linux-kernel - Re: [PATCH v1] media: videobuf2: Allow applications customize data offsets of capture buffers

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Message-ID: <1c1fda82-334a-04ec-fc2e-d1ea2da466e9@xs4all.nl>
Date:   Wed, 18 May 2022 12:26:26 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Nicolas Dufresne <nicolas@...fresne.ca>,
        Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Benjamin Gaignard <benjamin.gaignard@...labora.com>,
        Andrzej Pietrasiewicz <andrzej.p@...labora.com>,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Boris Brezillon <bbrezillon@...labora.com>,
        Daniel Almeida <daniel.almeida@...labora.com>,
        Sebastian Fricke <sebastian.fricke@...labora.com>,
        Laura Nao <laura.nao@...labora.com>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dmitry Osipenko <digetx@...il.com>
Subject: Re: [PATCH v1] media: videobuf2: Allow applications customize data
 offsets of capture buffers



On 3/25/22 13:32, Nicolas Dufresne wrote:
> Le jeudi 24 mars 2022 à 21:20 +0300, Dmitry Osipenko a écrit :
>> The root of the problem is that DRM UAPI is more flexible and allows to
>> customize offsets for both S/MPLANEs, while V4L doesn't allow to do it
>> at all. I'm exploring all the potential options, so far neither of the
>> proposed variants is ideal.
> 
> In GStreamer kmssink, the way DRM is used, is that if you have 2 planes in your
> pixel format, but only received 1 DMABuf, we will pass this DMABuf twice (well
> GEM handles, but twice), with appropriate offset.
> 
> With this in mind, the idea for V4L2 could be to always resort to MPLANE for
> this purpose. The tricky part for userland is that it needs to know the dual
> pixel format and map that accordingly. That is a bit difficult and this is
> something Helen was trying to address with the v4l2_buffer_ext (that and
> allowing space to store DRM Modifiers in the future).

FYI: here is Helen's last patch series. Since Helen is no longer active in
the media subsystem, someone else who is sufficiently motivated would have to
take over.

https://patchwork.linuxtv.org/project/linux-media/cover/20210114180738.1758707-1-helen.koike@collabora.com/

I'm not enthusiastic about messing with data_offset: it was - in hindsight - a
bad idea.

Regards,

	Hans

> 
> The second challenge is the overhead. In DRM, as we "prime" the DMABuf into
> handles, this gives a kernel object to store any relevant information about the
> buffer. So having it duplicate can be done at no cost. In V4L2, the driver would
> need to handle that more often. Specially that despite the recommendation
> (except for primary buffer decoder, were this is mandatory), we don't force a
> strict DMABuf / Buffer IDX  mapping.
> 
> Nicolas