reverse prime produces corrupted output on specific resolutions

[Aleksander Miera]

The issue has initially been discovered for resolution 1366x768 on a DisplayLink (evdi)-backed device, but applies also to UDL, as well as reverse prime setups (rendering on Intel, output through NVidia).

The issue manifests with 1366x768 and several other resolutions only, due to the fact that the GPU seems to use 64-aligned framebuffer stride. This is "accidentally" fine with the most popular VGA resolutions (e.g. assuming 32bits/4bytes per pixel and FullHD, the stride would be 1920*4 -> 7680, which is divisible by 64 w/o a remainder, yet 1366*4=5464, 5464/64 = 85.375). The proposed fix is to update the stride with the GPU-provided value instead of the Xorg-calculated one at the stage of sharing the pixmap between GPUs. Doing that at earlier stage would probably require distinguishing between allocation of a FB and a generic pixmap, which is the same for Xorg.

Issue for sure was seen on 18.04 and 19.04, probably still there for 20.04.

Upstream report:
https://gitlab.freedesktop.org/xorg/xserver/-/issues/889

And posted (not yet integrated) fix:
https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/496

[Daniel van Vugt (vanvugt)]

Please note that 19.04 is no longer supported. It would be nice to know for sure if 20.04 is still a problem.

[Aleksander Miera (amiera)]

Just reproduced on 20.04.
xserver-xorg-core:
  Installed: 2:1.20.8-2ubuntu2.2
  Candidate: 2:1.20.8-2ubuntu2.2

No reproduction on 20.10 build, but it was tested on a different machine and issue might be hw-dependent. Te be confirmed on the same computer soon.

[Aleksander Miera (amiera)]

Managed to reproduce on fresh download 20.10, too. OTOH, I remember seeing that gone in some cases on 20.04. I wonder what the exact cases are, as apparently this seems more complicated than "reproduces always".

[Aleksander Miera (amiera)]

@vanvugt is there any chance this gets merged? Preferably on Xorg's end, if you can give me a hint on whom to tag there it would be most appreciated.

[Daniel van Vugt (vanvugt)]

It appears the Xorg developers already know about it. I don't know enough about it myself to recommend Ubuntu jumps ahead of them on this.

[Aleksander Miera (amiera)]

Fixed in upstream master
https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/529

[Aleksander Miera (amiera)]

Fixed in upstream 1.20
https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/539