Re: framebuffer corruption due to overlapping stp instructions on arm64

Richard Earnshaw (lists)
2018-08-03T14:17:42Z

On 03/08/18 14:31, Mikulas Patocka wrote:
>
>
> On Fri, 3 Aug 2018, Andrew Pinski wrote:
>
>> On Thu, Aug 2, 2018 at 12:31 PM Mikulas Patocka <mpatocka@redhat.com> wrote:
>>>
>>> Hi
>>>
>>> I tried to use a PCIe graphics card on the MacchiatoBIN board and I hit a
>>> strange problem.
>>>
>>> When I use the links browser in graphics mode on the framebuffer, I get
>>> occasional pixel corruption. Links does memcpy, memset and 4-byte writes
>>> on the framebuffer - nothing else.
>>>
>>> I found out that the pixel corruption is caused by overlapping unaligned
>>> stp instructions inside memcpy. In order to avoid branching, the arm64
>>> memcpy implementation may write the same destination twice with different
>>> alignment. If I put "dmb sy" between the overlapping stp instructions, the
>>> pixel corruption goes away.
>>>
>>> This seems like a hardware bug. Is it a known errata? Do you have any
>>> workarounds for it?
>>
>> Yes fix Links not to use memcpy on the framebuffer.
>> It is undefined behavior to use device memory with memcpy.
>>
>> Thanks,
>> Andrew Pinski
>
> Links can be fixed easily - but there is exterme amount of code that
> accesses videoram via C pointers in the Xserver and in the GPU drivers.
> How do you intend to fix that?
>
> What should we use instead of direct access or memcpy? Libc doesn't
> provide any macros or functions for framebuffer access. Using hardcoded
> assembler doesn't make the the programs portable.
>
> Mikulas
>

Dialing back the optimization levels when building the Xserver so the
compilers plays by its rules is one thing. Dialing back the
optimizations in the C library to handle a non-conforming program is
quite another. That affects every program on the system, even if it
turns out to be a server with no graphics system.

R.