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

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Date:   Fri, 25 Mar 2022 13:11:13 +0000
From:   Dave Stevenson <dave.stevenson@...pberrypi.com>
To:     Nicolas Dufresne <nicolas@...fresne.ca>
Cc:     Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        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>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Osipenko <digetx@...il.com>
Subject: Re: [PATCH v1] media: videobuf2: Allow applications customize data
 offsets of capture buffers

Hi Nicolas & Dmitry

On Fri, 25 Mar 2022 at 12:32, Nicolas Dufresne <nicolas@...fresne.ca> 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).
>
> 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.

To throw another use case of data offsets into the mix, I'm keeping a
watching eye on implementing stereoscopic capture into libcamera where
we want to present the same buffer to the ISP twice (once for each
eye) with either a vertical or horizontal offset between the two
passes.
Adding a data_offset of either a half line or half the frame is the
easiest way around that one, although it could potentially be
accommodated through the selection API setting a compose region
instead.

  Dave