drm/i915/dp: Skip computing a non-DSC link config if DSC is needed

Computing the non-DSC mode link config is redundant once it's determined
that DSC will be needed, so skip computing it. In a follow-up patch this
simplifies setting the link limits which are dependent on the DSC vs.
non-DSC mode.

While at it sanitize the debug print about the MST DSC fallback path,
making it similar to the SST DSC one.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230921195159.2646027-4-imre.deak@intel.com

drivers/gpu/drm/i915/display/intel_dp.c

@@ -2247,7 +2247,8 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
 	struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
 	struct link_config_limits limits;
 	bool joiner_needs_dsc = false;
-	int ret;
+	bool dsc_needed;
+	int ret = 0;
 
 	intel_dp_compute_config_limits(intel_dp, pipe_config,
 				       respect_downstream_limits, &limits);
@@ -2263,13 +2264,20 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
 	 */
 	joiner_needs_dsc = DISPLAY_VER(i915) < 13 && pipe_config->bigjoiner_pipes;
 
-	/*
-	 * Optimize for slow and wide for everything, because there are some
-	 * eDP 1.3 and 1.4 panels don't work well with fast and narrow.
-	 */
-	ret = intel_dp_compute_link_config_wide(intel_dp, pipe_config, conn_state, &limits);
+	dsc_needed = joiner_needs_dsc || intel_dp->force_dsc_en;
+
+	if (!dsc_needed) {
+		/*
+		 * Optimize for slow and wide for everything, because there are some
+		 * eDP 1.3 and 1.4 panels don't work well with fast and narrow.
+		 */
+		ret = intel_dp_compute_link_config_wide(intel_dp, pipe_config,
+							conn_state, &limits);
+		if (ret)
+			dsc_needed = true;
+	}
 
-	if (ret || joiner_needs_dsc || intel_dp->force_dsc_en) {
+	if (dsc_needed) {
 		drm_dbg_kms(&i915->drm, "Try DSC (fallback=%s, joiner=%s, force=%s)\n",
 			    str_yes_no(ret), str_yes_no(joiner_needs_dsc),
 			    str_yes_no(intel_dp->force_dsc_en));

drivers/gpu/drm/i915/display/intel_dp_mst.c

@@ -337,7 +337,8 @@ static int intel_dp_mst_compute_config(struct intel_encoder *encoder,
 	const struct drm_display_mode *adjusted_mode =
 		&pipe_config->hw.adjusted_mode;
 	struct link_config_limits limits;
-	int ret;
+	bool dsc_needed;
+	int ret = 0;
 
 	if (adjusted_mode->flags & DRM_MODE_FLAG_DBLSCAN)
 		return -EINVAL;
@@ -348,15 +349,25 @@ static int intel_dp_mst_compute_config(struct intel_encoder *encoder,
 
 	intel_dp_mst_compute_config_limits(intel_dp, pipe_config, &limits);
 
-	ret = intel_dp_mst_compute_link_config(encoder, pipe_config,
-					       conn_state, &limits);
+	dsc_needed = intel_dp->force_dsc_en;
 
-	if (ret == -EDEADLK)
-		return ret;
+	if (!dsc_needed) {
+		ret = intel_dp_mst_compute_link_config(encoder, pipe_config,
+						       conn_state, &limits);
+
+		if (ret == -EDEADLK)
+			return ret;
+
+		if (ret)
+			dsc_needed = true;
+	}
 
 	/* enable compression if the mode doesn't fit available BW */
-	drm_dbg_kms(&dev_priv->drm, "Force DSC en = %d\n", intel_dp->force_dsc_en);
-	if (ret || intel_dp->force_dsc_en) {
+	if (dsc_needed) {
+		drm_dbg_kms(&dev_priv->drm, "Try DSC (fallback=%s, force=%s)\n",
+			    str_yes_no(ret),
+			    str_yes_no(intel_dp->force_dsc_en));
+
 		/*
 		 * FIXME: As bpc is hardcoded to 8, as mentioned above,
 		 * WARN and ignore the debug flag force_dsc_bpc for now.