type -p position data_file
  -i ==> extract from the 'image' a specific 'data_file'
  -T ==> set image type to 'type'
  -p ==> 'position' (starting at 0) of the 'data_file' inside the 'image'

Installing a Linux Image:

To downloading a U-Boot image over the serial (console) interface, you must convert the image to S-Record format:

objcopy -I binary -O srec examples/image examples/image.srec

The 'objcopy' does not understand the information in the U-Boot image header, so the resulting S-Record file will be relative to address 0x00000000. To load it to a given address, you need to specify the target address as 'offset' parameter with the 'loads' command.

Example: install the image to address 0x40100000 (which on the TQM8xxL is in the first Flash bank):

=> erase 40100000 401FFFFF

.......... done
Erased 8 sectors

=> loads 40100000
## Ready for S-Record download ...
~>examples/image.srec
1 2 3 4 5 6 7 8 9 10 11 12 13 ...
...
15989 15990 15991 15992
[file transfer complete]
[connected]
## Start Addr = 0x00000000

You can check the success of the download using the 'iminfo' command; this includes a checksum verification so you can be sure no data corruption happened:

=> imi 40100000

## Checking Image at 40100000 ...
   Image Name:   2.2.13 for initrd on TQM850L
   Image Type:   PowerPC Linux Kernel Image (gzip compressed)
   Data Size:    335725 Bytes = 327 kB = 0 MB
   Load Address: 00000000
   Entry Point:  0000000c
   Verifying Checksum ... OK

Boot Linux:

The "bootm" command is used to boot an application that is stored in memory (RAM or Flash). In case of a Linux kernel image, the contents of the "bootargs" environment variable is passed to the kernel as parameters. You can check and modify this variable using the "printenv" and "setenv" commands:

=> printenv bootargs
bootargs=root=/dev/ram

=> setenv bootargs root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2

=> printenv bootargs
bootargs=root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2

=> bootm 40020000
## Booting Linux kernel at 40020000 ...
   Image Name:   2.2.13 for NFS on TQM850L
   Image Type:   PowerPC Linux Kernel Image (gzip compressed)
   Data Size:    381681 Bytes = 372 kB = 0 MB
   Load Address: 00000000
   Entry Point:  0000000c
   Verifying Checksum ... OK
   Uncompressing Kernel Image ... OK
Linux version 2.2.13 (wd@denx.local.net) (gcc version 2.95.2 19991024 (release)) #1 Wed Jul 19 02:35:17 MEST 2000
Boot arguments: root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2
time_init: decrementer frequency = 187500000/60
Calibrating delay loop... 49.77 BogoMIPS
Memory: 15208k available (700k kernel code, 444k data, 32k init) [c0000000,c1000000]
...

If you want to boot a Linux kernel with initial RAM disk, you pass the memory addresses of both the kernel and the initrd image (PPBCOOT format!) to the "bootm" command:

=> imi 40100000 40200000

## Checking Image at 40100000 ...
   Image Name:   2.2.13 for initrd on TQM850L
   Image Type:   PowerPC Linux Kernel Image (gzip compressed)
   Data Size:    335725 Bytes = 327 kB = 0 MB
   Load Address: 00000000
   Entry Point:  0000000c
   Verifying Checksum ... OK

## Checking Image at 40200000 ...
   Image Name:   Simple Ramdisk Image
   Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
   Data Size:    566530 Bytes = 553 kB = 0 MB
   Load Address: 00000000
   Entry Point:  00000000
   Verifying Checksum ... OK

=> bootm 40100000 40200000
## Booting Linux kernel at 40100000 ...
   Image Name:   2.2.13 for initrd on TQM850L
   Image Type:   PowerPC Linux Kernel Image (gzip compressed)
   Data Size:    335725 Bytes = 327 kB = 0 MB
   Load Address: 00000000
   Entry Point:  0000000c
   Verifying Checksum ... OK
   Uncompressing Kernel Image ... OK
## Loading RAMDisk Image at 40200000 ...
   Image Name:   Simple Ramdisk Image
   Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
   Data Size:    566530 Bytes = 553 kB = 0 MB
   Load Address: 00000000
   Entry Point:  00000000
   Verifying Checksum ... OK
   Loading Ramdisk ... OK
Linux version 2.2.13 (wd@denx.local.net) (gcc version 2.95.2 19991024 (release)) #1 Wed Jul 19 02:32:08 MEST 2000
Boot arguments: root=/dev/ram
time_init: decrementer frequency = 187500000/60
Calibrating delay loop... 49.77 BogoMIPS
...
RAMDISK: Compressed image found at block 0
VFS: Mounted root (ext2 filesystem).

