drm/i915/dp: Add support for "4:2:0 also" modes for DP

Currently we only support "4:2:0 also" modes on native HDMI.
Extend that support for DP as well.

With all the HDMI DFP TMDS clock handling sorted out this
is now going to work for both native DP and DP->HDMI
converters. As with native HDMI we first check if RGB
output is possible, and if not we try YCbCr 4:2:0 instead.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220322120015.28074-10-ville.syrjala@linux.intel.comReviewed-by: Uma Shankar <uma.shankar@intel.com>

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

@@ -868,6 +868,8 @@ intel_dp_mode_valid_downstream(struct intel_connector *connector,
 {
 	struct intel_dp *intel_dp = intel_attached_dp(connector);
 	const struct drm_display_info *info = &connector->base.display_info;
+	enum drm_mode_status status;
+	bool ycbcr_420_only;
 
 	/* If PCON supports FRL MODE, check FRL bandwidth constraints */
 	if (intel_dp->dfp.pcon_max_frl_bw) {
@@ -892,9 +894,25 @@ intel_dp_mode_valid_downstream(struct intel_connector *connector,
 	    target_clock > intel_dp->dfp.max_dotclock)
 		return MODE_CLOCK_HIGH;
 
+	ycbcr_420_only = drm_mode_is_420_only(info, mode);
+
 	/* Assume 8bpc for the DP++/HDMI/DVI TMDS clock check */
-	return intel_dp_tmds_clock_valid(intel_dp, target_clock, 8,
-					 drm_mode_is_420_only(info, mode));
+	status = intel_dp_tmds_clock_valid(intel_dp, target_clock,
+					   8, ycbcr_420_only);
+
+	if (status != MODE_OK) {
+		if (ycbcr_420_only ||
+		    !connector->base.ycbcr_420_allowed ||
+		    !drm_mode_is_420_also(info, mode))
+			return status;
+
+		status = intel_dp_tmds_clock_valid(intel_dp, target_clock,
+						   8, true);
+		if (status != MODE_OK)
+			return status;
+	}
+
+	return MODE_OK;
 }
 
 static bool intel_dp_need_bigjoiner(struct intel_dp *intel_dp,
@@ -1880,6 +1898,43 @@ static bool intel_dp_has_audio(struct intel_encoder *encoder,
 		return intel_conn_state->force_audio == HDMI_AUDIO_ON;
 }
 
+static int
+intel_dp_compute_output_format(struct intel_encoder *encoder,
+			       struct intel_crtc_state *crtc_state,
+			       struct drm_connector_state *conn_state)
+{
+	struct drm_i915_private *i915 = to_i915(encoder->base.dev);
+	struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
+	struct intel_connector *connector = intel_dp->attached_connector;
+	const struct drm_display_info *info = &connector->base.display_info;
+	const struct drm_display_mode *adjusted_mode = &crtc_state->hw.adjusted_mode;
+	bool ycbcr_420_only;
+	int ret;
+
+	ycbcr_420_only = drm_mode_is_420_only(info, adjusted_mode);
+
+	crtc_state->output_format = intel_dp_output_format(connector, ycbcr_420_only);
+
+	if (ycbcr_420_only && !intel_dp_is_ycbcr420(intel_dp, crtc_state)) {
+		drm_dbg_kms(&i915->drm,
+			    "YCbCr 4:2:0 mode but YCbCr 4:2:0 output not possible. Falling back to RGB.\n");
+		crtc_state->output_format = INTEL_OUTPUT_FORMAT_RGB;
+	}
+
+	ret = intel_dp_compute_link_config(encoder, crtc_state, conn_state);
+	if (ret) {
+		if (intel_dp_is_ycbcr420(intel_dp, crtc_state) ||
+		    !connector->base.ycbcr_420_allowed ||
+		    !drm_mode_is_420_also(info, adjusted_mode))
+			return ret;
+
+		crtc_state->output_format = intel_dp_output_format(connector, true);
+		ret = intel_dp_compute_link_config(encoder, crtc_state, conn_state);
+	}
+
+	return ret;
+}
+
 int
 intel_dp_compute_config(struct intel_encoder *encoder,
 			struct intel_crtc_state *pipe_config,
@@ -1890,7 +1945,6 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 	struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
 	const struct drm_display_mode *fixed_mode;
 	struct intel_connector *connector = intel_dp->attached_connector;
-	const struct drm_display_info *info = &connector->base.display_info;
 	bool constant_n = drm_dp_has_quirk(&intel_dp->desc, DP_DPCD_QUIRK_CONSTANT_N);
 	int ret = 0, output_bpp;
 
@@ -1919,11 +1973,8 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 	if (intel_dp_hdisplay_bad(dev_priv, adjusted_mode->crtc_hdisplay))
 		return -EINVAL;
 
-	pipe_config->output_format =
-		intel_dp_output_format(connector, drm_mode_is_420_only(info, adjusted_mode));
-
-	ret = intel_dp_compute_link_config(encoder, pipe_config, conn_state);
-	if (ret < 0)
+	ret = intel_dp_compute_output_format(encoder, pipe_config, conn_state);
+	if (ret)
 		return ret;
 
 	if ((intel_dp_is_edp(intel_dp) && fixed_mode) ||