linux-kernel - Re: [PATCH] media: Clarify the meaning of file descriptors in VIDIOC

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Date:   Thu, 20 Jun 2019 13:50:30 +0900
From:   Alexandre Courbot <acourbot@...omium.org>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Linux Media Mailing List <linux-media@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Hirokazu Honda <hiroh@...omium.org>
Subject: Re: [PATCH] media: Clarify the meaning of file descriptors in VIDIOC_DQBUF

On Wed, Jun 12, 2019 at 6:36 PM Tomasz Figa <tfiga@...omium.org> wrote:
>
> When the application calls VIDIOC_DQBUF with the DMABUF memory type, the
> v4l2_buffer structure (or v4l2_plane structures) are filled with DMA-buf
> file descriptors. However, the current documentation does not explain
> whether those are new file descriptors referring to the same DMA-bufs or
> just the same integers as passed to VIDIOC_QBUF back in time. Clarify
> the documentation that it's the latter.
>
> Signed-off-by: Tomasz Figa <tfiga@...omium.org>

That's a welcome precision indeed.

Reviewed-by: Alexandre Courbot <acourbot@...omium.org>

> ---
>  Documentation/media/uapi/v4l/vidioc-qbuf.rst | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/Documentation/media/uapi/v4l/vidioc-qbuf.rst b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> index dbf7b445a27b..407302d80684 100644
> --- a/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> +++ b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
> @@ -139,6 +139,14 @@ may continue as normal, but should be aware that data in the dequeued
>  buffer might be corrupted. When using the multi-planar API, the planes
>  array must be passed in as well.
>
> +If the application sets the ``memory`` field to ``V4L2_MEMORY_DMABUF`` to
> +dequeue a :ref:`DMABUF <dmabuf>` buffer, the driver fills the ``m.fd`` field
> +with a file descriptor numerically the same as the one given to ``VIDIOC_QBUF``
> +when the buffer was enqueued. No new file descriptor is created at dequeue time
> +and the value is only for the application convenience. When the multi-planar
> +API is used the ``m.fd`` fields of the passed array of struct
> +:c:type:`v4l2_plane` are filled instead.
> +
>  By default ``VIDIOC_DQBUF`` blocks when no buffer is in the outgoing
>  queue. When the ``O_NONBLOCK`` flag was given to the
>  :ref:`open() <func-open>` function, ``VIDIOC_DQBUF`` returns
> --
> 2.22.0.rc2.383.gf4fbbf30c2-goog
>