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    SubjectRe: Re: Re: [PATCH v5 08/44] drm/connector: hdmi: Add Broadcast RGB property
    On Fri, Feb 02, 2024 at 05:40:47PM +0200, Ville Syrjälä wrote:
    > 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.

    Right. And the limited range option is called "Limited 16:235" despite
    being usable on bpc > 8 bits. Naming errors occurs, and history happens
    to make names inconsistent too, that's fine and not an argument in
    itself.

    > Full range YCbCr is a much rarer beast so we've never bothered
    > to enable it.

    vc4 supports 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.

    This is for HDMI only.

    > I don't recall right now what the HDMI requirements are.

    HDMI has supported it for a while, and it's defined (for example) in the
    HDMI 1.4 spec in Section 6.6 - Video Quantization Ranges. It supports
    limited and full range on both RGB and YCbCr, as long as the EDIDs state
    so and the Infoframes signal it.

    Maxime
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