drm/i915: Filter gmch fifo underruns in the shared handler

This simplifies the code in the vlv irq handler. Also this now
means that we correctly filter underruns on gen2-4.

And as the real upshot I need to document one less function for
the fifo underrun code.

v2: Shorten one long line.
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -1749,9 +1749,9 @@ static void valleyview_pipestat_irq_handler(struct drm_device *dev, u32 iir)
 		 * we need to be careful that we only handle what we want to
 		 * handle.
 		 */
-		mask = 0;
-		if (__cpu_fifo_underrun_reporting_enabled(dev_priv, pipe))
-			mask |= PIPE_FIFO_UNDERRUN_STATUS;
+
+		/* fifo underruns are filterered in the underrun handler. */
+		mask = PIPE_FIFO_UNDERRUN_STATUS;
 
 		switch (pipe) {
 		case PIPE_A:

drivers/gpu/drm/i915/intel_drv.h

@@ -766,8 +766,6 @@ void intel_cpu_fifo_underrun_irq_handler(struct drm_i915_private *dev_priv,
 void intel_pch_fifo_underrun_irq_handler(struct drm_i915_private *dev_priv,
 					 enum transcoder pch_transcoder);
 void i9xx_check_fifo_underruns(struct drm_i915_private *dev_priv);
-bool __cpu_fifo_underrun_reporting_enabled(struct drm_i915_private *dev_priv,
-					   enum pipe pipe);
 
 /* i915_irq.c */
 void gen5_enable_gt_irq(struct drm_i915_private *dev_priv, uint32_t mask);

drivers/gpu/drm/i915/intel_fifo_underrun.c

@@ -252,7 +252,8 @@ bool intel_set_cpu_fifo_underrun_reporting(struct drm_i915_private *dev_priv,
 	return ret;
 }
 
-bool __cpu_fifo_underrun_reporting_enabled(struct drm_i915_private *dev_priv,
+static bool
+__cpu_fifo_underrun_reporting_enabled(struct drm_i915_private *dev_priv,
 				      enum pipe pipe)
 {
 	struct drm_crtc *crtc = dev_priv->pipe_to_crtc_mapping[pipe];
@@ -312,6 +313,11 @@ bool intel_set_pch_fifo_underrun_reporting(struct drm_i915_private *dev_priv,
 void intel_cpu_fifo_underrun_irq_handler(struct drm_i915_private *dev_priv,
 					 enum pipe pipe)
 {
+	/* GMCH can't disable fifo underruns, filter them. */
+	if (HAS_GMCH_DISPLAY(dev_priv->dev) &&
+	    !__cpu_fifo_underrun_reporting_enabled(dev_priv, pipe))
+		return;
+
 	if (intel_set_cpu_fifo_underrun_reporting(dev_priv, pipe, false))
 		DRM_ERROR("CPU pipe %c FIFO underrun\n",
 			  pipe_name(pipe));