[PATCH v2] drm/i915: Eliminate race from gen2/3 page flip interrupt handling

[PATCH v2] drm/i915: Eliminate race from gen2/3 page flip interrupt handling

[ville.syrjala]

From: Ville Syrjälä <ville.syrjala@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 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)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_irq.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 9fde49a..2aea737 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -2284,8 +2284,11 @@ 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);
-				intel_finish_page_flip(dev, 0);
-				flip_mask &= ~I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT;
+
+				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;
+				}
 			}
 		}
 
@@ -2293,8 +2296,11 @@ static irqreturn_t i8xx_irq_handler(int irq, void *arg)
 		    drm_handle_vblank(dev, 1)) {
 			if (iir & I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT) {
 				intel_prepare_page_flip(dev, 1);
-				intel_finish_page_flip(dev, 1);
-				flip_mask &= ~I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT;
+
+				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;
+				}
 			}
 		}
 
@@ -2491,8 +2497,17 @@ 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);
-					intel_finish_page_flip(dev, pipe);
-					flip_mask &= ~flip[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];
+					}
 				}
 			}
 
-- 
1.7.12.4

_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
http://lists.freedesktop.org/mailman/listinfo/intel-gfx