[PATCH] drm: document userspace expectations around the Colorspace connector property

[Pekka Paalanen]

On Mon, 12 Feb 2024 17:50:36 +0100
Sebastian Wick <sebastian.wick at redhat.com> wrote:

> On Mon, Feb 12, 2024 at 11:10:15AM +0200, Pekka Paalanen wrote:
> > On Fri,  9 Feb 2024 17:53:07 +0100
> > Xaver Hugl <xaver.hugl at kde.org> wrote:
> >   
> > > Signed-off-by: Xaver Hugl <xaver.hugl at kde.org>
> > > ---
> > >  drivers/gpu/drm/drm_connector.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> > > index b0516505f7ae..01e13984629b 100644
> > > --- a/drivers/gpu/drm/drm_connector.c
> > > +++ b/drivers/gpu/drm/drm_connector.c
> > > @@ -2158,6 +2158,14 @@ EXPORT_SYMBOL(drm_mode_create_aspect_ratio_property);
> > >   *     one supported. Sink supported colorspaces should be retrieved by
> > >   *     userspace from EDID and driver will not explicitly expose them.
> > >   *
> > > + *     As userspace can't currently know whether or not the output is using
> > > + *     RGB or YCC signalling, the driver must translate properties to their
> > > + *     relevant RGB or YCC counterparts, depending on the actually used
> > > + *     signalling. Property values that are only valid for either YCC or RGB
> > > + *     and have no equivalent for the other signalling type must not be
> > > + *     exposed as supported, unless the driver can guarantee it never uses
> > > + *     the signalling that doesn't match the property.
> > > + *
> > >   *     Basically the expectation from userspace is:
> > >   *      - Set up CRTC DEGAMMA/CTM/GAMMA to convert to some sink
> > >   *        colorspace  
> > 
> > While this addition is good, I have another question:
> > 
> > Does "Colorspace" property choose also the RGB->YCbCr matrix that the
> > driver will use when it happens to use YCbCr signalling?
> > 
> > So far we have only been talking about the primaries and white point.  
> 
> Uh, yeah, good point. The InfoFrames do affect both the YCbCr conversion
> and the transfer characteristics that the sink expects. Drivers should
> do the RGB to YCbCr conversion with the new matrix. I think (and very
> much hope) that drivers don't rely on the TF for any internal processing
> but if they did, they also should use the one the sink expects.

Oh, is transfer function also somehow associated here? That would be a
third thing. Are there any "Colorspace" values that target a display
with not gamma 2.2 TF (possibly extended/mirrored)?

The only TF metadata control I've known so far is the one in
HDR_OUTPUT_METADATA.


Thanks,
pq
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