xref: /kernel/linux/linux-5.10/Documentation/userspace-api/media/v4l/vidioc-expbuf.rst (revision 8c2ecf20)

.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L

.. _VIDIOC_EXPBUF:

*******************
ioctl VIDIOC_EXPBUF
*******************

Name
====

VIDIOC_EXPBUF - Export a buffer as a DMABUF file descriptor.

Synopsis
========

.. c:macro:: VIDIOC_EXPBUF

``int ioctl(int fd, VIDIOC_EXPBUF, struct v4l2_exportbuffer *argp)``

Arguments
=========

``fd``
    File descriptor returned by :c:func:`open()`.

``argp``
    Pointer to struct :c:type:`v4l2_exportbuffer`.

Description
===========

This ioctl is an extension to the :ref:`memory mapping <mmap>` I/O
method, therefore it is available only for ``V4L2_MEMORY_MMAP`` buffers.
It can be used to export a buffer as a DMABUF file at any time after
buffers have been allocated with the
:ref:`VIDIOC_REQBUFS` ioctl.

To export a buffer, applications fill struct
:c:type:`v4l2_exportbuffer`. The ``type`` field is
set to the same buffer type as was previously used with struct
:c:type:`v4l2_requestbuffers` ``type``.
Applications must also set the ``index`` field. Valid index numbers
range from zero to the number of buffers allocated with
:ref:`VIDIOC_REQBUFS` (struct
:c:type:`v4l2_requestbuffers` ``count``) minus
one. For the multi-planar API, applications set the ``plane`` field to
the index of the plane to be exported. Valid planes range from zero to
the maximal number of valid planes for the currently active format. For
the single-planar API, applications must set ``plane`` to zero.
Additional flags may be posted in the ``flags`` field. Refer to a manual
for open() for details. Currently only O_CLOEXEC, O_RDONLY, O_WRONLY,
and O_RDWR are supported. All other fields must be set to zero. In the
case of multi-planar API, every plane is exported separately using
multiple :ref:`VIDIOC_EXPBUF` calls.

After calling :ref:`VIDIOC_EXPBUF` the ``fd`` field will be set by a
driver. This is a DMABUF file descriptor. The application may pass it to
other DMABUF-aware devices. Refer to :ref:`DMABUF importing <dmabuf>`
for details about importing DMABUF files into V4L2 nodes. It is
recommended to close a DMABUF file when it is no longer used to allow
the associated memory to be reclaimed.

Examples
========

.. code-block:: c

    int buffer_export(int v4lfd, enum v4l2_buf_type bt, int index, int *dmafd)
    {
	struct v4l2_exportbuffer expbuf;

	memset(&expbuf, 0, sizeof(expbuf));
	expbuf.type = bt;
	expbuf.index = index;
	if (ioctl(v4lfd, VIDIOC_EXPBUF, &expbuf) == -1) {
	    perror("VIDIOC_EXPBUF");
	    return -1;
	}

	*dmafd = expbuf.fd;

	return 0;
    }

.. code-block:: c

    int buffer_export_mp(int v4lfd, enum v4l2_buf_type bt, int index,
	int dmafd[], int n_planes)
    {
	int i;

	for (i = 0; i < n_planes; ++i) {
	    struct v4l2_exportbuffer expbuf;

	    memset(&expbuf, 0, sizeof(expbuf));
	    expbuf.type = bt;
	    expbuf.index = index;
	    expbuf.plane = i;
	    if (ioctl(v4lfd, VIDIOC_EXPBUF, &expbuf) == -1) {
		perror("VIDIOC_EXPBUF");
		while (i)
		    close(dmafd[--i]);
		return -1;
	    }
	    dmafd[i] = expbuf.fd;
	}

	return 0;
    }

.. c:type:: v4l2_exportbuffer

.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|

.. flat-table:: struct v4l2_exportbuffer
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2

    * - __u32
      - ``type``
      - Type of the buffer, same as struct
	:c:type:`v4l2_format` ``type`` or struct
	:c:type:`v4l2_requestbuffers` ``type``, set
	by the application. See :c:type:`v4l2_buf_type`
    * - __u32
      - ``index``
      - Number of the buffer, set by the application. This field is only
	used for :ref:`memory mapping <mmap>` I/O and can range from
	zero to the number of buffers allocated with the
	:ref:`VIDIOC_REQBUFS` and/or
	:ref:`VIDIOC_CREATE_BUFS` ioctls.
    * - __u32
      - ``plane``
      - Index of the plane to be exported when using the multi-planar API.
	Otherwise this value must be set to zero.
    * - __u32
      - ``flags``
      - Flags for the newly created file, currently only ``O_CLOEXEC``,
	``O_RDONLY``, ``O_WRONLY``, and ``O_RDWR`` are supported, refer to
	the manual of open() for more details.
    * - __s32
      - ``fd``
      - The DMABUF file descriptor associated with a buffer. Set by the
	driver.
    * - __u32
      - ``reserved[11]``
      - Reserved field for future use. Drivers and applications must set
	the array to zero.

Return Value
============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.

EINVAL
    A queue is not in MMAP mode or DMABUF exporting is not supported or
    ``flags`` or ``type`` or ``index`` or ``plane`` fields are invalid.