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

[Nicolas Frattaroli <nicolas.frattaroli@collabora.com>]

On Wednesday, 25 March 2026 12:03:07 Central European Standard Time 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.
> 
> 

Alright, so in v12 I'll do the following:

- Add a line to all YCBCR connector formats that specifies they're
  limited range as long as Broadcast RGB is limited. Whether it's limited
  range when Broadcast RGB is full is purposefully left undefined.
  In the future, we can expand this to state they're limited range by
  default unless some other property is set. If we're not re-using
  Broadcast RGB for that, this will work out fine, because users who
  don't know about the eventual new property won't have this behaviour
  changed. If we do re-use "Broadcast RGB" for that, then only users
  relying on things we explicitly left undefined will get surprise
  full range YCBCR.
- Add a line to the RGB connector format that specifies its range
  depends on the "Broadcast RGB" property

This is a bit of a mess, because it's entirely reasonable that a
future YCBCR range property would want to default to full range
so that users get the most color out of their monitors. But with
this description of the connector color formats, we can't do that.

If there are alternate suggestions, I'm open for them. We can't
really rename "Broadcast RGB" but if I had a time machine, that'd
be my first choice.

Kind regards,
Nicolas Frattaroli