drm_hdmi_connector_mode_valid assumes modes are only valid if they work
with RGB. The reality is more complex however: YCbCr 4:2:0
chroma-subsampled modes only require half the pixel clock that the same
mode would require in RGB.

This leads to drm_hdmi_connector_mode_valid rejecting perfectly valid
420-only or 420-also modes.

Fix this by checking whether the mode is 420-capable first. If so, then
proceed by checking it with HDMI_COLORSPACE_YUV420 so long as the
connector has legalized 420, otherwise error out. If the mode is not
420-capable, check with RGB as was previously always the case.

Fixes: 47368ab437fd ("drm/display: hdmi: add generic mode_valid helper")
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
---
 drivers/gpu/drm/display/drm_hdmi_state_helper.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/display/drm_hdmi_state_helper.c b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
index 1ea3b9c93aa5..ceabfdb30f7a 100644
--- a/drivers/gpu/drm/display/drm_hdmi_state_helper.c
+++ b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
@@ -898,8 +898,21 @@ drm_hdmi_connector_mode_valid(struct drm_connector *connector,
 			      const struct drm_display_mode *mode)
 {
 	unsigned long long clock;
+	enum hdmi_colorspace fmt;
+
+	if (drm_mode_is_420_only(&connector->display_info, mode)) {
+		if (connector->ycbcr_420_allowed)
+			fmt = HDMI_COLORSPACE_YUV420;
+		else
+			return MODE_NO_420;
+	} else if (drm_mode_is_420_also(&connector->display_info, mode) &&
+		   connector->ycbcr_420_allowed) {
+		fmt = HDMI_COLORSPACE_YUV420;
+	} else {
+		fmt = HDMI_COLORSPACE_RGB;
+	}
 
-	clock = drm_hdmi_compute_mode_clock(mode, 8, HDMI_COLORSPACE_RGB);
+	clock = drm_hdmi_compute_mode_clock(mode, 8, fmt);
 	if (!clock)
 		return MODE_ERROR;
 

-- 
2.52.0