Re: framebuffer corruption due to overlapping stp instructions on arm64

Ramana Radhakrishnan
2018-08-06T08:04:37Z

On Sun, Aug 5, 2018 at 10:36 PM, Pavel Machek <pavel@ucw.cz> 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.
>
> No, I don't think so. Why do you think so?

It is undefined behaviour in the architecture.

Ramana