[PATCH v14 09/28] drm/display: hdmi-state-helper: Try subsampling in mode_valid

Nicolas Frattaroli
2026-04-23T19:05:45Z

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 DRM_OUTPUT_COLOR_FORMAT_YCBCR420 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")
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
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 954f8b2973fc..5cc30da30989 100644
--- a/drivers/gpu/drm/display/drm_hdmi_state_helper.c
+++ b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
@@ -942,8 +942,21 @@ drm_hdmi_connector_mode_valid(struct drm_connector *connector,
const struct drm_display_mode *mode)
{
unsigned long long clock;
+ enum drm_output_color_format fmt;
+
+ if (drm_mode_is_420_only(&connector->display_info, mode)) {
+ if (connector->ycbcr_420_allowed)
+ fmt = DRM_OUTPUT_COLOR_FORMAT_YCBCR420;
+ else
+ return MODE_NO_420;
+ } else if (drm_mode_is_420_also(&connector->display_info, mode) &&
+ connector->ycbcr_420_allowed) {
+ fmt = DRM_OUTPUT_COLOR_FORMAT_YCBCR420;
+ } else {
+ fmt = DRM_OUTPUT_COLOR_FORMAT_RGB444;
+ }

- clock = drm_hdmi_compute_mode_clock(mode, 8, DRM_OUTPUT_COLOR_FORMAT_RGB444);
+ clock = drm_hdmi_compute_mode_clock(mode, 8, fmt);
if (!clock)
return MODE_ERROR;

--
2.53.0