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Date: Fri, 2 Feb 2024 16:59:30 +0100
From: Maxime Ripard <mripard@kernel.org>
To: Ville =?utf-8?B?U3lyasOkbMOk?= <ville.syrjala@linux.intel.com>
Cc: Sebastian Wick <sebastian.wick@redhat.com>, 
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>, Thomas Zimmermann <tzimmermann@suse.de>, 
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	Jonathan Corbet <corbet@lwn.net>, Sandy Huang <hjc@rock-chips.com>, 
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Subject: Re: Re: Re: [PATCH v5 08/44] drm/connector: hdmi: Add Broadcast RGB
 property
Message-ID: <hez2m57ogqx3yyqk45tzdkvxvhrbdepgm244i4m2aty2xhf5b5@acqgvmxhmmvr>
References: <20231207-kms-hdmi-connector-state-v5-0-6538e19d634d@kernel.org>
 <20231207-kms-hdmi-connector-state-v5-8-6538e19d634d@kernel.org>
 <20240115143308.GA159345@toolbox>
 <20240115143720.GA160656@toolbox>
 <73peztbeeikb3fg6coxu3punxllgtyrmgco34tnxkojtsjbr3s@26bud3sjbcez>
 <Zb0M_2093UwPXK8y@intel.com>
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On Fri, Feb 02, 2024 at 05:40:47PM +0200, Ville Syrj=E4l=E4 wrote:
> On Fri, Feb 02, 2024 at 02:01:39PM +0100, Maxime Ripard wrote:
> > Hi,
> >=20
> > 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) u=
sed.
> > > > > + *      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 sug=
gests,
> > > > > + *              this works for any number of bits-per-component.
> > > > > + *
> > > > > + *      Drivers can set up this property by calling
> > > > > + *      drm_connector_attach_broadcast_rgb_property().
> > > > > + *
> > > >=20
> > > > This is a good time to document this in more detail. There might be=
 two
> > > > different things being affected:
> > > >=20
> > > > 1. The signalling (InfoFrame/SDP/...)
> > > > 2. The color pipeline processing
> > > >=20
> > > > All values of Broadcast RGB always affect the color pipeline proces=
sing
> > > > such that a full-range input to the CRTC is converted to either ful=
l- or
> > > > limited-range, depending on what the monitor is supposed to accept.
> > > >=20
> > > > When automatic is selected, does that mean that there is no signall=
ing,
> > > > or that the signalling matches what the monitor is supposed to acce=
pt
> > > > according to the spec? Also, is this really HDMI specific?
> > > >=20
> > > > When full or limited is selected and the monitor doesn't support the
> > > > signalling, what happens?
> > >=20
> > > 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?
> >=20
> > 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.
>=20
> 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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