drm/i915: Remove IMR masking during interrupt handler, and restart it if needed.

The IMR masking was a technique recommended for avoiding getting stuck with
no interrupts generated again in MSI mode.  It kept new IIR bits from getting
set between the IIR read and the IIR write, which would have otherwise
prevented an MSI from ever getting generated again.  However, this caused a
problem for vblank as the IMR mask would keep the pipe event interrupt from
getting reflected in IIR, even after the IMR mask was brought back down.

Instead, just check the state of IIR after we ack the interrupts we're going
to handle, and restart if we didn't get IIR all the way to zero.
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -168,46 +168,42 @@ irqreturn_t i915_driver_irq_handler(DRM_IRQ_ARGS)
 {
 	struct drm_device *dev = (struct drm_device *) arg;
 	drm_i915_private_t *dev_priv = (drm_i915_private_t *) dev->dev_private;
-	u32 iir;
-	u32 pipea_stats = 0, pipeb_stats = 0;
+	u32 iir, new_iir;
+	u32 pipea_stats, pipeb_stats;
 	int vblank = 0;
 	unsigned long irqflags;
 
-	spin_lock_irqsave(&dev_priv->user_irq_lock, irqflags);
 	atomic_inc(&dev_priv->irq_received);
 
-	if (dev->pdev->msi_enabled)
-		I915_WRITE(IMR, ~0);
 	iir = I915_READ(IIR);
 
-	if (iir == 0) {
-		if (dev->pdev->msi_enabled) {
-			I915_WRITE(IMR, dev_priv->irq_mask_reg);
-			(void) I915_READ(IMR);
-		}
-		spin_unlock_irqrestore(&dev_priv->user_irq_lock, irqflags);
+	if (iir == 0)
 		return IRQ_NONE;
-	}
 
+	do {
+		pipea_stats = 0;
+		pipeb_stats = 0;
 		/*
 		 * Clear the PIPE(A|B)STAT regs before the IIR
 		 */
 		if (iir & I915_DISPLAY_PIPE_A_EVENT_INTERRUPT) {
+			spin_lock_irqsave(&dev_priv->user_irq_lock, irqflags);
 			pipea_stats = I915_READ(PIPEASTAT);
 			I915_WRITE(PIPEASTAT, pipea_stats);
+			spin_unlock_irqrestore(&dev_priv->user_irq_lock,
+					       irqflags);
 		}
 
 		if (iir & I915_DISPLAY_PIPE_B_EVENT_INTERRUPT) {
+			spin_lock_irqsave(&dev_priv->user_irq_lock, irqflags);
 			pipeb_stats = I915_READ(PIPEBSTAT);
 			I915_WRITE(PIPEBSTAT, pipeb_stats);
+			spin_unlock_irqrestore(&dev_priv->user_irq_lock,
+					       irqflags);
 		}
 
 		I915_WRITE(IIR, iir);
-	if (dev->pdev->msi_enabled)
-		I915_WRITE(IMR, dev_priv->irq_mask_reg);
-	(void) I915_READ(IIR); /* Flush posted writes */
-
-	spin_unlock_irqrestore(&dev_priv->user_irq_lock, irqflags);
+		new_iir = I915_READ(IIR); /* Flush posted writes */
 
 		if (dev_priv->sarea_priv)
 			dev_priv->sarea_priv->last_dispatch =
@@ -232,6 +228,24 @@ irqreturn_t i915_driver_irq_handler(DRM_IRQ_ARGS)
 		    (iir & I915_ASLE_INTERRUPT))
 			opregion_asle_intr(dev);
 
+		/* With MSI, interrupts are only generated when iir
+		 * transitions from zero to nonzero.  If another bit got
+		 * set while we were handling the existing iir bits, then
+		 * we would never get another interrupt.
+		 *
+		 * This is fine on non-MSI as well, as if we hit this path
+		 * we avoid exiting the interrupt handler only to generate
+		 * another one.
+		 *
+		 * Note that for MSI this could cause a stray interrupt report
+		 * if an interrupt landed in the time between writing IIR and
+		 * the posting read.  This should be rare enough to never
+		 * trigger the 99% of 100,000 interrupts test for disabling
+		 * stray interrupts.
+		 */
+		iir = new_iir;
+	} while (iir != 0);
+
 	return IRQ_HANDLED;
 }