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

From: Ville Syrjälä <[email-masked]>
Date: Thu, 26 Mar 2026 15:07:15 +0200

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