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> --- 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
