[Intel-gfx] [PATCH] drm/i915: Eliminate race from gen2/3 page flip interrupt handling

[Chris Wilson]

On Mon, Feb 18, 2013 at 01:57:51PM +0200, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> If the interrupt handler were to process a previous vblank interrupt and
> the following flip pending interrupt at the same time, the page flip
> would be complete too soon.
> 
> To eliminate this race check the live pending flip status from the ISR
> register before finishing the page flip.

Ok, that makes a lot of sense.

/* We detect FlipDone by looking for the change in PendingFlip from '1'
 * to '0' on the following vblank, i.e. IIR has the Pendingflip
 * asserted following the MI_DISPLAY_FLIP, but ISR is deasserted, hence
 * the flip is completed (no longer pending). Since this doesn't raise an
 * interrupt per-se, we watch for the change at vblank.
 */
 
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
Reviewed-by: Chris Wilson <chris at chris-wils

Time to give it a quick test and make sure it doesn't break a ton of
assumptions... :)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre