1 .. Permission is granted to copy, distribute and/or modify this
2 .. document under the terms of the GNU Free Documentation License,
3 .. Version 1.1 or any later version published by the Free Software
4 .. Foundation, with no Invariant Sections, no Front-Cover Texts
5 .. and no Back-Cover Texts. A copy of the license is included at
6 .. Documentation/userspace-api/media/fdl-appendix.rst.
8 .. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
12 *****************************
13 Streaming I/O (User Pointers)
14 *****************************
16 Input and output devices support this I/O method when the
17 ``V4L2_CAP_STREAMING`` flag in the ``capabilities`` field of struct
18 :c:type:`v4l2_capability` returned by the
19 :ref:`VIDIOC_QUERYCAP` ioctl is set. If the
20 particular user pointer method (not only memory mapping) is supported
21 must be determined by calling the :ref:`VIDIOC_REQBUFS` ioctl
22 with the memory type set to ``V4L2_MEMORY_USERPTR``.
24 This I/O method combines advantages of the read/write and memory mapping
25 methods. Buffers (planes) are allocated by the application itself, and
26 can reside for example in virtual or shared memory. Only pointers to
27 data are exchanged, these pointers and meta-information are passed in
28 struct :c:type:`v4l2_buffer` (or in struct
29 :c:type:`v4l2_plane` in the multi-planar API case). The
30 driver must be switched into user pointer I/O mode by calling the
31 :ref:`VIDIOC_REQBUFS` with the desired buffer type.
32 No buffers (planes) are allocated beforehand, consequently they are not
33 indexed and cannot be queried like mapped buffers with the
34 :ref:`VIDIOC_QUERYBUF <VIDIOC_QUERYBUF>` ioctl.
36 Example: Initiating streaming I/O with user pointers
37 ====================================================
41 struct v4l2_requestbuffers reqbuf;
43 memset (&reqbuf, 0, sizeof (reqbuf));
44 reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
45 reqbuf.memory = V4L2_MEMORY_USERPTR;
47 if (ioctl (fd, VIDIOC_REQBUFS, &reqbuf) == -1) {
49 printf ("Video capturing or user pointer streaming is not supported\\n");
51 perror ("VIDIOC_REQBUFS");
56 Buffer (plane) addresses and sizes are passed on the fly with the
57 :ref:`VIDIOC_QBUF <VIDIOC_QBUF>` ioctl. Although buffers are commonly
58 cycled, applications can pass different addresses and sizes at each
59 :ref:`VIDIOC_QBUF <VIDIOC_QBUF>` call. If required by the hardware the
60 driver swaps memory pages within physical memory to create a continuous
61 area of memory. This happens transparently to the application in the
62 virtual memory subsystem of the kernel. When buffer pages have been
63 swapped out to disk they are brought back and finally locked in physical
64 memory for DMA. [#f1]_
66 Filled or displayed buffers are dequeued with the
67 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. The driver can unlock the
68 memory pages at any time between the completion of the DMA and this
69 ioctl. The memory is also unlocked when
70 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` is called,
71 :ref:`VIDIOC_REQBUFS`, or when the device is closed.
72 Applications must take care not to free buffers without dequeuing.
73 Firstly, the buffers remain locked for longer, wasting physical memory.
74 Secondly the driver will not be notified when the memory is returned to
75 the application's free list and subsequently reused for other purposes,
76 possibly completing the requested DMA and overwriting valuable data.
78 For capturing applications it is customary to enqueue a number of empty
79 buffers, to start capturing and enter the read loop. Here the
80 application waits until a filled buffer can be dequeued, and re-enqueues
81 the buffer when the data is no longer needed. Output applications fill
82 and enqueue buffers, when enough buffers are stacked up output is
83 started. In the write loop, when the application runs out of free
84 buffers it must wait until an empty buffer can be dequeued and reused.
85 Two methods exist to suspend execution of the application until one or
86 more buffers can be dequeued. By default :ref:`VIDIOC_DQBUF
87 <VIDIOC_QBUF>` blocks when no buffer is in the outgoing queue. When the
88 ``O_NONBLOCK`` flag was given to the :ref:`open() <func-open>` function,
89 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` returns immediately with an ``EAGAIN``
90 error code when no buffer is available. The :ref:`select()
91 <func-select>` or :ref:`poll() <func-poll>` function are always
94 To start and stop capturing or output applications call the
95 :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` and
96 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctl.
100 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` removes all buffers from
101 both queues and unlocks all buffers as a side effect. Since there is no
102 notion of doing anything "now" on a multitasking system, if an
103 application needs to synchronize with another event it should examine
104 the struct :c:type:`v4l2_buffer` ``timestamp`` of captured or
107 Drivers implementing user pointer I/O must support the
108 :ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`, :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`,
109 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
110 and :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctls, the
111 :ref:`select() <func-select>` and :ref:`poll() <func-poll>` function. [#f2]_
114 We expect that frequently used buffers are typically not swapped out.
115 Anyway, the process of swapping, locking or generating scatter-gather
116 lists may be time consuming. The delay can be masked by the depth of
117 the incoming buffer queue, and perhaps by maintaining caches assuming
118 a buffer will be soon enqueued again. On the other hand, to optimize
119 memory usage drivers can limit the number of buffers locked in
120 advance and recycle the most recently used buffers first. Of course,
121 the pages of empty buffers in the incoming queue need not be saved to
122 disk. Output buffers must be saved on the incoming and outgoing queue
123 because an application may share them with other processes.
126 At the driver level :ref:`select() <func-select>` and :ref:`poll() <func-poll>` are
127 the same, and :ref:`select() <func-select>` is too important to be optional.
128 The rest should be evident.