drm/i915: Fix races in gen4 page flip interrupt handling

Use the gen3 logic for handling page flip interrupts on gen4.

Unfortuantely this kills the stall_check since that looks like it can
easily trigger too early. With the current logic the stall check would
kick in on the first vblank after the flip has been submitted to the
ring. If the CS takes longer than that to process the commands in the
ring, the stall check will cause the page flip to be complete too
early. That doesn't sound like a very good idea. Something better
should be deviced if we still need the stall check. For now, mark
i915_pageflip_stall_check() as unused.

v2: Fix irq enable_mask and add __always_unused (Chris Wilson)

References: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/1116587
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

@@ -1547,7 +1547,7 @@ void i915_handle_error(struct drm_device *dev, bool wedged)
 	queue_work(dev_priv->wq, &dev_priv->gpu_error.work);
 }
 
-static void i915_pageflip_stall_check(struct drm_device *dev, int pipe)
+static void __always_unused i915_pageflip_stall_check(struct drm_device *dev, int pipe)
 {
 	drm_i915_private_t *dev_priv = dev->dev_private;
 	struct drm_crtc *crtc = dev_priv->pipe_to_crtc_mapping[pipe];
@@ -2598,6 +2598,8 @@ static int i965_irq_postinstall(struct drm_device *dev)
 			       I915_RENDER_COMMAND_PARSER_ERROR_INTERRUPT);
 
 	enable_mask = ~dev_priv->irq_mask;
+	enable_mask &= ~(I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT |
+			 I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT);
 	enable_mask |= I915_USER_INTERRUPT;
 
 	if (IS_G4X(dev))
@@ -2684,6 +2686,13 @@ static irqreturn_t i965_irq_handler(int irq, void *arg)
 	unsigned long irqflags;
 	int irq_received;
 	int ret = IRQ_NONE, pipe;
+	u32 flip[2] = {
+		I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT,
+		I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT
+	};
+	u32 flip_mask =
+		I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT |
+		I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT;
 
 	atomic_inc(&dev_priv->irq_received);
 
@@ -2692,7 +2701,7 @@ static irqreturn_t i965_irq_handler(int irq, void *arg)
 	for (;;) {
 		bool blc_event = false;
 
-		irq_received = iir != 0;
+		irq_received = (iir & ~flip_mask) != 0;
 
 		/* Can't rely on pipestat interrupt bit in iir as it might
 		 * have been cleared after the pipestat interrupt was received.
@@ -2739,25 +2748,25 @@ static irqreturn_t i965_irq_handler(int irq, void *arg)
 			I915_READ(PORT_HOTPLUG_STAT);
 		}
 
-		I915_WRITE(IIR, iir);
+		I915_WRITE(IIR, iir & ~flip_mask);
 		new_iir = I915_READ(IIR); /* Flush posted writes */
 
 		if (iir & I915_USER_INTERRUPT)
 			notify_ring(dev, &dev_priv->ring[RCS]);
 		if (iir & I915_BSD_USER_INTERRUPT)
 			notify_ring(dev, &dev_priv->ring[VCS]);
 
-		if (iir & I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT)
-			intel_prepare_page_flip(dev, 0);
-
-		if (iir & I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT)
-			intel_prepare_page_flip(dev, 1);
-
 		for_each_pipe(pipe) {
 			if (pipe_stats[pipe] & PIPE_START_VBLANK_INTERRUPT_STATUS &&
 			    drm_handle_vblank(dev, pipe)) {
-				i915_pageflip_stall_check(dev, pipe);
-				intel_finish_page_flip(dev, pipe);
+				if (iir & flip[pipe]) {
+					intel_prepare_page_flip(dev, pipe);
+
+					if ((I915_READ(ISR) & flip[pipe]) == 0) {
+						intel_finish_page_flip(dev, pipe);
+						flip_mask &= ~flip[pipe];
+					}
+				}
 			}
 
 			if (pipe_stats[pipe] & PIPE_LEGACY_BLC_EVENT_STATUS)