On Monday, 8 June 2026 13:30:22 Central European Summer Time
[email protected] wrote:
> 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 <[email protected]>
>
> 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?
Finally, we have automated the ultimate Linux kernel review response:
"while you're at it, can you fix everything else wrong in this code?"
(No, this does not expose any pre-existing issue; the validation is
more restrictive than it was before. That it's still not restrictive
enough in all cases does not introduce a new problem.)
>
> > + 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");
>
>
