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

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] The function `sink_supports_format_bpc` explicitly permits 16 BPC for RGB444 and YCBCR444 formats without verifying if the sink's EDID actually supports 48-bit deep color.
--

commit 767fa3cc832e8b93f697ad91bcc66e5b3b1ed85d
Author: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>

drm/display: hdmi: Only allow BPC values of 8, 10, 12 and 16

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.

> diff --git a/drivers/gpu/drm/display/drm_hdmi_state_helper.c b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> index 4867edbf2622b..b06499dbfe8b3 100644
> --- a/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> +++ b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> @@ -419,6 +419,16 @@ sink_supports_format_bpc(const struct drm_connector *connector,
>  		return false;
>  	}
>  
> +	switch (bpc) {
> +	case 8:
> +	case 10:
> +	case 12:
> +	case 16:

[Severity: High]
This is a pre-existing issue, but does explicitly allowing 16 BPC here
expose a missing validation check for 48-bit deep color further down in
sink_supports_format_bpc()?

When checking DRM_OUTPUT_COLOR_FORMAT_RGB444 and
DRM_OUTPUT_COLOR_FORMAT_YCBCR444, the code validates 10 BPC and 12 BPC
against the EDID flags, but completely omits checking 16 BPC against
DRM_EDID_HDMI_DC_48:

drivers/gpu/drm/display/drm_hdmi_state_helper.c:sink_supports_format_bpc() {
	...
	case DRM_OUTPUT_COLOR_FORMAT_RGB444:
		...
		if (bpc == 10 && !(info->edid_hdmi_rgb444_dc_modes & DRM_EDID_HDMI_DC_30)) {
			drm_dbg_kms(dev, "10 BPC but sink doesn't support Deep Color 30.\n");
			return false;
		}

		if (bpc == 12 && !(info->edid_hdmi_rgb444_dc_modes & DRM_EDID_HDMI_DC_36)) {
			drm_dbg_kms(dev, "12 BPC but sink doesn't support Deep Color 36.\n");
			return false;
		}

		drm_dbg_kms(dev, "RGB format supported in that configuration.\n");
		return true;
	...
}

If the TMDS clock limit permits it, the driver might incorrectly conclude
that the sink supports 16 BPC and program the hardware accordingly, which
could result in a black screen if the monitor doesn't actually support
48-bit deep color.

Should a check for (info->edid_hdmi_rgb444_dc_modes & DRM_EDID_HDMI_DC_48)
be added for the RGB444 and YCBCR444 cases?

> +		break;
> +	default:
> +		return false;
> +	}
> +
>  	if (!info->is_hdmi &&
>  	    (format != DRM_OUTPUT_COLOR_FORMAT_RGB444 || bpc != 8)) {
>  		drm_dbg_kms(dev, "DVI Monitors require an RGB output at 8 bpc\n");

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260608-hdmi-max-bpc-fix-v1-0-6e8dcebc7274@collabora.com?part=1