framebuffer corruption due to overlapping stp instructions on arm64

Mikulas Patocka
2018-08-02T19:31:22Z

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?

I tried AMD card (HD 6350) and NVidia (NVS 285) and both exhibit the same
corruption. OpenGL doesn't work (it results in artifacts on the AMD card
and lock-up on the NVidia card), but it's quite expected if even simple
writing to the framebuffer doesn't work.

Mikulas