bash#

Boot Linux and pass a flat device tree:

First, U-Boot must be compiled with the appropriate defines. See the section titled "Linux Kernel Interface" above for a more in depth explanation. The following is an example of how to start a kernel and pass an updated flat device tree:

=> print oftaddr oftaddr=0x300000 => print oft oft=oftrees/mpc8540ads.dtb => tftp $oftaddr $oft Speed: 1000, full duplex Using TSEC0 device TFTP from server 192.168.1.1; our IP address is 192.168.1.101 Filename 'oftrees/mpc8540ads.dtb'. Load address: 0x300000 Loading: # done Bytes transferred = 4106 (100a hex) => tftp $loadaddr $bootfile Speed: 1000, full duplex Using TSEC0 device TFTP from server 192.168.1.1; our IP address is 192.168.1.2 Filename 'uImage'. Load address: 0x200000 Loading:############ done Bytes transferred = 1029407 (fb51f hex) => print loadaddr loadaddr=200000 => print oftaddr oftaddr=0x300000 => bootm $loadaddr - $oftaddr

Booting image at 00200000 ...

Image Name: Linux-2.6.17-dirty Image Type: PowerPC Linux Kernel Image (gzip compressed) Data Size: 1029343 Bytes = 1005.2 kB Load Address: 00000000 Entry Point: 00000000 Verifying Checksum ... OK Uncompressing Kernel Image ... OK Booting using flat device tree at 0x300000 Using MPC85xx ADS machine description Memory CAM mapping: CAM0=256Mb, CAM1=256Mb, CAM2=0Mb residual: 0Mb [snip]

More About U-Boot Image Types:

U-Boot supports the following image types:

"Standalone Programs" are directly runnable in the environment provided by U-Boot; it is expected that (if they behave well) you can continue to work in U-Boot after return from the Standalone Program. "OS Kernel Images" are usually images of some Embedded OS which will take over control completely. Usually these programs will install their own set of exception handlers, device drivers, set up the MMU, etc. - this means, that you cannot expect to re-enter U-Boot except by resetting the CPU. "RAMDisk Images" are more or less just data blocks, and their parameters (address, size) are passed to an OS kernel that is being started. "Multi-File Images" contain several images, typically an OS (Linux) kernel image and one or more data images like RAMDisks. This construct is useful for instance when you want to boot over the network using BOOTP etc., where the boot server provides just a single image file, but you want to get for instance an OS kernel and a RAMDisk image.

"Multi-File Images" start with a list of image sizes, each
image size (in bytes) specified by an "uint32_t" in network
byte order. This list is terminated by an "(uint32_t)0".
Immediately after the terminating 0 follow the images, one by
one, all aligned on "uint32_t" boundaries (size rounded up to
a multiple of 4 bytes).

"Firmware Images" are binary images containing firmware (like U-Boot or FPGA images) which usually will be programmed to flash memory.

"Script files" are command sequences that will be executed by U-Boot's command interpreter; this feature is especially useful when you configure U-Boot to use a real shell (hush) as command interpreter.

Booting the Linux zImage:

On some platforms, it's possible to boot Linux zImage. This is done using the "bootz" command. The syntax of "bootz" command is the same as the syntax of "bootm" command.

Note, defining the CONFIG_SUPPORT_RAW_INITRD allows user to supply kernel with raw initrd images. The syntax is slightly different, the address of the initrd must be augmented by it's size, in the following format: ":".

Standalone HOWTO:

One of the features of U-Boot is that you can dynamically load and run "standalone" applications, which can use some resources of U-Boot like console I/O functions or interrupt services.

Two simple examples are included with the sources:

"Hello World" Demo:

'examples/hello_world.c' contains a small "Hello World" Demo application; it is automatically compiled when you build U-Boot. It's configured to run at address 0x00040004, so you can play with it like that:

=> loads
## Ready for S-Record download ...
~>examples/hello_world.srec
1 2 3 4 5 6 7 8 9 10 11 ...
[file transfer complete]
[connected]
## Start Addr = 0x00040004

=> go 40004 Hello World! This is a test.
## Starting application at 0x00040004 ...
Hello World
argc = 7
argv[0] = "40004"
argv[1] = "Hello"
argv[2] = "World!"
argv[3] = "This"
argv[4] = "is"
argv[5] = "a"
argv[6] = "test."
argv[7] = "<NULL>"
Hit any key to exit ...

