MCP2210 driver for Linux

This is a Linux device driver for the Microchip mcp2210 USB-to-SPI bridge. It currently builds as an out-of-tree module and includes a userland utility for testing, configuration, control and spi messaging via spidev. (There are other userland utilities for doing spi through spidev).

This is a USB interface driver, NOT a usbhid driver! The MCP2210 isn't a device that interfaces with humans, and they only choose the HID interface to avoid the hassle of writing and maintaining drivers for every other platform. However, it means that you use this whole layer of software, protocols, drivers, etc that you don't really need. Therefore, this driver skips that layer and communicates directly with the device via standard interrupt URBs (not hid reports).

Unlike other libraries, this driver connects the SPI devices directly to the spi sub-system, allowing you to choose whatever spi protocol driver you wish, with the default being spidev (which allows you to interact with the SPI device from userspace).

RTFM

God loves people who rtfm. It's fun, healthy and besides, everybody's doing it.

• MCP2210 Product page
• MCP2210 Datasheet
• spidev

Configuring, Building and Installing

If you have your kernel sources installed and all is right with the world, building and installing (with a default configuration) is as simple as:

make all && make modules_install

To explicitly say where your sources are, use:

KERNELDIR=/usr/scr/abcdefg make

This is an out-of-tree module, so if you don't fancy the default configuration, run make out-of-tree-autoconf.h to have it generated from the template and edit to your needs. You can refer to Kconfig for descriptions of the various config options, but there is currently no mechanism to use the Kconfig for a make menuconfig; the Kconfig exists primarily for mainline integration.

Note that at this time, there is no install target for the userland utilities and to run mcp2210-util, you must explicitly specify an LD_LIBRARY_PATH. Example:

/home/daniel/proj/mcp2210$ make -j4
.... make output.....
/home/daniel/proj/mcp2210$ LD_LIBRARY_PATH=`pwd`/user user/mcp2210-util --help

Cross-Compiling

I'm not going to tell you how to setup a cross-compiling toolchain, but here is how I build for my Raspberry Pi. Please note that the USB drivers for the Pi are currently problematic, especially prior the commit "USB fix using a FIQ to implement split transactions", which has greatly improved things. So the below MCP2210_QUIRKS=1 cpp variable should be omitted if you aren't using a pi.

#!/bin/bash

export KERNELDIR=/home/daniel/proj/kernel/rpi.3.10
export ARCH=arm
export CROSS_COMPILE=/usr/bin/armv6j-hardfloat-linux-gnueabi-
export CPPFLAGS="-DMCP2210_QUIRKS=1"

CFLAGS="-O2 -g3 -pipe -march=armv6j -mfpu=vfp -mfloat-abi=hard" make -j4 -C user &&
make -j4 "$@" &&
make mcp2210.s &&
scp -p mcp2210.ko user/libmcp2210.so user/mcp2210 root@pi:bin/

At the very least, your KERNELDIR should have a valid .config and you should run make modules_prepare. You'll need to enable the following in your .config (either modules or built-ins will do):

• CONFIG_SPI - if you want to use the MCP2210 as an spi master
• CONFIG_SPI_SPIDEV - if you want to use the kernel's handy-dandy spidev driver and the userspace utility's spi functionality. If you will only be using spi protocol drivers specific to your peripherals, then you do not need this option.
• CONFIG_GPIOLIB - if you want to use the gpio interface or use gpios for chip selects (as opposed to Microchip's mechanism, which is faster if it serves your needs)

You don't need to make mcp2210.s if you don't care to examine the disassembly. The CFLAGS supplied when building the userspace program aren't inferred if not supplied (although it really doesn't contain any floating point calculations at the moment). I use -j4 because I have a quad core processor, tune to your preferences.

Installing the Driver

Aside from the usual modprobe/insmod, this part is unfortunately a pain in the ass. This is because the hid-generic driver uses these values to select its devices:

static const struct hid_device_id hid_table[] = {
    { HID_DEVICE(HID_BUS_ANY, HID_GROUP_GENERIC, HID_ANY_ID, HID_ANY_ID) },
    { }
};

This prevents the mcp2210 driver from being selected as a candidate, even if the vid/pid explicitly match. Currently, the work-around is to run the script user/mcp2210_bind.sh as root (you must have sysfs mounted). This uses sysfs files to tell the usbhid driver to unbind from the mcp2210 device so that the mcp2210 driver can probe it. If you know of a way to do this via udev rules, please notify me! (just create an issue via the issue tracker).

