Re: [PATCH v9 05/19] drm/display: hdmi-state-helper: Try subsampling in mode_valid

Dmitry Baryshkov
2026-03-01T16:49:14Z

On Fri, Feb 27, 2026 at 08:20:10PM +0100, Nicolas Frattaroli wrote:
> 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>
> 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(-)

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>

--
With best wishes
Dmitry