## Application terminated, rc = 0x0

Another example, which demonstrates how to register a CPM interrupt handler with the U-Boot code, can be found in 'examples/timer.c'. Here, a CPM timer is set up to generate an interrupt every second. The interrupt service routine is trivial, just printing a '.' character, but this is just a demo program. The application can be controlled by the following keys:

? - print current values og the CPM Timer registers
b - enable interrupts and start timer
e - stop timer and disable interrupts
q - quit application

=> loads
## Ready for S-Record download ...
~>examples/timer.srec
1 2 3 4 5 6 7 8 9 10 11 ...
[file transfer complete]
[connected]
## Start Addr = 0x00040004

=> go 40004
## Starting application at 0x00040004 ...
TIMERS=0xfff00980
Using timer 1
  tgcr @ 0xfff00980, tmr @ 0xfff00990, trr @ 0xfff00994, tcr @ 0xfff00998, tcn @ 0xfff0099c, ter @ 0xfff009b0

Hit 'b': [q, b, e, ?] Set interval 1000000 us Enabling timer Hit '?': [q, b, e, ?] ........ tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0xef6, ter=0x0 Hit '?': [q, b, e, ?] . tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x2ad4, ter=0x0 Hit '?': [q, b, e, ?] . tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x1efc, ter=0x0 Hit '?': [q, b, e, ?] . tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x169d, ter=0x0 Hit 'e': [q, b, e, ?] ...Stopping timer Hit 'q': [q, b, e, ?] ## Application terminated, rc = 0x0

Minicom warning:

Over time, many people have reported problems when trying to use the "minicom" terminal emulation program for serial download. I (wd) consider minicom to be broken, and recommend not to use it. Under Unix, I recommend to use C-Kermit for general purpose use (and especially for kermit binary protocol download ("loadb" command), and use "cu" for S-Record download ("loads" command). See http://www.denx.de/wiki/view/DULG/SystemSetup#Section_4.3. for help with kermit.

Nevertheless, if you absolutely want to use it try adding this configuration to your "File transfer protocols" section:

   Name    Program          Name U/D FullScr IO-Red. Multi
X  kermit  /usr/bin/kermit -i -l %l -s   Y    U    Y       N      N
Y  kermit  /usr/bin/kermit -i -l %l -r   N    D    Y       N      N

NetBSD Notes:

Starting at version 0.9.2, U-Boot supports NetBSD both as host (build U-Boot) and target system (boots NetBSD/mpc8xx).

Building requires a cross environment; it is known to work on NetBSD/i386 with the cross-powerpc-netbsd-1.3 package (you will also need gmake since the Makefiles are not compatible with BSD make). Note that the cross-powerpc package does not install include files; attempting to build U-Boot will fail because <machine/ansi.h> is missing. This file has to be installed and patched manually:

# cd /usr/pkg/cross/powerpc-netbsd/include
# mkdir powerpc
# ln -s powerpc machine
# cp /usr/src/sys/arch/powerpc/include/ansi.h powerpc/ansi.h
# ${EDIT} powerpc/ansi.h    ## must remove __va_list, _BSD_VA_LIST

Native builds don't work due to incompatibilities between native and U-Boot include files.

Booting assumes that (the first part of) the image booted is a stage-2 loader which in turn loads and then invokes the kernel proper. Loader sources will eventually appear in the NetBSD source tree (probably in sys/arc/mpc8xx/stand/u-boot_stage2/); in the meantime, see ftp://ftp.denx.de/pub/u-boot/ppcboot_stage2.tar.gz

Implementation Internals:

The following is not intended to be a complete description of every implementation detail. However, it should help to understand the inner workings of U-Boot and make it easier to port it to custom hardware.

