So, this might be due to differing radeon_surface layouts based on the RADEON_SURF_SCANOUT flag.
tvoss can apparently reproduce this, and it goes away when he *doesn't* add the SCANOUT flag to the gbm bo in Mir¹. XMir never sets RADEON_SURF_SCANOUT, so it's plausible that this is the issue.
¹: Luckily, it appears that this buffer still happens to be scanoutable ☺
So, this might be due to differing radeon_surface layouts based on the RADEON_SURF_SCANOUT flag.
tvoss can apparently reproduce this, and it goes away when he *doesn't* add the SCANOUT flag to the gbm bo in Mir¹. XMir never sets RADEON_ SURF_SCANOUT, so it's plausible that this is the issue.
¹: Luckily, it appears that this buffer still happens to be scanoutable ☺