Configuration & Setup

As you may be aware, SPI does not offer a mechanism to automatically detect and configure peripherals. Typically, a particular chip is hard-wired to an SPI master and the driver for that master knows whats conntect to it. In Linux, SPI slave devices are configred via struct spi_board_info objects. But for USB-to-SPI protocol bridges like the MCP2210, we can't know how its board is wired -- we need that information to come from somewhere else.

The MCP2210 has an embedded EEPROM (most likely 512 bytes in size), part of which it uses for its own internal settings and 256 bytes which it exposes as "user-EEPROM". In its internal settings (3.1.1 through 3.1.11 in the datasheet), the MCP2210 stores its inital "power up" values for all parameters:

• Mode of each pin (GPIO, SPI or dedicated)
• Direction and value (for outputs) of each GPIO
• The idle and active values of all chip select lines (only affects pins configured as SPI)
• SPI mode, bitrate, and timing parameters
• Some access control settings (for changing the persistent settings)
• USB power control: self or host powered, requested power
• USB vid, pid, mfg name, product name, etc.

While this is a pleasntly comprehensive feature set, it is still not enough to fully configure & probe any board wiring because:

1. We only have transfer settings for a single SPI device.
2. We still aren't telling the USB host what chip is connected to each chip select.

The mcp2210 driver offers two separate mechanisms to provide that information and configure the device: one from userland and an auto-configuration mechanism that utilizes the user area of the on-chip EEPROM.

Configuring from Userland (Manual)

When ioctl control is enabled (CONFIG_MCP2210_IOCTL), the mcp2210 driver will create a /dev node upon a successful probe with the name usb2spi_bridge<x> (where x is a number) which can be used to interact with the driver. Userspace configuration is fairly straight-forward:

user/mcp2210-util set config <mask>

For mask, you OR together the values for the configuration option(s) you want to set:

"But where the hell do you specify the settings!?" I hear you ask. Yeah, well currently, you have to modify user/settings.h, which is auto-generated from user/settings-example.h the first time you run make and recompile. (If you don't like this, then please write an xml / json / something interface and send me a patch or pull request!) This is pretty straight forward, consisting of simple static const structs initialized with named (C99) initializers.

WARNING: Modifying nvram_access_control can permanently lock you chip! Don't change it unless you really mean to do this and understand what you're doing.

Automatic Configuration

When the driver probes, it queries the device for (among other things) its power-up chip settings (section 3.1.1) and the power-up spi transfer settings (section 3.1.2). If Creek support is enabled (via CONFIG_MCP2210_CREEK), the first four bytes of the user EEPROM area are also read. If these match a "magic number", then the remainder of the user-EEPROM is read and decoded into a struct mcp2210_board_config object which contains all of the information (timings for each SPI device, name of the spi protocol driver, label, etc.) to allow probing the spi_master.

For details on the encoding format, see the comments in mcp2210-creek.h (see also creek_encode() and creek_decode())

NOTICE: At this time, the Creek binary format is unstable and subject to change without a version bump.

Storing and Viewing Auto-Configure Data

All support for creating this encoding from your struct mcp2210_board_config in user/settings.h is in the userspace utility program.

1. Edit user/settings.h to your needs and recomple mcp2210-util
2. Run the following command to encode to config.dat:

mcp2210-util encode > config.dat

3. ls -l config.dat to determine its size (yes, the utility should eventually do this for you)
4. Run the following command to store it to the user-EEPROM (replace size with the size of the file)

mcp2210-util eeprom write size=<size> addr=0 < config.dat

Decoding is the inverse. However, the creek data doesn't include any data available in the "chip settings" (because this would be a redundant waste of limited space) so your chip settings in settings.h (or at least the pin modes) must match what was used when the encoding was generated.

1. Make sure my_chip_settings in user/settings.h is correct.
2. Run the following command:

mcp2210-util decode < config.dat