Re: [PATCH v2 01/14] drm/edid: Don't clear YUV422 if using deep color

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

On Wed, Dec 15, 2021 at 01:43:53PM +0100, Maxime Ripard wrote:
> The current code, when parsing the EDID Deep Color depths, that the
> YUV422 cannot be used, referring to the HDMI 1.3 Specification.
> 
> This specification, in its section 6.2.4, indeed states:
> 
>   For each supported Deep Color mode, RGB 4:4:4 shall be supported and
>   optionally YCBCR 4:4:4 may be supported.
> 
>   YCBCR 4:2:2 is not permitted for any Deep Color mode.
> 
> This indeed can be interpreted like the code does, but the HDMI 1.4
> specification further clarifies that statement in its section 6.2.4:
> 
>   For each supported Deep Color mode, RGB 4:4:4 shall be supported and
>   optionally YCBCR 4:4:4 may be supported.
> 
>   YCBCR 4:2:2 is also 36-bit mode but does not require the further use
>   of the Deep Color modes described in section 6.5.2 and 6.5.3.
> 
> This means that, even though YUV422 can be used with 12 bit per color,
> it shouldn't be treated as a deep color mode.
> 
> This deviates from the interpretation of the code and comment, so let's
> fix those.
> 
> Fixes: d0c94692e0a3 ("drm/edid: Parse and handle HDMI deep color modes.")
> Signed-off-by: Maxime Ripard <maxime@cerno.tech>
> ---
>  drivers/gpu/drm/drm_edid.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 12893e7be89b..e57d1b8cdaaa 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -5106,10 +5106,9 @@ static void drm_parse_hdmi_deep_color_info(struct drm_connector *connector,
>  
>  	/*
>  	 * Deep color support mandates RGB444 support for all video
> -	 * modes and forbids YCRCB422 support for all video modes per
> -	 * HDMI 1.3 spec.
> +	 * modes.
>  	 */
> -	info->color_formats = DRM_COLOR_FORMAT_RGB444;
> +	info->color_formats |= DRM_COLOR_FORMAT_RGB444;
>  
>  	/* YCRCB444 is optional according to spec. */
>  	if (hdmi[6] & DRM_EDID_HDMI_DC_Y444) {

This whole code seems pretty much wrong. What it tries to do (I think)
is make sure we don't use deep color with YCbCr 4:4:4 unless supported.
But what it actually does is also disable YCbCr 4:4:4 8bpc when deep
color is not supported for YCbCr 4:4:4.

I think what we want is to just get rid of this color_formats stuff here
entirely and instead have some kind of separate tracking of RGB 4:4:4 vs.
YCbCr 4:4:4 deep color capabilities.

-- 
Ville Syrjälä
Intel