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Date: Mon, 12 Feb 2024 11:01:07 +0100
From: Maxime Ripard <mripard@kernel.org>
To: Sebastian Wick <sebastian.wick@redhat.com>
Cc: Ville =?utf-8?B?U3lyasOkbMOk?= <ville.syrjala@linux.intel.com>, 
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>, Thomas Zimmermann <tzimmermann@suse.de>, 
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Subject: Re: Re: Re: Re: Re: [PATCH v5 08/44] drm/connector: hdmi: Add
 Broadcast RGB property
Message-ID: <ahfl6f72lpgpsbnrbgvbsh4db4npr2hh36kua2c6krh544hv5r@dndw4hz2mu2g>
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>
 <hez2m57ogqx3yyqk45tzdkvxvhrbdepgm244i4m2aty2xhf5b5@acqgvmxhmmvr>
 <Zb0aYAapkxQ2kopt@intel.com>
 <zml6j27skvjmbrfyz7agy5waxajv4p4asbemeexelm3wuv4o7j@xkd2wvnxhbuc>
 <20240209203435.GB996172@toolbox>
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On Fri, Feb 09, 2024 at 09:34:35PM +0100, Sebastian Wick wrote:
> On Mon, Feb 05, 2024 at 10:39:38AM +0100, Maxime Ripard wrote:
> > On Fri, Feb 02, 2024 at 06:37:52PM +0200, Ville Syrj=E4l=E4 wrote:
> > > On Fri, Feb 02, 2024 at 04:59:30PM +0100, Maxime Ripard wrote:
> > > > 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 Li=
mited) used.
> > > > > > > > > + *      Infoframes will be generated according to that v=
alue.
> > > > > > > > > + *
> > > > > > > > > + *      The value of this property can be one of the fol=
lowing:
> > > > > > > > > + *
> > > > > > > > > + *      Automatic:
> > > > > > > > > + *              RGB Range is selected automatically base=
d 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-co=
mponent.
> > > > > > > > > + *
> > > > > > > > > + *      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 pipelin=
e processing
> > > > > > > > such that a full-range input to the CRTC is converted to ei=
ther full- or
> > > > > > > > limited-range, depending on what the monitor is supposed to=
 accept.
> > > > > > > >=20
> > > > > > > > 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?
> > > > > > > >=20
> > > > > > > > When full or limited is selected and the monitor doesn't su=
pport the
> > > > > > > > signalling, what happens?
> > > > > > >=20
> > > > > > > Forgot to mention: user-space still has no control over RGB v=
s 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 rang=
e 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.
> > > >=20
> > > > Right. And the limited range option is called "Limited 16:235" desp=
ite
> > > > being usable on bpc > 8 bits. Naming errors occurs, and history hap=
pens
> > > > to make names inconsistent too, that's fine and not an argument in
> > > > itself.
> > > >=20
> > > > > Full range YCbCr is a much rarer beast so we've never bothered
> > > > > to enable it.
> > > >=20
> > > > vc4 supports it.
> > >=20
> > > Someone implemented it incorrectly then.
> >=20
> > Incorrectly according to what documentation / specification? I'm sorry,
> > but I find it super ironic that i915 gets to do its own thing, not
> > document any of it, and when people try to clean things up they get told
> > that we got it all wrong.
>=20
> FWIW, this was an i915 property and if another driver uses the same
> property name it must have the same behavior. Yes, it isn't standardized
> and yes, it's not documented (hence this effort here) but it's still on
> vc4 to make the property compatible.

How is it not compatible? It's a superset of what i915 provides, but
it's strictly compatible with it.

I would argue that i915 is the broken one since userspace could force a
full range output, but since the driver takes the YUV vs RGB decision
itself and only supports limited range for YUV, the driver would
effectively ignore that user-space property, without the user-space
being able to tell it was ignored in the first place.

> Trying to make the property handle YCbCr is very much in the "let's try
> to fix the property" territory that I want to avoid, so I'm in favor of
> adjusting vc4.

Breaking the ABI in the process. For something that is explicitly
supported by the spec, the driver, and the hardware. On a property that
never said it wasn't meant to be used that way, and with semantics based
on a driver that never provided a way to check those restrictions in the
first place.

And it's not like i915 is going to use that code anyway.

Maxime

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