drm/i915: Suppress spurious CPU FIFO underruns on ILK-IVB

We still get spurious pipe underruns on ILK/SNB/IVB under two
circumstances when dealing with PCH ports:
* When the pipe has been disabled, but FDI RX/TX is still enabled
* During FDI link training

Both cases seem to happen at least when we do VGA+HDMI cloning
from the same pipe. I don't think I've seen them when not cloning,
but can't be 100% sure.

Disable underrun reporting around those places to eliminate the
dmesg errors.

Testcase: igt/kms_setmode/basic-clone-single-crtc
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448050160-14124-1-git-send-email-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_display.c

@@ -4137,6 +4137,12 @@ static void ironlake_pch_enable(struct drm_crtc *crtc)
 	I915_WRITE(FDI_RX_TUSIZE1(pipe),
 		   I915_READ(PIPE_DATA_M1(pipe)) & TU_SIZE_MASK);
 
+	/*
+	 * Sometimes spurious CPU pipe underruns happen during FDI
+	 * training, at least with VGA+HDMI cloning. Suppress them.
+	 */
+	intel_set_cpu_fifo_underrun_reporting(dev_priv, pipe, false);
+
 	/* For PCH output, training FDI link */
 	dev_priv->display.fdi_link_train(crtc);
 
@@ -4170,6 +4176,8 @@ static void ironlake_pch_enable(struct drm_crtc *crtc)
 
 	intel_fdi_normal_train(crtc);
 
+	intel_set_cpu_fifo_underrun_reporting(dev_priv, pipe, true);
+
 	/* For PCH DP, enable TRANS_DP_CTL */
 	if (HAS_PCH_CPT(dev) && intel_crtc->config->has_dp_encoder) {
 		const struct drm_display_mode *adjusted_mode =
@@ -5062,12 +5070,22 @@ static void ironlake_crtc_disable(struct drm_crtc *crtc)
 	drm_crtc_vblank_off(crtc);
 	assert_vblank_disabled(crtc);
 
+	/*
+	 * Sometimes spurious CPU pipe underruns happen when the
+	 * pipe is already disabled, but FDI RX/TX is still enabled.
+	 * Happens at least with VGA+HDMI cloning. Suppress them.
+	 */
+	if (intel_crtc->config->has_pch_encoder)
+		intel_set_cpu_fifo_underrun_reporting(dev_priv, pipe, false);
+
 	intel_disable_pipe(intel_crtc);
 
 	ironlake_pfit_disable(intel_crtc, false);
 
-	if (intel_crtc->config->has_pch_encoder)
+	if (intel_crtc->config->has_pch_encoder) {
 		ironlake_fdi_disable(crtc);
+		intel_set_cpu_fifo_underrun_reporting(dev_priv, pipe, true);
+	}
 
 	for_each_encoder_on_crtc(dev, crtc, encoder)
 		if (encoder->post_disable)