linux-kernel - Re: [PATCH v3 3/9] drm/vc4: hdmi: Add Broadcast RGB property to allow override of RGB range

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Message-ID: <tnkfcgkoobxyyhnsmoyayohip73wl6axnd7liefsvof6qwgovd@ropob4set3nl>
Date:   Mon, 23 Oct 2023 16:58:12 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Daniel Vetter <daniel@...ll.ch>, Emma Anholt <emma@...olt.net>,
        David Airlie <airlied@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>
Subject: Re: [PATCH v3 3/9] drm/vc4: hdmi: Add Broadcast RGB property to
 allow override of RGB range

Hi Hans,

On Thu, Oct 19, 2023 at 10:26:40AM +0200, Hans Verkuil wrote:
> Hi Maxime,
> 
> On 19/10/2023 10:02, Maxime Ripard wrote:
> > Hi,
> > 
> > On Wed, Oct 11, 2023 at 03:23:18PM +0200, Daniel Vetter wrote:
> >> On Mon, 6 Mar 2023 at 11:49, Maxime Ripard <maxime@...no.tech> wrote:
> >>>
> >>> From: Dave Stevenson <dave.stevenson@...pberrypi.com>
> >>>
> >>> Copy Intel's "Broadcast RGB" property semantics to add manual override
> >>> of the HDMI pixel range for monitors that don't abide by the content
> >>> of the AVI Infoframe.
> >>>
> >>> Signed-off-by: Dave Stevenson <dave.stevenson@...pberrypi.com>
> >>> Signed-off-by: Maxime Ripard <maxime@...no.tech>
> >>
> >> Stumbled over this grepping around, but would have been nice to lift
> >> this into drm code and document the property. It's one of the legacy
> >> ones from the table of horrors after all ...
> >>
> >> Shouldn't be an uapi problem because it's copypasted to much, just not great.
> > 
> > We already discussed it on IRC, but just for the record I have a current
> > series that should address exactly that:
> > 
> > https://lore.kernel.org/dri-devel/20230920-kms-hdmi-connector-state-v2-3-17932daddd7d@kernel.org/
> > 
> > Maxime
> 
> I've pasted a snippet from that patch below for a quick review:
> 
> >  /**
> >   * DOC: HDMI connector properties
> >   *
> > + * Broadcast RGB (HDMI Specific):
> 
> Full vs Limited is actually not HDMI specific, DisplayPort can signal this as
> well for whatever it is worth.

Sure, what I (and the original patch I guess) meant is that it's only
ever used on HDMI connectors these days. If that ever changes, then we
can update the doc.

> > + *      Indicates the RGB Range (Full vs Limited) used.
> 
> RGB Range -> RGB Quantization Range
> 
> > + *
> > + *      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:
> 
> It is very unfortunate that this is called "Limited 16:235" instead of just "Limited"
> since for color component bit depths > 8 these values are different.
> 
> I have no idea if it is possible to add an alias "Limited" that you can use instead.
> In any case, this should document that it works just as well for higher bit depths,
> but with different limits.

I had a look and it doesn't look like the property infrastructure can
deal with aliases. I'll add something in the doc

Thanks!
Maxime

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