Re: [PATCH v2 1/2] drm/display: hdmi: Only allow BPC values of 8, 10, 12 and 16

[Maxime Ripard <mripard@kernel.org>]

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Hi Nicolas,

On Thu, Jun 18, 2026 at 08:54:23PM +0200, Nicolas Frattaroli wrote:
> As per the comment in sink_supports_format_bpc(), CTA-861-F defines that
> only bits-per-channel values of 8, 10, 12 and 16 are allowed for HDMI.
> Allowing more than this has surprising consequences for the atomic check
> phase. The HDMI state helpers may accidentally conclude that a sink
> supports 11bpc if a caller asks for it.
> 
> Fix this by exiting early if the bpc value doesn't match one of those
> given in the standard.
> 
> Fixes: 26ff1c38fc29 ("drm/connector: hdmi: Compute bpc and format automatically")
> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> ---
>  drivers/gpu/drm/display/drm_hdmi_state_helper.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/display/drm_hdmi_state_helper.c b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> index ce17eeefc2da..0bf6102244f7 100644
> --- a/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> +++ b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> @@ -420,6 +420,16 @@ sink_supports_format_bpc(const struct drm_connector *connector,
>  		return false;
>  	}
>  
> +	switch (bpc) {
> +	case 8:
> +	case 10:
> +	case 12:
> +	case 16:
> +		break;
> +	default:
> +		return false;
> +	}
> +

Sorry for not noticing in your first version, but I'd move this before
the VIC 1 check, and would add a comment to refer to the spec (possibly
by splitting the comment already there for the VIC 1 check).

We should also log debug message on failure like we do for every other
rejection path.

Maxime

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