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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _userp:
+
+*****************************
+Streaming I/O (User Pointers)
+*****************************
+
+Input and output devices support this I/O method when the
+``V4L2_CAP_STREAMING`` flag in the ``capabilities`` field of struct
+:ref:`v4l2_capability <v4l2-capability>` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl is set. If the
+particular user pointer method (not only memory mapping) is supported
+must be determined by calling the :ref:`VIDIOC_REQBUFS` ioctl
+with the memory type set to ``V4L2_MEMORY_USERPTR``.
+
+This I/O method combines advantages of the read/write and memory mapping
+methods. Buffers (planes) are allocated by the application itself, and
+can reside for example in virtual or shared memory. Only pointers to
+data are exchanged, these pointers and meta-information are passed in
+struct :ref:`v4l2_buffer <v4l2-buffer>` (or in struct
+:ref:`v4l2_plane <v4l2-plane>` in the multi-planar API case). The
+driver must be switched into user pointer I/O mode by calling the
+:ref:`VIDIOC_REQBUFS` with the desired buffer type.
+No buffers (planes) are allocated beforehand, consequently they are not
+indexed and cannot be queried like mapped buffers with the
+:ref:`VIDIOC_QUERYBUF <VIDIOC_QUERYBUF>` ioctl.
+
+Example: Initiating streaming I/O with user pointers
+====================================================
+
+.. code-block:: c
+
+    struct v4l2_requestbuffers reqbuf;
+
+    memset (&reqbuf, 0, sizeof (reqbuf));
+    reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+    reqbuf.memory = V4L2_MEMORY_USERPTR;
+
+    if (ioctl (fd, VIDIOC_REQBUFS, &reqbuf) == -1) {
+	if (errno == EINVAL)
+	    printf ("Video capturing or user pointer streaming is not supported\\n");
+	else
+	    perror ("VIDIOC_REQBUFS");
+
+	exit (EXIT_FAILURE);
+    }
+
+Buffer (plane) addresses and sizes are passed on the fly with the
+:ref:`VIDIOC_QBUF <VIDIOC_QBUF>` ioctl. Although buffers are commonly
+cycled, applications can pass different addresses and sizes at each
+:ref:`VIDIOC_QBUF <VIDIOC_QBUF>` call. If required by the hardware the
+driver swaps memory pages within physical memory to create a continuous
+area of memory. This happens transparently to the application in the
+virtual memory subsystem of the kernel. When buffer pages have been
+swapped out to disk they are brought back and finally locked in physical
+memory for DMA. [#f1]_
+
+Filled or displayed buffers are dequeued with the
+:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. The driver can unlock the
+memory pages 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`, or when the device is closed.
+Applications must take care not to free buffers without dequeuing. For
+once, the buffers remain locked until further, wasting physical memory.
+Second the driver will not be notified when the memory is returned to
+the application's free list and subsequently reused for other purposes,
+possibly completing the requested DMA and overwriting valuable data.
+
+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 :ref:`open() <func-open>` function,
+:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` returns immediately with an ``EAGAIN``
+error code when no buffer is available. The :ref:`select()
+<func-select>` or :ref:`poll() <func-poll>` function are always
+available.
+
+To start and stop capturing or output applications call the
+:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` and
+:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctl.
+
+.. 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 :ref:`v4l2_buffer <v4l2-buffer>` ``timestamp`` of captured or
+   outputted buffers.
+
+Drivers implementing user pointer 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, the
+:ref:`select() <func-select>` and :ref:`poll() <func-poll>` function. [#f2]_
+
+.. [#f1]
+   We expect that frequently used buffers are typically not swapped out.
+   Anyway, the process of swapping, locking or generating scatter-gather
+   lists may be time consuming. The delay can be masked by the depth of
+   the incoming buffer queue, and perhaps by maintaining caches assuming
+   a buffer will be soon enqueued again. On the other hand, to optimize
+   memory usage drivers can limit the number of buffers locked in
+   advance and recycle the most recently used buffers first. Of course,
+   the pages of empty buffers in the incoming queue need not be saved to
+   disk. Output buffers must be saved on the incoming and outgoing queue
+   because an application may share them with other processes.
+
+.. [#f2]
+   At the driver level :ref:`select() <func-select>` and :ref:`poll() <func-poll>` are
+   the same, and :ref:`select() <func-select>` is too important to be optional.
+   The rest should be evident.