drm/i915/display: Use YCbCr420 as fallback when RGB fails

When encoder validation of a display mode fails, retry with less bandwidth
heavy YCbCr420 color mode, if available. This enables some HDMI 1.4 setups
to support 4k60Hz output, which previously failed silently.

AMDGPU had nearly the exact same issue. This problem description is
therefore copied from my commit message of the AMDGPU patch.

On some setups, while the monitor and the gpu support display modes with
pixel clocks of up to 600MHz, the link encoder might not. This prevents
YCbCr444 and RGB encoding for 4k60Hz, but YCbCr420 encoding might still be
possible. However, which color mode is used is decided before the link
encoder capabilities are checked. This patch fixes the problem by retrying
to find a display mode with YCbCr420 enforced and using it, if it is
valid.
Signed-off-by: Werner Sembach <wse@tuxedocomputers.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210510133349.14491-4-wse@tuxedocomputers.com

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

@@ -1900,6 +1900,7 @@ intel_hdmi_mode_valid(struct drm_connector *connector,
 	int clock = mode->clock;
 	int max_dotclk = to_i915(connector->dev)->max_dotclk_freq;
 	bool has_hdmi_sink = intel_has_hdmi_sink(hdmi, connector->state);
+	bool ycbcr_420_only;
 
 	if (mode->flags & DRM_MODE_FLAG_DBLSCAN)
 		return MODE_NO_DBLESCAN;
@@ -1916,12 +1917,22 @@ intel_hdmi_mode_valid(struct drm_connector *connector,
 		clock *= 2;
 	}
 
-	if (drm_mode_is_420_only(&connector->display_info, mode))
+	ycbcr_420_only = drm_mode_is_420_only(&connector->display_info, mode);
+	if (ycbcr_420_only)
 		clock /= 2;
 
 	status = intel_hdmi_mode_clock_valid(hdmi, clock, has_hdmi_sink);
-	if (status != MODE_OK)
-		return status;
+	if (status != MODE_OK) {
+		if (ycbcr_420_only ||
+		    !connector->ycbcr_420_allowed ||
+		    !drm_mode_is_420_also(&connector->display_info, mode))
+			return status;
+
+		clock /= 2;
+		status = intel_hdmi_mode_clock_valid(hdmi, clock, has_hdmi_sink);
+		if (status != MODE_OK)
+			return status;
+	}
 
 	return intel_mode_valid_max_plane_size(dev_priv, mode, false);
 }
@@ -2129,6 +2140,14 @@ static int intel_hdmi_compute_output_format(struct intel_encoder *encoder,
 	}
 
 	ret = intel_hdmi_compute_clock(encoder, crtc_state);
+	if (ret) {
+		if (crtc_state->output_format != INTEL_OUTPUT_FORMAT_YCBCR420 &&
+		    connector->ycbcr_420_allowed &&
+		    drm_mode_is_420_also(&connector->display_info, adjusted_mode)) {
+			crtc_state->output_format = INTEL_OUTPUT_FORMAT_YCBCR420;
+			ret = intel_hdmi_compute_clock(encoder, crtc_state);
+		}
+	}
 
 	return ret;
 }