Re: [PATCH v11 03/22] drm: Add new general DRM property "color format"

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

On Wed, Mar 25, 2026 at 01:03:07PM +0200, Ville Syrjälä wrote:
> On Wed, Mar 25, 2026 at 09:24:27AM +0100, Maxime Ripard wrote:
> > On Tue, Mar 24, 2026 at 09:53:35PM +0200, Ville Syrjälä wrote:
> > > On Tue, Mar 24, 2026 at 08:10:11PM +0100, Nicolas Frattaroli wrote:
> > > > On Tuesday, 24 March 2026 18:00:45 Central European Standard Time Ville Syrjälä wrote:
> > > > > On Tue, Mar 24, 2026 at 05:01:07PM +0100, Nicolas Frattaroli wrote:
> > > > > > +enum drm_connector_color_format {
> > > > > > +	/**
> > > > > > +	 * @DRM_CONNECTOR_COLOR_FORMAT_AUTO: The driver or display protocol
> > > > > > +	 * helpers should pick a suitable color format. All implementations of a
> > > > > > +	 * specific display protocol must behave the same way with "AUTO", but
> > > > > > +	 * different display protocols do not necessarily have the same "AUTO"
> > > > > > +	 * semantics.
> > > > > > +	 *
> > > > > > +	 * For HDMI, "AUTO" picks RGB, but falls back to YCbCr 4:2:0 if the
> > > > > > +	 * bandwidth required for full-scale RGB is not available, or the mode
> > > > > > +	 * is YCbCr 4:2:0-only, as long as the mode and output both support
> > > > > > +	 * YCbCr 4:2:0.
> > > > > > +	 *
> > > > > > +	 * For display protocols other than HDMI, the recursive bridge chain
> > > > > > +	 * format selection picks the first chain of bridge formats that works,
> > > > > > +	 * as has already been the case before the introduction of the "color
> > > > > > +	 * format" property. Non-HDMI bridges should therefore either sort their
> > > > > > +	 * bus output formats by preference, or agree on a unified auto format
> > > > > > +	 * selection logic that's implemented in a common state helper (like
> > > > > > +	 * how HDMI does it).
> > > > > > +	 */
> > > > > > +	DRM_CONNECTOR_COLOR_FORMAT_AUTO = 0,
> > > > > > +
> > > > > > +	/**
> > > > > > +	 * @DRM_CONNECTOR_COLOR_FORMAT_RGB444: RGB output format
> > > > > > +	 */
> > > > > > +	DRM_CONNECTOR_COLOR_FORMAT_RGB444,
> > > > > > +
> > > > > > +	/**
> > > > > > +	 * @DRM_CONNECTOR_COLOR_FORMAT_YCBCR444: YCbCr 4:4:4 output format (ie.
> > > > > > +	 * not subsampled)
> > > > > > +	 */
> > > > > > +	DRM_CONNECTOR_COLOR_FORMAT_YCBCR444,
> > > > > > +
> > > > > > +	/**
> > > > > > +	 * @DRM_CONNECTOR_COLOR_FORMAT_YCBCR422: YCbCr 4:2:2 output format (ie.
> > > > > > +	 * with horizontal subsampling)
> > > > > > +	 */
> > > > > > +	DRM_CONNECTOR_COLOR_FORMAT_YCBCR422,
> > > > > > +
> > > > > > +	/**
> > > > > > +	 * @DRM_CONNECTOR_COLOR_FORMAT_YCBCR420: YCbCr 4:2:0 output format (ie.
> > > > > > +	 * with horizontal and vertical subsampling)
> > > > > > +	 */
> > > > > > +	DRM_CONNECTOR_COLOR_FORMAT_YCBCR420,
> > > > > 
> > > > > Seems like this should document what the quantization range
> > > > > should be for each format.
> > > > > 
> > > > 
> > > > I don't think so? If you want per-component bit depth values,
> > > > DRM_FORMAT_* defines would be the appropriate values to use. This
> > > > enum is more abstract than that, and is there to communicate
> > > > YUV vs. RGB and chroma subsampling, with bit depth being handled
> > > > by other properties.
> > > > 
> > > > If you mean the factor used for subsampling, then that'd only be
> > > > relevant if YCBCR410 was supported where one chroma plane isn't
> > > > halved but quartered in resolution. I suspect 4:1:0 will never
> > > > be added; no digital display protocol standard supports it to my
> > > > knowledge, and hopefully none ever will.
> > > 
> > > No, I mean the quantization range (16-235 vs. 0-255 etc).
> > > 
> > > The i915 behaviour is that YCbCr is always limited range,
> > > RGB can either be full or limited range depending on the 
> > > "Broadcast RGB" property and other related factors.
> > 
> > So far the HDMI state has both the format and quantization range as
> > different fields. I'm not sure we need to document the range in the
> > format field, maybe only mention it's not part of the format but has a
> > field of its own?
> 
> I think we only have it for RGB (on some drivers only?). For YCbCr
> I think the assumption is limited range everywhere.
> 
> But I'm not really concerned about documenting struct members.
> What I'm talking about is the *uapi* docs. Surely userspace
> will want to know what the new property actually does so the
> uapi needs to be documented properly. And down the line some
> new driver might also implement the wrong behaviour if there
> is no clear specification.
> 
> So I'm thinking (or perhaps hoping) the rule might be something like:
> - YCbCr limited range 
> - RGB full range if "Broadcast RGB" property is not present
> - RGB full or limited range based on the "Broadcast RGB" property
>   if it's present
> 
> I think the "Broadcast RGB" property itself might also be lacking
> proper uapi docs, so that may need to be remedied as well.

Oh, and I think a bunch of infoframe code still needs changes to 
set up the quantization range properly for YCbCr. i915 does handle
that part correctly, but eg. hdmi_generate_avi_infoframe() does not.
I didn't spot any changes to that in the series.

-- 
Ville Syrjälä
Intel