Framebuffer not at expected address, messing up memory map

[hoglet67]

On a 512MB Pi, the framebuffer is at a bus address of: 0x5EB64000

Which is an ARM address of: 0x1EB64000

cpu_mem is set to 16 (MB)

I would have thought this meant the top 16MB was used by the GPU (0x1F000000-0x1FFFFFFF on a 512MB machine), and that the frame buffer would be created within this memory.

(The FB should be 640x480x2 bytes = 0x307000)

This is going to mess up our memory map on a 256MB machine.

Setting gpu_mem to 16, 32 or 64 doesn't affect this.

Setting total_mem=384 changes it to 0x56B64000 / 0x16B64000 (which is expected).

Our memory map is currently:

# Memory map to stay compatible with 256MB Pi Models
#
# 0x00000000   0MB - Start of RAM
# 0xD8000000 216MB - ARM Basic (allow 4MB)
# 0x0DC00000 220MB - Stack space (allow 16 x 1MB)
# 0x0EC00000 236MB - Pi Kernel (allow 4MB)
# 0x0F000000 240MB - Top of cached RAM, start of GPU Memory
# 0x10000000 256MB - Top of physical RAM on 256MB Pi Model

So on a 256MB machine the Frame Buffer will end up at 0x0EB64000, which will collide with the stack space (!!)

Tried, by setting total_mem=256 and the system fails to boot.

[hoglet67]

Command line reported by the Pi say:

           CMD_LINE : bcm2708_fb.fbwidth=1600 bcm2708_fb.fbheight=1200 bcm2708_fb.fbswap=1 dma.dmachans=0x7f35 bcm2708.boardrev=0x900093 bcm2708.serial=0x11807111 bcm2708.uart_clock=48000000 bcm2708.disk_led_gpio=47 smsc95xx.macaddr=B8:27:EB:80:71:11 vc_mem.mem_base=0x1ec00000 vc_mem.mem_size=0x20000000  copro=0 copro1_speed=3 copro3_speed=4 tube_delay=0 elk_mode=0

It looks like the frame buffer is being positioned just below vc_mem.mem_base=0x1ec00000

Not sure whether this is cause or effect.

[hoglet67]

Reduced the size of the stacks from 1MB to 0.5MB.

# Memory map to stay compatible with 256MB Pi Models
#
# 0x00000000   0MB - Start of RAM
# 0x0D800000 216MB - ARM Basic (allow 4MB)
# 0x0DC00000 220MB - Stack space (allow 16 x 0.5MB)
# 0x0E400000 228MB - Pi Kernel (allow 4MB)
# 0x0E800000 232MB - Frame Buffer (allow 8MB)
# 0x0E800000 232MB - Top of cached RAM
# 0x0F000000 240MB - Start of GPU Memory
# 0x10000000 256MB - Top of physical RAM on 256MB Pi Model