Re: [PATCH 5/5] drm/i915: Allow vblank interrupts during modeset and eliminate some vblank races

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Feb 21, 2014 at 09:03:35PM +0200, ville.syrjala@linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> Tell the drm core vblank code to reject drm_vblank_get()s only between
> drm_vblank_off() and drm_vblank_on() calls, and sprinkle the appropriate
> drm_vblank_on() calls to the .crtc_enable() hooks. At this time I kept
> the off calls in their current position, and added the on calls to the
> end of .crtc_enable(). Later on these will be moved inwards a bit to
> allow vblank interrupts during plane enable/disable steps.
> 
> We can kill of the drm_vblank_{pre,post}_modeset() calls since those are
> there simply to make drm_vblank_get() fail during a modeset. The way
> they do it is by grabbing a vblank reference, and after drm_vblank_off()
> gets called this will results in drm_vblank_get() failing due to the
> elevated refcount while vblank interrupts are disabled. Unfortunately
> this means there's no point during modeset where the behaviour can be
> restored back to the normal state until the vblank refcount drops to 0.
> There's no gurantee of that happening even after the modeset has
> completed, so simply dropping the drm_vblank_{pre,post}_modeset() calls
> is the best option. The new reject mechanism will take care of things
> in a much more consistent and race free manner.
> 
> Testcase: igt/kms_flip/{dpms,modeset}-vs-vblank-race

QA hit the new tests and filed a bug.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75593

<snip>

-- 
Ville Syrjälä
Intel OTC