drm/i915: Retry reading the PCH FDI receiver ISR

According to the PRM (Vol3P2), the PCH FDI receiver ISR read for bit lock
should be retried at least once. This patch retries the read 5 times
with a small delay in between reads. I've had reports of display corruption
on resume with "FDI train 1 fail!", so I'm hoping that adding this retry
will mitigate the issue.
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -2537,7 +2537,7 @@ static void gen6_fdi_link_train(struct drm_crtc *crtc)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
 	int pipe = intel_crtc->pipe;
-	u32 reg, temp, i;
+	u32 reg, temp, i, retry;
 
 	/* Train 1: umask FDI RX Interrupt symbol_lock and bit_lock bit
 	   for train result */
@@ -2589,15 +2589,19 @@ static void gen6_fdi_link_train(struct drm_crtc *crtc)
 		POSTING_READ(reg);
 		udelay(500);
 
+		for (retry = 0; retry < 5; retry++) {
 			reg = FDI_RX_IIR(pipe);
 			temp = I915_READ(reg);
 			DRM_DEBUG_KMS("FDI_RX_IIR 0x%x\n", temp);
-
 			if (temp & FDI_RX_BIT_LOCK) {
 				I915_WRITE(reg, temp | FDI_RX_BIT_LOCK);
 				DRM_DEBUG_KMS("FDI train 1 done.\n");
 				break;
 			}
+			udelay(50);
+		}
+		if (retry < 5)
+			break;
 	}
 	if (i == 4)
 		DRM_ERROR("FDI train 1 fail!\n");
@@ -2638,15 +2642,19 @@ static void gen6_fdi_link_train(struct drm_crtc *crtc)
 		POSTING_READ(reg);
 		udelay(500);
 
+		for (retry = 0; retry < 5; retry++) {
 			reg = FDI_RX_IIR(pipe);
 			temp = I915_READ(reg);
 			DRM_DEBUG_KMS("FDI_RX_IIR 0x%x\n", temp);
-
 			if (temp & FDI_RX_SYMBOL_LOCK) {
 				I915_WRITE(reg, temp | FDI_RX_SYMBOL_LOCK);
 				DRM_DEBUG_KMS("FDI train 2 done.\n");
 				break;
 			}
+			udelay(50);
+		}
+		if (retry < 5)
+			break;
 	}
 	if (i == 4)
 		DRM_ERROR("FDI train 2 fail!\n");