linux-kernel - Re: [PATCH RFC v3 12/37] drm/connector: hdmi: Create Infoframe DebugFS entries

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Message-ID: <b720744a-64d0-4d1c-8457-e6250e768efe@xs4all.nl>
Date:   Mon, 6 Nov 2023 15:55:15 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Maxime Ripard <mripard@...nel.org>
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>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-rockchip@...ts.infradead.org, linux-sunxi@...ts.linux.dev
Subject: Re: [PATCH RFC v3 12/37] drm/connector: hdmi: Create Infoframe
 DebugFS entries

On 06/11/2023 15:44, Maxime Ripard wrote:
> Hi Hans,
> 
> On Fri, Nov 03, 2023 at 10:05:18AM +0100, Hans Verkuil wrote:
>> Hi Maxime,
>>
>> Thank you for posting v3, this time it runs fine on my RPi 4, thank you for
>> fixing that.
>>
>> I'll start working on a conformity checker for this.
> 
> Awesome :)

And anyone who is interested can find the work-in-progress for that here:

git://linuxtv.org/hverkuil/edid-decode.git

Use the -I option to point to the file containing the infoframe to parse
it. Later it will also check the conformity of the infoframe against the
display's EDID. That's why this is part of edid-decode since you need an
EDID parser to verify if the InfoFrames are also conform the standards.

> 
>>> +static int create_hdmi_infoframe_files(struct drm_connector *connector,
>>> +				       struct dentry *parent)
>>> +{
>>> +	int ret;
>>> +
>>> +	ret = CREATE_HDMI_INFOFRAME_FILE(connector, parent, audio);
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	ret = CREATE_HDMI_INFOFRAME_FILE(connector, parent, avi);
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	ret = CREATE_HDMI_INFOFRAME_FILE(connector, parent, drm);
>>
>> Hmm, I had to look into the code to figure out that 'drm' stands for
>> Dynamic Range and Mastering InfoFrame. While entirely correct, it is
>> also very confusing in the context of the 'drm' subsystem.
>>
>> I am not quite certain what the best approach is here.
>>
>> Internally in the drm code it is talking about 'hdr' or 'hdr metadata',
>> but that's a bit confusing as well since there is also an HDR Dynamic
>> Metadata Extended InfoFrame defined in CTA-861, even though support for
>> that is not (yet) implemented in drm.
>>
>> At minimum there should be a comment in the code explaining what drm
>> stands for in this context.
>>
>> One option to consider is renaming this file to hdr_drm, thus indicating
>> that this is HDR related.
> 
> I ended up doing both, thanks for the suggestion
> 
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	ret = CREATE_HDMI_INFOFRAME_FILE(connector, parent, spd);
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	ret = CREATE_HDMI_INFOFRAME_FILE(connector, parent, vendor);
>>
>> There may be multiple vendor specific InfoFrames in the future, so how
>> would that be handled? Perhaps add a comment here that currently only one
>> vendor specific InfoFrame is supported, but suggest how to handle multiple
>> VSIFs in the future.
>>
>> What would actually be nice (although probably not that easy to fix) is if
>> the name of the file would be "vendor-XXXXXX' where 'XXXXXX' is the IEEE OUI
>> number.
> 
> I guess it's not entirely clear to me what that would look like. In
> order for the framework to create the debugfs files, we would need some
> enumeration mechanism (probably through a callback?), and then the
> driver would generate the entire content of that file.
> 
> Which makes me question whether the framework should be initialised at
> all? Maybe the simpler would be to just have drivers maintain their own
> debugfs files and storing the content in their own, private, structure.

It's a good question. And I also would like to use this for HDMI receivers
(so using it in the media subsystem).

Let me try and find some time this week or next week to dig into this a bit
more and see if I can implement it on the media side as well. See what would
be the best approach here.

It would not surprise me if the debugfs part should move to video/hdmi.c
since that's already used by both drm and media.

Regards,

	Hans

> 
> Maxime