Re: framebuffer corruption due to overlapping stp instructions on arm64

[Ard Biesheuvel <ard dot biesheuvel at linaro dot org>]

On 6 August 2018 at 12:31, Mikulas Patocka <mpatocka@redhat.com> wrote:
>
>
> On Mon, 6 Aug 2018, Ard Biesheuvel wrote:
>
>> On 6 August 2018 at 10:02, Mikulas Patocka <mpatocka@redhat.com> wrote:
>> >
>> >
>> > On Sun, 5 Aug 2018, Florian Weimer wrote:
>> >
>> >> On 08/04/2018 01:04 PM, Mikulas Patocka wrote:
>> >> > There's plenty of memcpy's in the graphics stack. No one will be rewriting
>> >> > all the graphics drivers because of tiny market share that ARM has in
>> >> > desktop computers. So if you refuse to fix things and blame everyone else,
>> >> > you can as well announce that you don't want to have PCIe graphics on ARM
>> >> > at all.
>> >>
>> >> The POWER toolchain maintainers said pretty much the same thing not too
>> >> long ago. I wonder how many architectures need to fail until the
>> >> graphics stack is finally fixed.
>> >>
>> >> Thanks,
>> >> Florian
>> >
>> > If you say that your architecture doesn't support unaligned accesses at
>> > all, there's no problem - the compiler won't generate them and the libc
>> > won't contain them.
>> >
>> > But if you say that your architecture supports unaligned accesses except
>> > for the framebuffer, then you have a problem - the compiler can't know
>> > which pointers point to the framebuffer and libc can't know either - you
>> > caused this problem by your architectural decision.
>> >
>> > You can use 'volatile' to suppress memory optimizations, but it's
>> > impossible to go through the whole Linux graphics stack and add volatile
>> > to every pointer that may point to videoram. Even if you succeesed, new
>> > videoram accesses without volatile will appear after a year of
>> > development.
>> >
>> > See for example the macros READ_ONCE and WRITE_ONCE in Linux kernel - they
>> > should be used when there's concurrent access to the particular variable,
>> > but mainstream architectures don't require them, so many kernel developers
>> > are omitting them in their code.
>> >
>> > If you are building a supercomputer with a particular GPU, you can force
>> > the GPU vendor to provide POWER-compliant drivers. If you are building a
>> > workstation where the user can plug any GPU, forcing developers will go
>> > nowhere. You have to emulate the unaligned accesses and make sure that the
>> > next versions of your architecture support them in hardware.
>> >
>>
>> I have the feeling this discussion is going off the rails again.
>>
>> The original report is about corruption when doing overlapping writes.
>> Matt Sealey said you cannot have PCI outbound windows with memory
>> semantics on ARM, and so you should be using device mappings (which do
>> not tolerate unaligned accesses)
>>
>> In this context, 'device mapping' does not mean 'any non-DRAM region',
>> but it refers to a particular type of MMU mapping attribute defined by
>> the ARM architecture.
>>
>> I think we can all agree that memcpy() should be usable on any region
>> of memory that has true memory semantics, even if it is backed by VRAM
>> on a graphics card.
>>
>> The question is if PCIe can provide such regions on ARM.
>
> I think there are three possible solutions:
>
> 1. provide an alternative memcpy implementation that doesn't do unaligned
> accesses and recompile the graphics software with -mstrict-align
>
> 2. map the PCI BAR as device memory and emulate the unaligned instructions
>
> 3. find some hardware workaround that could insert delays between the PCIe
> accesses (but the hardware engineers need to cooperate on this instead of
> asserting that they refuse tu support it)
>

Are we talking about a quirk for the Armada 8040 or about PCIe on ARM
in general?

If the latter, I still haven't seen an explanation why the particulars
of AMBA justify overlapped writes being dropped at will by the
interconnect.