Initial Stack, Global Data:

The implementation of U-Boot is complicated by the fact that U-Boot starts running out of ROM (flash memory), usually without access to system RAM (because the memory controller is not initialized yet). This means that we don't have writable Data or BSS segments, and BSS is not initialized as zero. To be able to get a C environment working at all, we have to allocate at least a minimal stack. Implementation options for this are defined and restricted by the CPU used: Some CPU models provide on-chip memory (like the IMMR area on MPC8xx and MPC826x processors), on others (parts of) the data cache can be locked as (mis-) used as memory, etc.

Chris Hallinan posted a good summary of these issues to the
U-Boot mailing list:

Subject: RE: [U-Boot-Users] RE: More On Memory Bank x (nothingness)?
From: "Chris Hallinan" <clh@net1plus.com>
Date: Mon, 10 Feb 2003 16:43:46 -0500 (22:43 MET)
...

Correct me if I'm wrong, folks, but the way I understand it
is this: Using DCACHE as initial RAM for Stack, etc, does not
require any physical RAM backing up the cache. The cleverness
is that the cache is being used as a temporary supply of
necessary storage before the SDRAM controller is setup. It's
beyond the scope of this list to explain the details, but you
can see how this works by studying the cache architecture and
operation in the architecture and processor-specific manuals.

OCM is On Chip Memory, which I believe the 405GP has 4K. It
is another option for the system designer to use as an
initial stack/RAM area prior to SDRAM being available. Either
option should work for you. Using CS 4 should be fine if your
board designers haven't used it for something that would
cause you grief during the initial boot! It is frequently not
used.

CONFIG_SYS_INIT_RAM_ADDR should be somewhere that won't interfere
with your processor/board/system design. The default value
you will find in any recent u-boot distribution in
walnut.h should work for you. I'd set it to a value larger
than your SDRAM module. If you have a 64MB SDRAM module, set
it above 400_0000. Just make sure your board has no resources
that are supposed to respond to that address! That code in
start.S has been around a while and should work as is when
you get the config right.

-Chris Hallinan
DS4.COM, Inc.

It is essential to remember this, since it has some impact on the C code for the initialization procedures:

• Initialized global data (data segment) is read-only. Do not attempt to write it.

• Do not use any uninitialized global data (or implicitly initialized as zero data - BSS segment) at all - this is undefined, initiali- zation is performed later (when relocating to RAM).

• Stack space is very limited. Avoid big data buffers or things like that.

Having only the stack as writable memory limits means we cannot use normal global data to share information between the code. But it turned out that the implementation of U-Boot can be greatly simplified by making a global data structure (gd_t) available to all functions. We could pass a pointer to this data as argument to all functions, but this would bloat the code. Instead we use a feature of the GCC compiler (Global Register Variables) to share the data: we place a pointer (gd) to the global data into a register which we reserve for this purpose.

When choosing a register for such a purpose we are restricted by the relevant (E)ABI specifications for the current architecture, and by GCC's implementation.

For PowerPC, the following registers have specific use: R1: stack pointer R2: reserved for system use R3-R4: parameter passing and return values R5-R10: parameter passing R13: small data area pointer R30: GOT pointer R31: frame pointer

(U-Boot also uses R12 as internal GOT pointer. r12
is a volatile register so r12 needs to be reset when
going back and forth between asm and C)

==> U-Boot will use R2 to hold a pointer to the global data

Note: on PPC, we could use a static initializer (since the
address of the global data structure is known at compile time),
but it turned out that reserving a register results in somewhat
smaller code - although the code savings are not that big (on
average for all boards 752 bytes for the whole U-Boot image,
624 text + 127 data).

On ARM, the following registers are used:

R0: function argument word/integer result
R1-R3:  function argument word
R9: platform specific
R10:    stack limit (used only if stack checking is enabled)
R11:    argument (frame) pointer
R12:    temporary workspace
R13:    stack pointer
R14:    link register
R15:    program counter

==> U-Boot will use R9 to hold a pointer to the global data

Note: on ARM, only R_ARM_RELATIVE relocations are supported.

On Nios II, the ABI is documented here: http://www.altera.com/literature/hb/nios2/n2cpu_nii51016.pdf

==> U-Boot will use gp to hold a pointer to the global data

Note: on Nios II, we give "-G0" option to gcc and don't use gp
to access small data sections, so gp is free.

On NDS32, the following registers are used:

R0-R1:  argument/return
R2-R5:  argument
R15:    temporary register for assembler
R16:    trampoline register
R28:    frame pointer (FP)
R29:    global pointer (GP)
R30:    link register (LP)
R31:    stack pointer (SP)
PC: program counter (PC)

==> U-Boot will use R10 to hold a pointer to the global data

NOTE: DECLARE_GLOBAL_DATA_PTR must be used with file-global scope, or current versions of GCC may "optimize" the code too much.

