drm/i915: Always read pipestat in irq_handler

Because we write pipestat before iir, it's possible that a pipestat
interrupt will occur between the pipestat write and the iir write. This
leaves pipestat with an interrupt status not visible in iir. This may cause
an interrupt flood as we never clear the pipestat event.
Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -170,37 +170,54 @@ irqreturn_t i915_driver_irq_handler(DRM_IRQ_ARGS)
 	drm_i915_private_t *dev_priv = (drm_i915_private_t *) dev->dev_private;
 	u32 iir, new_iir;
 	u32 pipea_stats, pipeb_stats;
+	u32 vblank_status;
+	u32 vblank_enable;
 	int vblank = 0;
 	unsigned long irqflags;
+	int irq_received;
+	int ret = IRQ_NONE;
 
 	atomic_inc(&dev_priv->irq_received);
 
 	iir = I915_READ(IIR);
 
-	if (iir == 0)
-		return IRQ_NONE;
+	if (IS_I965G(dev)) {
+		vblank_status = I915_START_VBLANK_INTERRUPT_STATUS;
+		vblank_enable = PIPE_START_VBLANK_INTERRUPT_ENABLE;
+	} else {
+		vblank_status = I915_VBLANK_INTERRUPT_STATUS;
+		vblank_enable = I915_VBLANK_INTERRUPT_ENABLE;
+	}
 
-	do {
-		pipea_stats = 0;
-		pipeb_stats = 0;
-		/*
-		 * Clear the PIPE(A|B)STAT regs before the IIR
+	for (;;) {
+		irq_received = iir != 0;
+
+		/* Can't rely on pipestat interrupt bit in iir as it might
+		 * have been cleared after the pipestat interrupt was received.
+		 * It doesn't set the bit in iir again, but it still produces
+		 * interrupts (for non-MSI).
 		 */
-		if (iir & I915_DISPLAY_PIPE_A_EVENT_INTERRUPT) {
 		spin_lock_irqsave(&dev_priv->user_irq_lock, irqflags);
 		pipea_stats = I915_READ(PIPEASTAT);
+		pipeb_stats = I915_READ(PIPEBSTAT);
+		/*
+		 * Clear the PIPE(A|B)STAT regs before the IIR
+		 */
+		if (pipea_stats & 0x8000ffff) {
 			I915_WRITE(PIPEASTAT, pipea_stats);
-			spin_unlock_irqrestore(&dev_priv->user_irq_lock,
-					       irqflags);
+			irq_received = 1;
 		}
 
-		if (iir & I915_DISPLAY_PIPE_B_EVENT_INTERRUPT) {
-			spin_lock_irqsave(&dev_priv->user_irq_lock, irqflags);
-			pipeb_stats = I915_READ(PIPEBSTAT);
+		if (pipeb_stats & 0x8000ffff) {
 			I915_WRITE(PIPEBSTAT, pipeb_stats);
-			spin_unlock_irqrestore(&dev_priv->user_irq_lock,
-					       irqflags);
+			irq_received = 1;
 		}
+		spin_unlock_irqrestore(&dev_priv->user_irq_lock, irqflags);
+
+		if (!irq_received)
+			break;
+
+		ret = IRQ_HANDLED;
 
 		I915_WRITE(IIR, iir);
 		new_iir = I915_READ(IIR); /* Flush posted writes */
@@ -214,12 +231,12 @@ irqreturn_t i915_driver_irq_handler(DRM_IRQ_ARGS)
 			DRM_WAKEUP(&dev_priv->irq_queue);
 		}
 
-		if (pipea_stats & I915_VBLANK_INTERRUPT_STATUS) {
+		if (pipea_stats & vblank_status) {
 			vblank++;
 			drm_handle_vblank(dev, 0);
 		}
 
-		if (pipeb_stats & I915_VBLANK_INTERRUPT_STATUS) {
+		if (pipeb_stats & vblank_status) {
 			vblank++;
 			drm_handle_vblank(dev, 1);
 		}
@@ -244,9 +261,9 @@ irqreturn_t i915_driver_irq_handler(DRM_IRQ_ARGS)
 		 * stray interrupts.
 		 */
 		iir = new_iir;
-	} while (iir != 0);
+	}
 
-	return IRQ_HANDLED;
+	return ret;
 }
 
 static int i915_emit_irq(struct drm_device * dev)