drm/i915: Don't mark PCH underrun reporting as disabled for transcoder B/C on LPT-H

Marking PCH transcoder FIFO underrun reporting as disabled for
transcoder B/C on LPT-H will block us from enabling the south error
interrupt. So let's only mark transcoder A underrun reporting as
disabled initially.

This is a little tricky to hit since you need a machine with LPT-H, and
the BIOS must enable either pipe B or C at boot. Then i915 would mark
the "transcoder B/C" underrun reporting as disabled and never enable it
again, meaning south interrupts would never get enabled either. The only
other interrupt in there is actually the poison interrupt which, if we
could ever trigger it, would just result in a little error in dmesg.

Here's the resulting change in SDEIMR on my HSW when I boot it with
multiple displays attached:
- (0x000c4004): 0xf115ffff
+ (0x000c4004): 0xf114ffff

My previous attempt [1] tried to fix this a little differently, but
Daniel requested I do this instead.

[1] https://lists.freedesktop.org/archives/intel-gfx/2015-November/081420.html

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470416417-15021-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -15771,6 +15771,13 @@ static bool intel_encoder_has_connectors(struct intel_encoder *encoder)
 	return false;
 }
 
+static bool has_pch_trancoder(struct drm_i915_private *dev_priv,
+			      enum transcoder pch_transcoder)
+{
+	return HAS_PCH_IBX(dev_priv) || HAS_PCH_CPT(dev_priv) ||
+		(HAS_PCH_LPT_H(dev_priv) && pch_transcoder == TRANSCODER_A);
+}
+
 static void intel_sanitize_crtc(struct intel_crtc *crtc)
 {
 	struct drm_device *dev = crtc->base.dev;
@@ -15849,7 +15856,17 @@ static void intel_sanitize_crtc(struct intel_crtc *crtc)
 		 * worst a fifo underrun happens which also sets this to false.
 		 */
 		crtc->cpu_fifo_underrun_disabled = true;
-		crtc->pch_fifo_underrun_disabled = true;
+		/*
+		 * We track the PCH trancoder underrun reporting state
+		 * within the crtc. With crtc for pipe A housing the underrun
+		 * reporting state for PCH transcoder A, crtc for pipe B housing
+		 * it for PCH transcoder B, etc. LPT-H has only PCH transcoder A,
+		 * and marking underrun reporting as disabled for the non-existing
+		 * PCH transcoders B and C would prevent enabling the south
+		 * error interrupt (see cpt_can_enable_serr_int()).
+		 */
+		if (has_pch_trancoder(dev_priv, (enum transcoder)crtc->pipe))
+			crtc->pch_fifo_underrun_disabled = true;
 	}
 }