drm/i915: Check for FIFO underuns when disabling reporting on gmch platforms

FIFO underruns don't generate an interrupt on gmch platforms, so we
should check whether there were any that we failed to notice when
we're disabling FIFO underrun reporting.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -306,16 +306,22 @@ static bool cpt_can_enable_serr_int(struct drm_device *dev)
 	return true;
 }
 
-static void i9xx_clear_fifo_underrun(struct drm_device *dev, enum pipe pipe)
+static void i9xx_set_fifo_underrun_reporting(struct drm_device *dev,
+					     enum pipe pipe, bool enable)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	u32 reg = PIPESTAT(pipe);
-	u32 pipestat = I915_READ(reg) & 0x7fff0000;
+	u32 pipestat = I915_READ(reg) & 0xffff0000;
 
 	assert_spin_locked(&dev_priv->irq_lock);
 
-	I915_WRITE(reg, pipestat | PIPE_FIFO_UNDERRUN_STATUS);
-	POSTING_READ(reg);
+	if (enable) {
+		I915_WRITE(reg, pipestat | PIPE_FIFO_UNDERRUN_STATUS);
+		POSTING_READ(reg);
+	} else {
+		if (pipestat & PIPE_FIFO_UNDERRUN_STATUS)
+			DRM_ERROR("pipe %c underrun\n", pipe_name(pipe));
+	}
 }
 
 static void ironlake_set_fifo_underrun_reporting(struct drm_device *dev,
@@ -472,8 +478,8 @@ static bool __intel_set_cpu_fifo_underrun_reporting(struct drm_device *dev,
 
 	intel_crtc->cpu_fifo_underrun_disabled = !enable;
 
-	if (enable && (INTEL_INFO(dev)->gen < 5 || IS_VALLEYVIEW(dev)))
-		i9xx_clear_fifo_underrun(dev, pipe);
+	if (INTEL_INFO(dev)->gen < 5 || IS_VALLEYVIEW(dev))
+		i9xx_set_fifo_underrun_reporting(dev, pipe, enable);
 	else if (IS_GEN5(dev) || IS_GEN6(dev))
 		ironlake_set_fifo_underrun_reporting(dev, pipe, enable);
 	else if (IS_GEN7(dev))