On Thu, Mar 26, 2026 at 01:44:03PM +0100, Nicolas Frattaroli wrote:
> 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.
"Broadcast RGB", as the name implies, only affects RGB output.
> 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
>
--
Ville Syrjälä
Intel
