Genode on RPI

[Tomasz Gajewski]

Hi,

I'll try to quickly move discussion about rpi to new thread to not
pollute thread about Roadmap 2019.


Bob Stewart <robjsstewart at gmail.com> writes:

> I'm working on implementing a base-hw kernel on a Cortex A53 platform
> (not Rpi) which is what the Rpi 3x is based on. So, that means a
> significant amount of work to convert from the existing Genode code
> supporting the armv7 architecture to support the armv8
> architecture. I'm focused initially on only the aarch64 instruction
> set, but will later add support for aarch32. This is a retirement
> project for me so my progress is not driven by any great need and is
> slow. After about six months of casual working I'm at the stage of
> debugging a virtual memory translation table enhancements need to
> support 64-bit memory (a new Level-0 table in the page table code in
> lpae.h). The means all the assembly code changes required have been
> made (and about 12 other areas of change) and I clean compile a from a
> log run script.

My plans did not include targetting 64 bit implementation as seems to be
too big target for amount of time I can devote to it. I just wanted to
fix/workaround some problems which I expected to find when dealing with
device drivers - not earlier. And only trying to use code available in
Genode already (so 32bit for ARMv7). If I correctly understand 32bit
instruction set for ARMv8 should be compatible with ARMv7. Am I wrong
with this assumption?

> For debugging at this level, you really only have two choices: Either
> delve into the ARM hardware debugging facilites (CoreSight) or insert
> small pieces of code to poke variable values or a trace number into a
> currently unused region of memory. Code like:
>
>             using ull = unsigned long long;
>             ull volatile * debug_ptr = (ull *)0x91000600;
>             *debug_ptr = MAX_ENTRIES;
>             debug_ptr += 0x01;
>             *debug_ptr = BLOCK_SIZE_LOG2;
>             debug_ptr += 0x01;
>
> if your booting into uboot to start up, you can inspect the memory
> region through uboots' md command. Breakpoints can be implemented with
> the assembly code line:  asm volatile("hlt #01\n"); . Objdump gives
> you the assembly code listing if you need it.

Do I correctly understand, that I can go back to u-boot after running
Genode with 'hlt #01' and analyze content of memory there? I would be
surprised but definitely it would be useful.

> I sincerely hope Genode does not implement a version of the Linux
> DT. It is an error prone approach developed many years ago in the
> embedded world. There are more elegant, syntax checkable,
> configuration description approaches available today.

Could you give some hints about those alternatives? I have no experience
in this area and what seems to be tempting in device tree is that when
you look into linux sources there are already dts files for all flavours
of RPI including mine. U-boot uses this too - I wanted to use source
from this due to smaller amount of code but that probably doesn't matter
much.

> Let me know if I can help with any compile/link issues you have, I
> probably came across most of them in my work to-date. I'm using a
> Linaro tool chain for aarch64. There is a separate one for aarch32
> which I don't currently use.



Currently my "problem" is that my RPI doesn't pass through marked line
in crt0.s

    /*****************************************************
     ** Setup multiprocessor-aware kernel stack-pointer **
     *****************************************************/

    mov sp, #0                  /* for boot cpu use id 0    */
--> cps #31                     /* change to system mode    */

    .global _start_setup_stack  /* entrypoint for all cpus */
    _start_setup_stack:


I think it is due to RPI firmware is leaving processor in HYP mode - not
SVC. But currently this is all I know.

I tried to put code from FOC that is used switch processor back to SVC
mode but as I don't understand details of it yet something is not
working. There is some similar code in Genode implementation for Arndale
board.

On the other hand - maybe running in HYP mode is what is better for
Genode? Some hints?



If I comment out section with 'cps #31' it goes further and I it does
not return from 'Bootstrap::Platform & p = Bootstrap::platform();' in
'extern "C" void init()' and last place which I found that is executing
is constructor: Bootstrap::Platform::Board::Board in
'repos/base-hw/src/bootstrap/spec/rpi/platform.cc'

I'll try to find out more and ask more specific questions about this
part.


Tomasz Gajewski