drm/i915: print computed bpp in dp link configuration

Pretty useful to debug our DP bandwidth woes.

v2: Also print out the required and available link bandwidth,
suggested by Chris Wilson.

v3: Also print out the input parameters so that diagnosing failures to
find a valid dp link configuration is possible.

v4: s/Display port/DP/ to shorten the output.
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -688,7 +688,7 @@ intel_dp_mode_fixup(struct drm_encoder *encoder, struct drm_display_mode *mode,
 	int lane_count, clock;
 	int max_lane_count = intel_dp_max_lane_count(intel_dp);
 	int max_clock = intel_dp_max_link_bw(intel_dp) == DP_LINK_BW_2_7 ? 1 : 0;
-	int bpp;
+	int bpp, mode_rate;
 	static int bws[2] = { DP_LINK_BW_1_62, DP_LINK_BW_2_7 };
 
 	if (is_edp(intel_dp) && intel_dp->panel_fixed_mode) {
@@ -702,24 +702,30 @@ intel_dp_mode_fixup(struct drm_encoder *encoder, struct drm_display_mode *mode,
 		mode->clock = intel_dp->panel_fixed_mode->clock;
 	}
 
+	DRM_DEBUG_KMS("DP link computation with max lane count %i "
+		      "max bw %02x pixel clock %iKHz\n",
+		      max_lane_count, bws[max_clock], mode->clock);
+
 	if (!intel_dp_adjust_dithering(intel_dp, mode, adjusted_mode))
 		return false;
 
 	bpp = adjusted_mode->private_flags & INTEL_MODE_DP_FORCE_6BPC ? 18 : 24;
+	mode_rate = intel_dp_link_required(mode->clock, bpp);
 
 	for (lane_count = 1; lane_count <= max_lane_count; lane_count <<= 1) {
 		for (clock = 0; clock <= max_clock; clock++) {
 			int link_avail = intel_dp_max_data_rate(intel_dp_link_clock(bws[clock]), lane_count);
 
-			if (intel_dp_link_required(mode->clock, bpp)
-					<= link_avail) {
+			if (mode_rate <= link_avail) {
 				intel_dp->link_bw = bws[clock];
 				intel_dp->lane_count = lane_count;
 				adjusted_mode->clock = intel_dp_link_clock(intel_dp->link_bw);
-				DRM_DEBUG_KMS("Display port link bw %02x lane "
-						"count %d clock %d\n",
+				DRM_DEBUG_KMS("DP link bw %02x lane "
+						"count %d clock %d bpp %d\n",
 				       intel_dp->link_bw, intel_dp->lane_count,
-				       adjusted_mode->clock);
+				       adjusted_mode->clock, bpp);
+				DRM_DEBUG_KMS("DP link bw required %i available %i\n",
+					      mode_rate, link_avail);
 				return true;
 			}
 		}