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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: V4L
+
+.. _dmabuf:
+
+************************************
+Streaming I/O (DMA buffer importing)
+************************************
+
+The DMABUF framework provides a generic method for sharing buffers
+between multiple devices. Device drivers that support DMABUF can export
+a DMA buffer to userspace as a file descriptor (known as the exporter
+role), import a DMA buffer from userspace using a file descriptor
+previously exported for a different or the same device (known as the
+importer role), or both. This section describes the DMABUF importer role
+API in V4L2.
+
+Refer to :ref:`DMABUF exporting <VIDIOC_EXPBUF>` for details about
+exporting V4L2 buffers as DMABUF file descriptors.
+
+Input and output devices support the streaming I/O method when the
+``V4L2_CAP_STREAMING`` flag in the ``capabilities`` field of struct
+:c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP <VIDIOC_QUERYCAP>` ioctl is set. Whether
+importing DMA buffers through DMABUF file descriptors is supported is
+determined by calling the :ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`
+ioctl with the memory type set to ``V4L2_MEMORY_DMABUF``.
+
+This I/O method is dedicated to sharing DMA buffers between different
+devices, which may be V4L devices or other video-related devices (e.g.
+DRM). Buffers (planes) are allocated by a driver on behalf of an
+application. Next, these buffers are exported to the application as file
+descriptors using an API which is specific for an allocator driver. Only
+such file descriptor are exchanged. The descriptors and meta-information
+are passed in struct :c:type:`v4l2_buffer` (or in struct
+:c:type:`v4l2_plane` in the multi-planar API case). The
+driver must be switched into DMABUF I/O mode by calling the
+:ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>` with the desired buffer type.
+
+Example: Initiating streaming I/O with DMABUF file descriptors
+==============================================================
+
+.. code-block:: c
+
+    struct v4l2_requestbuffers reqbuf;
+
+    memset(&reqbuf, 0, sizeof (reqbuf));
+    reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+    reqbuf.memory = V4L2_MEMORY_DMABUF;
+    reqbuf.count = 1;
+
+    if (ioctl(fd, VIDIOC_REQBUFS, &reqbuf) == -1) {
+	if (errno == EINVAL)
+	    printf("Video capturing or DMABUF streaming is not supported\\n");
+	else
+	    perror("VIDIOC_REQBUFS");
+
+	exit(EXIT_FAILURE);
+    }
+
+The buffer (plane) file descriptor is passed on the fly with the
+:ref:`VIDIOC_QBUF <VIDIOC_QBUF>` ioctl. In case of multiplanar
+buffers, every plane can be associated with a different DMABUF
+descriptor. Although buffers are commonly cycled, applications can pass
+a different DMABUF descriptor at each :ref:`VIDIOC_QBUF <VIDIOC_QBUF>` call.
+
+Example: Queueing DMABUF using single plane API
+===============================================
+
+.. code-block:: c
+
+    int buffer_queue(int v4lfd, int index, int dmafd)
+    {
+	struct v4l2_buffer buf;
+
+	memset(&buf, 0, sizeof buf);
+	buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+	buf.memory = V4L2_MEMORY_DMABUF;
+	buf.index = index;
+	buf.m.fd = dmafd;
+
+	if (ioctl(v4lfd, VIDIOC_QBUF, &buf) == -1) {
+	    perror("VIDIOC_QBUF");
+	    return -1;
+	}
+
+	return 0;
+    }
+
+Example 3.6. Queueing DMABUF using multi plane API
+==================================================
+
+.. code-block:: c
+
+    int buffer_queue_mp(int v4lfd, int index, int dmafd[], int n_planes)
+    {
+	struct v4l2_buffer buf;
+	struct v4l2_plane planes[VIDEO_MAX_PLANES];
+	int i;
+
+	memset(&buf, 0, sizeof buf);
+	buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
+	buf.memory = V4L2_MEMORY_DMABUF;
+	buf.index = index;
+	buf.m.planes = planes;
+	buf.length = n_planes;
+
+	memset(&planes, 0, sizeof planes);
+
+	for (i = 0; i < n_planes; ++i)
+	    buf.m.planes[i].m.fd = dmafd[i];
+
+	if (ioctl(v4lfd, VIDIOC_QBUF, &buf) == -1) {
+	    perror("VIDIOC_QBUF");
+	    return -1;
+	}
+
+	return 0;
+    }
+
+Captured or displayed buffers are dequeued with the
+:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. The driver can unlock the
+buffer at any time between the completion of the DMA and this ioctl. The
+memory is also unlocked when
+:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` is called,
+:ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`, or when the device is closed.
+
+For capturing applications it is customary to enqueue a number of empty
+buffers, to start capturing and enter the read loop. Here the
+application waits until a filled buffer can be dequeued, and re-enqueues
+the buffer when the data is no longer needed. Output applications fill
+and enqueue buffers, when enough buffers are stacked up output is
+started. In the write loop, when the application runs out of free
+buffers it must wait until an empty buffer can be dequeued and reused.
+Two methods exist to suspend execution of the application until one or
+more buffers can be dequeued. By default :ref:`VIDIOC_DQBUF
+<VIDIOC_QBUF>` blocks when no buffer is in the outgoing queue. When the
+``O_NONBLOCK`` flag was given to the :c:func:`open()` function,
+:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` returns immediately with an ``EAGAIN``
+error code when no buffer is available. The
+:c:func:`select()` and :c:func:`poll()`
+functions are always available.
+
+To start and stop capturing or displaying applications call the
+:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` and
+:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctls.
+
+.. note::
+
+   :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` removes all buffers from
+   both queues and unlocks all buffers as a side effect. Since there is no
+   notion of doing anything "now" on a multitasking system, if an
+   application needs to synchronize with another event it should examine
+   the struct :c:type:`v4l2_buffer` ``timestamp`` of captured or
+   outputted buffers.
+
+Drivers implementing DMABUF importing I/O must support the
+:ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`, :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`,
+:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_STREAMON
+<VIDIOC_STREAMON>` and :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctls,
+and the :c:func:`select()` and :c:func:`poll()`
+functions.