drm/i915: kill lpt pch transcoder->crtc mapping code for fifo underruns

It's racy: There's no guarantee that we won't walk this code (due to a
pch fifo underrun interrupt) while someone is changing the pointers
around.

The only reason we do this is to use the righ crtc for the pch fifo
underrun accounting. But we never expose this to userspace, so
essentially no one really cares if we use the "wrong" crtc.

So let's just rip it out.

With this patch fifo underrun code will always use crtc A for tracking
underruns on the (only) pch transcoder on LPT.

v2: Add a big comment explaining what's going on. Requested by Paulo.

v3: Fixup spelling in comment as spotted by Paulo.

Cc: Paulo Zanoni <przanoni@gmail.com>
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

@@ -202,13 +202,13 @@ static void ibx_display_interrupt_update(struct drm_i915_private *dev_priv,
 #define ibx_disable_display_interrupt(dev_priv, bits) \
 	ibx_display_interrupt_update((dev_priv), (bits), 0)
 
-static void ibx_set_fifo_underrun_reporting(struct intel_crtc *crtc,
+static void ibx_set_fifo_underrun_reporting(struct drm_device *dev,
+					    enum transcoder pch_transcoder,
 					    bool enable)
 {
-	struct drm_device *dev = crtc->base.dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
-	uint32_t bit = (crtc->pipe == PIPE_A) ? SDE_TRANSA_FIFO_UNDER :
-						SDE_TRANSB_FIFO_UNDER;
+	uint32_t bit = (pch_transcoder == TRANSCODER_A) ?
+		       SDE_TRANSA_FIFO_UNDER : SDE_TRANSB_FIFO_UNDER;
 
 	if (enable)
 		ibx_enable_display_interrupt(dev_priv, bit);
@@ -306,29 +306,19 @@ bool intel_set_pch_fifo_underrun_reporting(struct drm_device *dev,
 					   bool enable)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
-	enum pipe p;
-	struct drm_crtc *crtc;
-	struct intel_crtc *intel_crtc;
+	struct drm_crtc *crtc = dev_priv->pipe_to_crtc_mapping[pch_transcoder];
+	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
 	unsigned long flags;
 	bool ret;
 
-	if (HAS_PCH_LPT(dev)) {
-		crtc = NULL;
-		for_each_pipe(p) {
-			struct drm_crtc *c = dev_priv->pipe_to_crtc_mapping[p];
-			if (intel_pipe_has_type(c, INTEL_OUTPUT_ANALOG)) {
-				crtc = c;
-				break;
-			}
-		}
-		if (!crtc) {
-			DRM_ERROR("PCH FIFO underrun, but no CRTC using the PCH found\n");
-			return false;
-		}
-	} else {
-		crtc = dev_priv->pipe_to_crtc_mapping[pch_transcoder];
-	}
-	intel_crtc = to_intel_crtc(crtc);
+	/*
+	 * NOTE: Pre-LPT has a fixed cpu pipe -> pch transcoder mapping, but LPT
+	 * has only one pch transcoder A that all pipes can use. To avoid racy
+	 * pch transcoder -> pipe lookups from interrupt code simply store the
+	 * underrun statistics in crtc A. Since we never expose this anywhere
+	 * nor use it outside of the fifo underrun code here using the "wrong"
+	 * crtc on LPT won't cause issues.
+	 */
 
 	spin_lock_irqsave(&dev_priv->irq_lock, flags);
 
@@ -340,7 +330,7 @@ bool intel_set_pch_fifo_underrun_reporting(struct drm_device *dev,
 	intel_crtc->pch_fifo_underrun_disabled = !enable;
 
 	if (HAS_PCH_IBX(dev))
-		ibx_set_fifo_underrun_reporting(intel_crtc, enable);
+		ibx_set_fifo_underrun_reporting(dev, pch_transcoder, enable);
 	else
 		cpt_set_fifo_underrun_reporting(dev, pch_transcoder, enable);