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

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Date:   Thu, 19 Oct 2023 10:26:40 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Maxime Ripard <maxime@...no.tech>, Daniel Vetter <daniel@...ll.ch>
Cc:     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 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.

> + *      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.

Regards,

	Hans

> + *              Limited RGB Range is forced.
> + *
> + *      Drivers can set up this property by calling
> + *      drm_connector_attach_broadcast_rgb_property().
> + *
>   * content type (HDMI specific):
>   *	Indicates content type setting to be used in HDMI infoframes to indicate
>   *	content type for the external device, so that it adjusts its display