drm/i915: Clear the vblank status bit before polling for the next vblank

The vblank status bit is a sticky bit that must be cleared with a write
of '1' prior to polling for the next vblank.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>

jbarnes: I'd still rather see a lock, but I think you're right that
we don't generally wait in code that needs not to miss an interrupt.
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/gpu/drm/i915/intel_display.c

@@ -990,6 +990,22 @@ void intel_wait_for_vblank(struct drm_device *dev, int pipe)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	int pipestat_reg = (pipe == 0 ? PIPEASTAT : PIPEBSTAT);
 
+	/* Clear existing vblank status. Note this will clear any other
+	 * sticky status fields as well.
+	 *
+	 * This races with i915_driver_irq_handler() with the result
+	 * that either function could miss a vblank event.  Here it is not
+	 * fatal, as we will either wait upon the next vblank interrupt or
+	 * timeout.  Generally speaking intel_wait_for_vblank() is only
+	 * called during modeset at which time the GPU should be idle and
+	 * should *not* be performing page flips and thus not waiting on
+	 * vblanks...
+	 * Currently, the result of us stealing a vblank from the irq
+	 * handler is that a single frame will be skipped during swapbuffers.
+	 */
+	I915_WRITE(pipestat_reg,
+		   I915_READ(pipestat_reg) | PIPE_VBLANK_INTERRUPT_STATUS);
+
 	/* Wait for vblank interrupt bit to set */
 	if (wait_for((I915_READ(pipestat_reg) &
 		      PIPE_VBLANK_INTERRUPT_STATUS),