userp.rst (5749B)
1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _userp: 5 6***************************** 7Streaming I/O (User Pointers) 8***************************** 9 10Input and output devices support this I/O method when the 11``V4L2_CAP_STREAMING`` flag in the ``capabilities`` field of struct 12:c:type:`v4l2_capability` returned by the 13:ref:`VIDIOC_QUERYCAP` ioctl is set. If the 14particular user pointer method (not only memory mapping) is supported 15must be determined by calling the :ref:`VIDIOC_REQBUFS` ioctl 16with the memory type set to ``V4L2_MEMORY_USERPTR``. 17 18This I/O method combines advantages of the read/write and memory mapping 19methods. Buffers (planes) are allocated by the application itself, and 20can reside for example in virtual or shared memory. Only pointers to 21data are exchanged, these pointers and meta-information are passed in 22struct :c:type:`v4l2_buffer` (or in struct 23:c:type:`v4l2_plane` in the multi-planar API case). The 24driver must be switched into user pointer I/O mode by calling the 25:ref:`VIDIOC_REQBUFS` with the desired buffer type. 26No buffers (planes) are allocated beforehand, consequently they are not 27indexed and cannot be queried like mapped buffers with the 28:ref:`VIDIOC_QUERYBUF <VIDIOC_QUERYBUF>` ioctl. 29 30Example: Initiating streaming I/O with user pointers 31==================================================== 32 33.. code-block:: c 34 35 struct v4l2_requestbuffers reqbuf; 36 37 memset (&reqbuf, 0, sizeof (reqbuf)); 38 reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; 39 reqbuf.memory = V4L2_MEMORY_USERPTR; 40 41 if (ioctl (fd, VIDIOC_REQBUFS, &reqbuf) == -1) { 42 if (errno == EINVAL) 43 printf ("Video capturing or user pointer streaming is not supported\\n"); 44 else 45 perror ("VIDIOC_REQBUFS"); 46 47 exit (EXIT_FAILURE); 48 } 49 50Buffer (plane) addresses and sizes are passed on the fly with the 51:ref:`VIDIOC_QBUF <VIDIOC_QBUF>` ioctl. Although buffers are commonly 52cycled, applications can pass different addresses and sizes at each 53:ref:`VIDIOC_QBUF <VIDIOC_QBUF>` call. If required by the hardware the 54driver swaps memory pages within physical memory to create a continuous 55area of memory. This happens transparently to the application in the 56virtual memory subsystem of the kernel. When buffer pages have been 57swapped out to disk they are brought back and finally locked in physical 58memory for DMA. [#f1]_ 59 60Filled or displayed buffers are dequeued with the 61:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. The driver can unlock the 62memory pages at any time between the completion of the DMA and this 63ioctl. The memory is also unlocked when 64:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` is called, 65:ref:`VIDIOC_REQBUFS`, or when the device is closed. 66Applications must take care not to free buffers without dequeuing. 67Firstly, the buffers remain locked for longer, wasting physical memory. 68Secondly the driver will not be notified when the memory is returned to 69the application's free list and subsequently reused for other purposes, 70possibly completing the requested DMA and overwriting valuable data. 71 72For capturing applications it is customary to enqueue a number of empty 73buffers, to start capturing and enter the read loop. Here the 74application waits until a filled buffer can be dequeued, and re-enqueues 75the buffer when the data is no longer needed. Output applications fill 76and enqueue buffers, when enough buffers are stacked up output is 77started. In the write loop, when the application runs out of free 78buffers it must wait until an empty buffer can be dequeued and reused. 79Two methods exist to suspend execution of the application until one or 80more buffers can be dequeued. By default :ref:`VIDIOC_DQBUF 81<VIDIOC_QBUF>` blocks when no buffer is in the outgoing queue. When the 82``O_NONBLOCK`` flag was given to the :c:func:`open()` function, 83:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` returns immediately with an ``EAGAIN`` 84error code when no buffer is available. The :ref:`select() 85<func-select>` or :c:func:`poll()` function are always 86available. 87 88To start and stop capturing or output applications call the 89:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` and 90:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctl. 91 92.. note:: 93 94 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` removes all buffers from 95 both queues and unlocks all buffers as a side effect. Since there is no 96 notion of doing anything "now" on a multitasking system, if an 97 application needs to synchronize with another event it should examine 98 the struct :c:type:`v4l2_buffer` ``timestamp`` of captured or 99 outputted buffers. 100 101Drivers implementing user pointer I/O must support the 102:ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`, :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`, 103:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` 104and :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctls, the 105:c:func:`select()` and :c:func:`poll()` function. [#f2]_ 106 107.. [#f1] 108 We expect that frequently used buffers are typically not swapped out. 109 Anyway, the process of swapping, locking or generating scatter-gather 110 lists may be time consuming. The delay can be masked by the depth of 111 the incoming buffer queue, and perhaps by maintaining caches assuming 112 a buffer will be soon enqueued again. On the other hand, to optimize 113 memory usage drivers can limit the number of buffers locked in 114 advance and recycle the most recently used buffers first. Of course, 115 the pages of empty buffers in the incoming queue need not be saved to 116 disk. Output buffers must be saved on the incoming and outgoing queue 117 because an application may share them with other processes. 118 119.. [#f2] 120 At the driver level :c:func:`select()` and :c:func:`poll()` are 121 the same, and :c:func:`select()` is too important to be optional. 122 The rest should be evident.