(In reply to Michel Dänzer from comment #4)
> We need to find out why the kernel starts returning -EINVAL from the page
> flip and wait for vblank ioctls.
> 
> Can you try writing 255 to /sys/module/drm/parameters/debug before
> reproducing the problem, and attach the resulting dmesg output? (Note that
> writing non-0 to /sys/module/drm/parameters/debug will cause a lot of
> debugging output to be generated, so you'll want to write 0 to it again soon
> afterwards)

Got it. I'll attach dmesg as soon as I reproduce the issue.

> In case that doesn't reveal why -EINVAL is being returned, are you able to
> recompile the drm.ko / amdgpu.ko kernel modules with a patch applied?

Sure. Gentoo is there :)