On Monday, 8 June 2026 13:46:27 Central European Summer Time Nicolas Frattaroli
wrote:
> 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.)
Correction: there isn't even any problem here. DC48 is the highest
bpc, so the check is redundant anyway, which is likely also why it
was omitted.
Bad clanker! Bad bad bad!
>
> >
> > > + 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");
> >
> >
>
>
