On Wed, Jun 12, 2019 at 6:36 PM Tomasz Figa <tfiga@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> That's a welcome precision indeed. Reviewed-by: Alexandre Courbot <acourbot@xxxxxxxxxxxx> > --- > 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 >
