Re: framebuffer corruption due to overlapping stp instructions on arm64

["Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>]

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.