On RISC-V, the following registers are used:

x0: hard-wired zero (zero)
x1: return address (ra)
x2: stack pointer (sp)
x3: global pointer (gp)
x4: thread pointer (tp)
x5: link register (t0)
x8: frame pointer (fp)
x10-x11:    arguments/return values (a0-1)
x12-x17:    arguments (a2-7)
x28-31:  temporaries (t3-6)
pc: program counter (pc)

==> U-Boot will use gp to hold a pointer to the global data

Memory Management:

U-Boot runs in system state and uses physical addresses, i.e. the MMU is not used either for address mapping nor for memory protection.

The available memory is mapped to fixed addresses using the memory controller. In this process, a contiguous block is formed for each memory type (Flash, SDRAM, SRAM), even when it consists of several physical memory banks.

U-Boot is installed in the first 128 kB of the first Flash bank (on TQM8xxL modules this is the range 0x40000000 ... 0x4001FFFF). After booting and sizing and initializing DRAM, the code relocates itself to the upper end of DRAM. Immediately below the U-Boot code some memory is reserved for use by malloc() [see CONFIG_SYS_MALLOC_LEN configuration setting]. Below that, a structure with global Board Info data is placed, followed by the stack (growing downward).

Additionally, some exception handler code is copied to the low 8 kB of DRAM (0x00000000 ... 0x00001FFF).

So a typical memory configuration with 16 MB of DRAM could look like this:

0x0000 0000 Exception Vector code
      :
0x0000 1FFF
0x0000 2000 Free for Application Use
      :
      :

      :
      :
0x00FB FF20 Monitor Stack (Growing downward)
0x00FB FFAC Board Info Data and permanent copy of global data
0x00FC 0000 Malloc Arena
      :
0x00FD FFFF
0x00FE 0000 RAM Copy of Monitor Code
...     eventually: LCD or video framebuffer
...     eventually: pRAM (Protected RAM - unchanged by reset)
0x00FF FFFF [End of RAM]

System Initialization:

In the reset configuration, U-Boot starts at the reset entry point (on most PowerPC systems at address 0x00000100). Because of the reset configuration for CS0# this is a mirror of the on board Flash memory. To be able to re-map memory U-Boot then jumps to its link address. To be able to implement the initialization code in C, a (small!) initial stack is set up in the internal Dual Ported RAM (in case CPUs which provide such a feature like), or in a locked part of the data cache. After that, U-Boot initializes the CPU core, the caches and the SIU.

Next, all (potentially) available memory banks are mapped using a preliminary mapping. For example, we put them on 512 MB boundaries (multiples of 0x20000000: SDRAM on 0x00000000 and 0x20000000, Flash on 0x40000000 and 0x60000000, SRAM on 0x80000000). Then UPM A is programmed for SDRAM access. Using the temporary configuration, a simple memory test is run that determines the size of the SDRAM banks.

When there is more than one SDRAM bank, and the banks are of different size, the largest is mapped first. For equal size, the first bank (CS2#) is mapped first. The first mapping is always for address 0x00000000, with any additional banks following immediately to create contiguous memory starting from 0.

Then, the monitor installs itself at the upper end of the SDRAM area and allocates memory for use by malloc() and for the global Board Info data; also, the exception vector code is copied to the low RAM pages, and the final stack is set up.

Only after this relocation will you have a "normal" C environment; until that you are restricted in several ways, mostly because you are running from ROM, and because the code will have to be relocated to a new address in RAM.

