On Fri, Nov 28, 2025 at 10:05:42PM +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 modes.
>
> Fix this by checking whether the mode is 420-only 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-only, check with RGB as was previously always the case.
>
> Fixes: 47368ab437fd ("drm/display: hdmi: add generic mode_valid helper")
> Signed-off-by: Nicolas Frattaroli <[email protected]>
> ---
> drivers/gpu/drm/display/drm_hdmi_state_helper.c | 12 +++++++++++-
> 1 file changed, 11 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 5da956bdd68c..1800e00b30c5 100644
> --- a/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> +++ b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> @@ -892,8 +892,18 @@ 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 {
> + fmt = HDMI_COLORSPACE_RGB;
> + }
>
> - clock = drm_hdmi_compute_mode_clock(mode, 8, HDMI_COLORSPACE_RGB);
> + clock = drm_hdmi_compute_mode_clock(mode, 8, fmt);I agree on principle, but we need to have a test for this. Maxime
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