Bug 91831

I'm not entirely sure if this is the right place to mention this issue, but I'm fairly confident that the right people will see it here:

I'm using a Kabini GPU on a 32-bit (kernel + userland) system using linux 4.2 and have just decided to give the amdgpu kernel drm driver and xf86-video-amdgpu a try (previously I was using radeon). What I noticed is that screen output in X11 is garbled using these drivers - i.e. pixels are messed up / missing / corrupt in various places all over the screen.

I've seen this issue before in the radeon driver, there's actually a (resolved) bug report about it at https://bugs.freedesktop.org/show_bug.cgi?id=84627.

Back then, the bug was fixed in the kernel in linux/drivers/gpu/drm/radeon/radeon_object.c. Here's the relevant section (from radeon_bo_create()):

#ifdef CONFIG_X86_32
        /* XXX: Write-combined CPU mappings of GTT seem broken on 32-bit
         * See https://bugs.freedesktop.org/show_bug.cgi?id=84627
         */
        bo->flags &= ~RADEON_GEM_GTT_WC;

I'm not into this stuff enough, but I believe that in contrast to the radeon driver, the amdgpu driver does not disable write-combined CPU mappings on 32 bit kernels. The question is: Would it be possible and reasonable to change amdgpu to follow radeon in this regard and thus - hopefully - fix the garbled screen problem on 32 bit?

As a side note, the problem goes away when disabling hardware acceleration:

Option "NoAccel" "true"

Screen output looks good then. Still, this is probably not the best solution.