On Tue, Mar 31, 2026 at 12:33:00PM +0200, Nicolas Frattaroli wrote:
> On Tuesday, 31 March 2026 01:56:16 Central European Summer Time Ville Syrjälä
> wrote:
> > On Sat, Mar 28, 2026 at 02:49:04AM +0200, Ville Syrjälä wrote:
> > > On Fri, Mar 27, 2026 at 01:56:06PM +0100, Nicolas Frattaroli wrote:
> > > > On Thursday, 26 March 2026 18:58:25 Central European Standard Time
> > > > Ville Syrjälä wrote:
> > > > > On Thu, Mar 26, 2026 at 06:02:47PM +0100, Maxime Ripard wrote:
> > > > > > On Wed, Mar 25, 2026 at 08:43:15PM +0200, Ville Syrjälä wrote:
> > > > > > > On Wed, Mar 25, 2026 at 03:56:58PM +0100, Maxime Ripard wrote:
> > > > > > > > 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.
> > > > > > > >
> > > > > > > > Ack
> > > > > > > >
> > > > > > > > > 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
> > > > > > > >
> > > > > > > > Isn't it much more complicated than that for HDMI though? My
> > > > > > > > recollection was that any VIC but VIC1 would be limited range,
> > > > > > > > and
> > > > > > > > anything else full range?
> > > > > > >
> > > > > > > Do we have some driver that implements the CTA-861 CE vs. IT mode
> > > > > > > logic but doesn't expose the "Broadcast RGB" property? I was
> > > > > > > hoping
> > > > > > > those would always go hand in hand now.
> > > > > >
> > > > > > I'm not sure. i915 and the HDMI state helpers handle it properly (I
> > > > > > think?) but it looks like only vc4 registers the Broadcast RGB
> > > > > > property
> > > > > > and uses the HDMI state helpers.
> > > > > >
> > > > > > And it looks like amdgpu registers Broadcast RGB but doesn't use
> > > > > > drm_default_rgb_quant_range() which seems suspicious?
> > > > >
> > > > > If they want just manual full vs. limited then they should
> > > > > limit the property to not expose the "auto" option at all.
> > > > >
> > > > > amdgpu also ties this in with the "colorspace" property, which
> > > > > originally in i915 only controlled the infoframes/etc. But on
> > > > > amdgpu it now controls various aspects of output color
> > > > > transformation. The end result is that the property is a complete
> > > > > mess with most of the values making no sense. And for whatever
> > > > > reason everyone involved refused to remove/deprecate the
> > > > > nonsensical values :/
> > > > >
> > > > > Looks like this series should make sure the documentation for
> > > > > the "colorspace" property is in sync with the new property
> > > > > as well. Currently now it's giving conflicting information.
> > > > >
> > > >
> > > > I take it the problematic information is in
> > > >
> > > > * DOC: standard connector properties
> > > > *
> > > > * Colorspace:
> > > >
> > > > and probably specifically BT2020_YCC's (and BT2020_RGB's?) insistence
> > > > that they "produce RGB content".
> > > >
> > > > I think we probably just have to change the statement "The variants
> > > > BT2020_RGB and BT2020_YCC are equivalent and the driver chooses between
> > > > RGB and YCbCr on its own."
> > > >
> > > > The "on its own" here would get turned into "based on the color format
> > > > property".
> > > >
> > > > Speaking of i915, that patch is one of the very few (5) patches in
> > > > this series still lacking a review (hint hint nudge nudge). I'd like
> > > > to get some more feedback on the remaining patches before I send out
> > > > another revision, so that it's hopefully not just docs changes (I
> > > > know better than to think those patches must be perfect and won't
> > > > need revision.)
> > >
> > > The i915 code around this is already a big mess, and I don't really
> > > adding to that mess. So I think we'll need to do some refactoring before
> > > we add anything there. I already started typing something and so far
> > > it looks fairly straightforward, so I should have something soon.
> >
> > OK, posted something
> > https://lore.kernel.org/intel-gfx/[email protected]/T/#m7c349478ca6c856fbc68d5e2178f1aa31678a05f
>
> Thanks! I'll take a look at this today to get a more solid idea of
> where the pain points you highlighted are.
>
> I'll also rebase/reimplement my i915 color format implementation
> (sans the DP-MST part, as discussed) on top of this on the next
> revision. I was never fully happy with the current one due to the
> logic being shoehorned into the already existing i915 fallback
> format logic, so I'm quite happy to have another opportunity to
> implement it with less historic baggage.
>
> > Are the wayland/compositor/color management folks on board with
> > these new properties? I don't think I see the usual suspects on
> > the cc list.
>
> I don't know which precise group of people you refer to,
Off the top of my head, Pekka,Simon,Sebastian,Jonas,Xaver might be
relevant here. Added to Cc...
> but at
> least from the Collabora side of things, the userspace Wayland
> people are on board with these new properties. In Weston, we use
> it to implement the Weston frontend's "color-format" option in a
> WIP branch at
>
> https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/1859
>
> I've also been made aware that LibreELEC is aware, and will look
> into making use of it rather than their own kernel patches.
>
> Kind regards,
> Nicolas Frattaroli
>
> > >
> > > While doing that several questions came to my mind though:
> > >
> > > * More interactions with the colorspace property, but I sent
> > > a separate mail already about that
> > >
> > > * Which conversion matrix to use, and the answer I suspect
> > > should be "ask the colorspace property", as mentioned in the
> > > other mail
> > >
> > > * Should we flat out reject color formats (and I suppose also
> > > colorspace prop values) the sink doesn't claim to support?
> > >
> > > If yes, then I think we'll have to forget about adding anything
> > > to i915 MST code. The way the MST stuff works is that if one
> > > stream needs a modeset then all the related streams get modeset
> > > as well. Thus if the user replaces a monitor getting fed with a
> > > YCbCr stream just as another stream is being modeset, then the
> > > entire atomic commit could fail due to the YCbCr stream getting
> > > rejected.
> > >
> > > I think eventually we might have to invent some mechanism where
> > > all the input into the modeset computation is cached somehow,
> > > and said cache updated only on explicit userspace modesets.
> > > Either that or we have to come up with a way to skip some of
> > > the calculations that depend on external factors. Either way
> > > it's going to be a pain.
> > >
> > > OTOH if we don't mind feeding the sink with stuff it can't
> > > understand, then I suppose we might add YCbCr 4:4:4 support
> > > for MST. It shouldn't be any different from RGB apart from
> > > the RGB->YCbCr conversion, which is handled elsewhere. But
> > > YCbCr 4:2:0 is definitely out either way, the MST code has
> > > no support for that currently.
> > >
> >
> >
>
>
>
--
Ville Syrjälä
Intel
