[PATCH 3/6] drm: Fix drm_vblank_pre/post_modeset regression from Linux 4.4

[Vlastimil Babka]

On 02/08/2016 02:13 AM, Mario Kleiner wrote:
> Changes to drm_update_vblank_count() in Linux 4.4 broke the
> behaviour of the pre/post modeset functions as the new update
> code doesn't deal with hw vblank counter resets inbetween calls
> to drm_vblank_pre_modeset an drm_vblank_post_modeset, as it
> should.
>
> This causes mistreatment of such hw counter resets as counter
> wraparound, and thereby large forward jumps of the software
> vblank counter which in turn cause vblank event dispatching
> and vblank waits to fail/hang --> userspace clients hang.
>
> This symptom was reported on radeon-kms to cause a infinite
> hang of KDE Plasma 5 shell's login procedure, preventing users
> from logging in.
>
> Fix this by detecting when drm_update_vblank_count() is called
> inside a pre->post modeset interval. If so, clamp valid vblank
> increments to the safe values 0 and 1, pretty much restoring
> the update behavior of the old update code of Linux 4.3 and
> earlier. Also reset the last recorded hw vblank count at call
> to drm_vblank_post_modeset() to be safe against hw that after
> modesetting, dpms on etc. only fires its first vblank irq after
> drm_vblank_post_modeset() was already called.
>
> Reported-by: Vlastimil Babka <vbabka at suse.cz>

FWIW, I've applied the whole patchset to 4.4 and the kde5 login problem 
didn't occur. I can test the next version too.

Thanks,
Vlastimil