media: Clarify the meaning of file descriptors in VIDIOC_DQBUF

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@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

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