U-Boot Porting Guide:

[Based on messages by Jerry Van Baren in the U-Boot-Users mailing list, October 2002]

int main(int argc, char *argv[]) { sighandler_t no_more_time;

signal(SIGALRM, no_more_time);
alarm(PROJECT_DEADLINE - toSec (3 * WEEK));

if (available_money > available_manpower) {
    Pay consultant to port U-Boot;
    return 0;
}

Download latest U-Boot source;

Subscribe to u-boot mailing list;

if (clueless)
    email("Hi, I am new to U-Boot, how do I get started?");

while (learning) {
    Read the README file in the top level directory;
    Read http://www.denx.de/twiki/bin/view/DULG/Manual;
    Read applicable doc/*.README;
    Read the source, Luke;
    /* find . -name "*.[chS]" | xargs grep -i <keyword> */
}

if (available_money > toLocalCurrency ($2500))
    Buy a BDI3000;
else
    Add a lot of aggravation and time;

if (a similar board exists) {   /* hopefully... */
    cp -a board/<similar> board/<myboard>
    cp include/configs/<similar>.h include/configs/<myboard>.h
} else {
    Create your own board support subdirectory;
    Create your own board include/configs/<myboard>.h file;
}
Edit new board/<myboard> files
Edit new include/configs/<myboard>.h

while (!accepted) {
    while (!running) {
        do {
            Add / modify source code;
        } until (compiles);
        Debug;
        if (clueless)
            email("Hi, I am having problems...");
    }
    Send patch file to the U-Boot email list;
    if (reasonable critiques)
        Incorporate improvements from email list code review;
    else
        Defend code as written;
}

return 0;

}

void no_more_time (int sig) { hire_a_guru(); }

Coding Standards:

All contributions to U-Boot should conform to the Linux kernel coding style; see the kernel coding style guide at https://www.kernel.org/doc/html/latest/process/coding-style.html, and the script "scripts/Lindent" in your Linux kernel source directory.

Source files originating from a different project (for example the MTD subsystem) are generally exempt from these guidelines and are not reformatted to ease subsequent migration to newer versions of those sources.

Please note that U-Boot is implemented in C (and to some small parts in Assembler); no C++ is used, so please do not use C++ style comments (//) in your code.

Please also stick to the following formatting rules:

• remove any trailing white space
• use TAB characters for indentation and vertical alignment, not spaces
• make sure NOT to use DOS '\r\n' line feeds
• do not add more than 2 consecutive empty lines to source files
• do not add trailing empty lines to source files

Submissions which do not conform to the standards may be returned with a request to reformat the changes.

Submitting Patches:

Since the number of patches for U-Boot is growing, we need to establish some rules. Submissions which do not conform to these rules may be rejected, even when they contain important and valuable stuff.

Please see http://www.denx.de/wiki/U-Boot/Patches for details.

Patches shall be sent to the u-boot mailing list u-boot@lists.denx.de; see https://lists.denx.de/listinfo/u-boot

When you send a patch, please include the following information with it:

• For bug fixes: a description of the bug and how your patch fixes this bug. Please try to include a way of demonstrating that the patch actually fixes something.

• For new features: a description of the feature and your implementation.

• A CHANGELOG entry as plaintext (separate from the patch)

• For major contributions, add a MAINTAINERS file with your information and associated file and directory references.

• When you add support for a new board, don't forget to add a maintainer e-mail address to the boards.cfg file, too.

• If your patch adds new configuration options, don't forget to document these in the README file.

• The patch itself. If you are using git (which is strongly recommended) you can easily generate the patch using the "git format-patch". If you then use "git send-email" to send it to the U-Boot mailing list, you will avoid most of the common problems with some other mail clients.

  If you cannot use git, use "diff -purN OLD NEW". If your version of diff does not support these options, then get the latest version of GNU diff.

  The current directory when running this command shall be the parent directory of the U-Boot source tree (i. e. please make sure that your patch includes sufficient directory information for the affected files).

  We prefer patches as plain text. MIME attachments are discouraged, and compressed attachments must not be used.

• If one logical set of modifications affects or creates several files, all these changes shall be submitted in a SINGLE patch file.

• Changesets that contain different, unrelated modifications shall be submitted as SEPARATE patches, one patch per changeset.

Notes:

• Before sending the patch, run the buildman script on your patched source tree and make sure that no errors or warnings are reported for any of the boards.

• Keep your modifications to the necessary minimum: A patch containing several unrelated changes or arbitrary reformats will be returned with a request to re-formatting / split it.

• If you modify existing code, make sure that your new code does not add to the memory footprint of the code ;-) Small is beautiful! When adding new features, these should compile conditionally only (using #ifdef), and the resulting code with the new feature disabled must not need more memory than the old code without your modification.

• Remember that there is a size limit of 100 kB per message on the u-boot mailing list. Bigger patches will be moderated. If they are reasonable and not too big, they will be acknowledged. But patches bigger than the size limit should be avoided.