Re: framebuffer corruption due to overlapping stp instructions on arm64

[Ard Biesheuvel]

On 6 August 2018 at 12:42, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
>
> On Mon, 6 Aug 2018, Ard Biesheuvel wrote:
>
>> On 6 August 2018 at 12:31, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>> >
>> >
>> > On Mon, 6 Aug 2018, Ard Biesheuvel wrote:
>> >
>> >> On 6 August 2018 at 10:02, Mikulas Patocka <mpatocka@xxxxxxxxxx> 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?
>
> I don't know - there are not any other easily available PCIe ARM boards
> except for Armada 8040.
>

... indeed, and sadly, the ones that are available all have this
horrible Synopsys DesignWare PCIe IP that does not implement a true
root complex at all, but is simply repurposed endpoint IP with some
tweaks so it vaguely resembles a root complex.

But this is exactly why I am asking: I use a AMD Seattle Overdrive as
my main Linux development system, and it runs the gnome-shell stack
flawlessly (using the nouveau driver), as well as a UEFI framebuffer
using efifb. So my suspicion is that this is either a Synopsys IP
issue or an interconnect issue, and has nothing to do with the
impedance mismatch between AMBA and PCIe.