drm/i915: Eliminate race from gen2/3 page flip interrupt handling

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 completed too soon.

To eliminate this race, check the live pending flip status from the ISR
register before finishing the page flip.

v2: Added a comment explaining the logic (by Chris Wilson)
v3: Fix a typo in the comment
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -2284,19 +2284,25 @@ static irqreturn_t i8xx_irq_handler(int irq, void *arg)
 		    drm_handle_vblank(dev, 0)) {
 			if (iir & I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT) {
 				intel_prepare_page_flip(dev, 0);
+
+				if ((I915_READ16(ISR) & I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT) == 0) {
 					intel_finish_page_flip(dev, 0);
 					flip_mask &= ~I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT;
 				}
 			}
+		}
 
 		if (pipe_stats[1] & PIPE_VBLANK_INTERRUPT_STATUS &&
 		    drm_handle_vblank(dev, 1)) {
 			if (iir & I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT) {
 				intel_prepare_page_flip(dev, 1);
+
+				if ((I915_READ16(ISR) & I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT) == 0) {
 					intel_finish_page_flip(dev, 1);
 					flip_mask &= ~I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT;
 				}
 			}
+		}
 
 		iir = new_iir;
 	}
@@ -2491,10 +2497,19 @@ static irqreturn_t i915_irq_handler(int irq, void *arg)
 			    drm_handle_vblank(dev, pipe)) {
 				if (iir & flip[plane]) {
 					intel_prepare_page_flip(dev, plane);
+
+					/* 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.
+					 */
+					if ((I915_READ(ISR) & flip[plane]) == 0) {
 						intel_finish_page_flip(dev, pipe);
 						flip_mask &= ~flip[plane];
 					}
 				}
+			}
 
 			if (pipe_stats[pipe] & PIPE_LEGACY_BLC_EVENT_STATUS)
 				blc_event = true;