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

[Ville Syrjälä]

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 at linux.intel.com>
> Signed-off-by: Nicholas Sielicki <nicholas.sielicki at 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 at 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