Re: Re: [PATCH v5 08/44] drm/connector: hdmi: Add Broadcast RGB property

[Ville Syrjälä]

On Fri, Feb 02, 2024 at 02:01:39PM +0100, Maxime Ripard wrote:
> Hi,
> 
> On Mon, Jan 15, 2024 at 03:37:20PM +0100, Sebastian Wick wrote:
> > > >  /**
> > > >   * DOC: HDMI connector properties
> > > >   *
> > > > + * Broadcast RGB
> > > > + *      Indicates the RGB Quantization Range (Full vs Limited) used.
> > > > + *      Infoframes will be generated according to that value.
> > > > + *
> > > > + *      The value of this property can be one of the following:
> > > > + *
> > > > + *      Automatic:
> > > > + *              RGB Range is selected automatically based on the mode
> > > > + *              according to the HDMI specifications.
> > > > + *
> > > > + *      Full:
> > > > + *              Full RGB Range is forced.
> > > > + *
> > > > + *      Limited 16:235:
> > > > + *              Limited RGB Range is forced. Unlike the name suggests,
> > > > + *              this works for any number of bits-per-component.
> > > > + *
> > > > + *      Drivers can set up this property by calling
> > > > + *      drm_connector_attach_broadcast_rgb_property().
> > > > + *
> > > 
> > > This is a good time to document this in more detail. There might be two
> > > different things being affected:
> > > 
> > > 1. The signalling (InfoFrame/SDP/...)
> > > 2. The color pipeline processing
> > > 
> > > All values of Broadcast RGB always affect the color pipeline processing
> > > such that a full-range input to the CRTC is converted to either full- or
> > > limited-range, depending on what the monitor is supposed to accept.
> > > 
> > > When automatic is selected, does that mean that there is no signalling,
> > > or that the signalling matches what the monitor is supposed to accept
> > > according to the spec? Also, is this really HDMI specific?
> > > 
> > > When full or limited is selected and the monitor doesn't support the
> > > signalling, what happens?
> > 
> > Forgot to mention: user-space still has no control over RGB vs YCbCr on
> > the cable, so is this only affecting RGB? If not, how does it affect
> > YCbCr?
> 
> So I dug a bit into both the i915 and vc4 drivers, and it looks like if
> we're using a YCbCr format, i915 will always use a limited range while
> vc4 will follow the value of the property.

The property is literally called "Broadcast *RGB*".
That should explain why it's only affecting RGB.

Full range YCbCr is a much rarer beast so we've never bothered
to enable it. Eg. with DP it only became possible with the
introduction of the VSC SDP (and I don't recall if there's
additional capability checks that are also required). With
DP MSA signalling full range YCbCr is not possible at all.
I don't recall right now what the HDMI requirements are.

-- 
Ville Syrjälä
Intel