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

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Date: Fri, 2 Feb 2024 14:01:39 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Sebastian Wick <sebastian.wick@...hat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Daniel Vetter <daniel@...ll.ch>, Emma Anholt <emma@...olt.net>, Jonathan Corbet <corbet@....net>, 
	Sandy Huang <hjc@...k-chips.com>, Heiko Stübner <heiko@...ech.de>, 
	Chen-Yu Tsai <wens@...e.org>, Jernej Skrabec <jernej.skrabec@...il.com>, 
	Samuel Holland <samuel@...lland.org>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, Hans Verkuil <hverkuil@...all.nl>, 
	linux-rockchip@...ts.infradead.org, linux-sunxi@...ts.linux.dev, 
	linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org
Subject: Re: Re: [PATCH v5 08/44] drm/connector: hdmi: Add Broadcast RGB
 property

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.

I guess the best way to reconcile that would be to state that it also
controls the YCbCr range, and i915 can choose to reject/adjust the
configurations it can't support.

Does that make sense?

Maxime

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