raspberry pi: Problems with the dynamic work area

[Christian Mauderer]

Hello,

the patch


https://git.rtems.org/rtems/commit/?id=a4d7e4cee77d16b0e34ef543f0804e7eb2954137

introduced some problems. It tries to adapt the work area based on the
size of the available memory. In theory this is a good idea but the
implementation doesn't work. The line

      uint32_t mem = (spec.spec >> (4 + 4 + 8 + 4)) & 0xf;

always returns something > 8 on raspberries newer than the first pi 1
boards. The highest bit in this field indicates that it is a new style
revision code. See
https://www.raspberrypi.org/documentation/hardware/raspberrypi/revision-codes/README.md

So currently for all except for very old boards the code is skipped. On
old boards (like my pi 1) it will be executed. But the memory sizes in
the rpi_mem array are missing a few factors 1024 and therefore the
calculated address is entirely wrong and leads to an exception.

I tried to fix it (see attached patch) but I run into multiple new problems:

- The MMU is configured based on the values from the linker command
file. Therefore if the work area is set to something bigger, an access
violation happens.

- In the linker command file there is a stack, a nocache section and a
nocacheheap after the work section. Also all three are currently zero,
it seems dangerous to ignore them in case they are non-zero in the future.

I could think of multiple possible solutions:

1. Revert that patch entirely (and maybe some code it replaced) and just
live with 256MB on the Pi.

2. Basically 1 but make the size configurable with for example different
linker scripts.

3. Take a really detailed look at the problem and fix all problems
listed above.

I started with three but I would now favor one or two because the more I
look into the Pi hardware the less I like the processor and the
hardware. It's a great platform for running Linux where you don't have
to do anything. But the hardware and processor are both poorly
documented and not well supported by debug tools so that it's not really
a lot of fun to work with it on that level.

Best regards

Christian
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