Re: [PATCH] drm: disable deep color when EDID violates spec

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Jan 10, 2017 at 12:33:35PM +0100, Ernst Sjöstrand wrote:
> Isn't 10bpc very common among monitors, and 12bpc very rare? Or maybe I'm
> confusing the transport layer with the presentation capabilities...?
> Here are 201 monitors that claim 10bpc:
> http://pricespy.co.uk/category.php?l=s300859434&o=eg_401#prodlista

I suppose that refers to the panel? Not sure.

> 
> Regards
> //Ernst
> 
> 2017-01-10 11:52 GMT+01:00 Ville Syrjälä <ville.syrjala@linux.intel.com>:
> 
> > On Thu, Jan 05, 2017 at 05:45:23PM -0600, Nicholas Sielicki wrote:
> > > As per the HDMI 1.3 and 1.4 spec, "deep color modes" are color depths
> > > greater than 24 bits per pixel. The spec explicitly states, "All Deep
> > > Color modes are optional though if an HDMI Source or Sink supports any
> > > Deep Color mode, it shall support 36-bit mode." (6.2.4 Color Depth
> > > Requirements).
> > >
> > > I came across a monitor (Acer X233H) that supplies an illegal EDID where
> > > DC_30bit is set and DC_36bit is not set. The end result is badly garbled
> > > output because the driver is sending 36BPP when the monitor can't handle
> > > it.
> > >
> > > Much of the intel hardware is incapable of operating at any
> > > bit-per-component setting outside of 8 or 12, and the spec seems to
> > > imply that if any deep color support is found, then it is a safe
> > > assumption to operate at 12.
> > >
> > > This patch ensures that the EDID is within the spec (specifically, that
> > > DC_36bit is set) before committing to going forward with any deep
> > > colors.  There was already a check for this EDID inconsistency, but it
> > > resulted only in a warning and did not fall-back to safer settings.
> > >
> > > CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > Signed-off-by: Nicholas Sielicki <nicholas.sielicki@gmail.com>
> > > ---
> > >  drivers/gpu/drm/drm_edid.c | 35 +++++++++++++++++++++++------------
> > >  1 file changed, 23 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> > > index 336be31ff3de..42ce3f54d2dc 100644
> > > --- a/drivers/gpu/drm/drm_edid.c
> > > +++ b/drivers/gpu/drm/drm_edid.c
> > > @@ -3772,30 +3772,34 @@ static void drm_parse_hdmi_deep_color_info(struct
> > drm_connector *connector,
> > >  {
> > >       struct drm_display_info *info = &connector->display_info;
> > >       unsigned int dc_bpc = 0;
> > > +     u8 modes = 0;
> > >
> > >       /* HDMI supports at least 8 bpc */
> > >       info->bpc = 8;
> > >
> > > +     /* Ensure all DC modes are unset if we return early */
> > > +     info->edid_hdmi_dc_modes = 0;
> >
> > Clearing this in drm_add_display_info() should be sufficient since
> > this guy doesn't get called from anywhere else. So this part could
> > be droppped.
> >
> > Otherwise this feels like a decent way to handle the problem. We
> > could of course try to use the 10bpc (or whatever) deep color modes
> > the sink claims to support, but given that the people designing the
> > thing didn't bother reading the spec, it seems safer to just disable
> > deep color support entirely.
> >
> > Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >
> > > +
> > >       if (cea_db_payload_len(hdmi) < 6)
> > >               return;
> > >
> > >       if (hdmi[6] & DRM_EDID_HDMI_DC_30) {
> > >               dc_bpc = 10;
> > > -             info->edid_hdmi_dc_modes |= DRM_EDID_HDMI_DC_30;
> > > +             modes |= DRM_EDID_HDMI_DC_30;
> > >               DRM_DEBUG("%s: HDMI sink does deep color 30.\n",
> > >                         connector->name);
> > >       }
> > >
> > >       if (hdmi[6] & DRM_EDID_HDMI_DC_36) {
> > >               dc_bpc = 12;
> > > -             info->edid_hdmi_dc_modes |= DRM_EDID_HDMI_DC_36;
> > > +             modes |= DRM_EDID_HDMI_DC_36;
> > >               DRM_DEBUG("%s: HDMI sink does deep color 36.\n",
> > >                         connector->name);
> > >       }
> > >
> > >       if (hdmi[6] & DRM_EDID_HDMI_DC_48) {
> > >               dc_bpc = 16;
> > > -             info->edid_hdmi_dc_modes |= DRM_EDID_HDMI_DC_48;
> > > +             modes |= DRM_EDID_HDMI_DC_48;
> > >               DRM_DEBUG("%s: HDMI sink does deep color 48.\n",
> > >                         connector->name);
> > >       }
> > > @@ -3806,9 +3810,24 @@ static void drm_parse_hdmi_deep_color_info(struct
> > drm_connector *connector,
> > >               return;
> > >       }
> > >
> > > +     /*
> > > +      * All deep color modes are optional, but if a sink supports any
> > deep
> > > +      * color mode, it must support 36-bit mode. If this is found not
> > > +      * to be the case, sink is in violation of HDMI 1.3 / 1.4 spec and
> > it
> > > +      * is prudent to disable all deep color modes. Return here before
> > > +      * committing bpc and edid_hdmi_dc_modes.
> > > +      */
> > > +     if (!(modes & DRM_EDID_HDMI_DC_36)) {
> > > +             DRM_DEBUG("%s: HDMI sink should do DC_36, but does not!\n",
> > > +                       connector->name);
> > > +             return;
> > > +     }
> > > +
> > > +
> > >       DRM_DEBUG("%s: Assigning HDMI sink color depth as %d bpc.\n",
> > >                 connector->name, dc_bpc);
> > >       info->bpc = dc_bpc;
> > > +     info->edid_hdmi_dc_modes = modes;
> > >
> > >       /*
> > >        * Deep color support mandates RGB444 support for all video
> > > @@ -3823,15 +3842,6 @@ static void drm_parse_hdmi_deep_color_info(struct
> > drm_connector *connector,
> > >               DRM_DEBUG("%s: HDMI sink does YCRCB444 in deep color.\n",
> > >                         connector->name);
> > >       }
> > > -
> > > -     /*
> > > -      * Spec says that if any deep color mode is supported at all,
> > > -      * then deep color 36 bit must be supported.
> > > -      */
> > > -     if (!(hdmi[6] & DRM_EDID_HDMI_DC_36)) {
> > > -             DRM_DEBUG("%s: HDMI sink should do DC_36, but does not!\n",
> > > -                       connector->name);
> > > -     }
> > >  }
> > >
> > >  static void
> > > @@ -3895,6 +3905,7 @@ static void drm_add_display_info(struct
> > drm_connector *connector,
> > >       /* driver figures it out in this case */
> > >       info->bpc = 0;
> > >       info->color_formats = 0;
> > > +     info->edid_hdmi_dc_modes = 0;
> > >       info->cea_rev = 0;
> > >       info->max_tmds_clock = 0;
> > >       info->dvi_dual = false;
> > > --
> > > 2.11.0
> >
> > --
> > Ville Syrjälä
> > Intel OTC
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >

-- 
Ville Syrjälä
Intel OTC
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel