1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2 .. c:namespace:: V4L
3
4 .. _dmabuf:
5
6 ************************************
7 Streaming I/O (DMA buffer importing)
8 ************************************
9
10 The DMABUF framework provides a generic method for sharing buffers
11 between multiple devices. Device drivers that support DMABUF can export
12 a DMA buffer to userspace as a file descriptor (known as the exporter
13 role), import a DMA buffer from userspace using a file descriptor
14 previously exported for a different or the same device (known as the
15 importer role), or both. This section describes the DMABUF importer role
16 API in V4L2.
17
18 Refer to :ref:`DMABUF exporting <VIDIOC_EXPBUF>` for details about
19 exporting V4L2 buffers as DMABUF file descriptors.
20
21 Input and output devices support the streaming I/O method when the
22 ``V4L2_CAP_STREAMING`` flag in the ``capabilities`` field of struct
23 :c:type:`v4l2_capability` returned by the
24 :ref:`VIDIOC_QUERYCAP <VIDIOC_QUERYCAP>` ioctl is set. Whether
25 importing DMA buffers through DMABUF file descriptors is supported is
26 determined by calling the :ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`
27 ioctl with the memory type set to ``V4L2_MEMORY_DMABUF``.
28
29 This I/O method is dedicated to sharing DMA buffers between different
30 devices, which may be V4L devices or other video-related devices (e.g.
31 DRM). Buffers (planes) are allocated by a driver on behalf of an
32 application. Next, these buffers are exported to the application as file
33 descriptors using an API which is specific for an allocator driver. Only
34 such file descriptor are exchanged. The descriptors and meta-information
35 are passed in struct :c:type:`v4l2_buffer` (or in struct
36 :c:type:`v4l2_plane` in the multi-planar API case). The
37 driver must be switched into DMABUF I/O mode by calling the
38 :ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>` with the desired buffer type.
39
40 Example: Initiating streaming I/O with DMABUF file descriptors
41 ==============================================================
42
43 .. code-block:: c
44
45 struct v4l2_requestbuffers reqbuf;
46
47 memset(&reqbuf, 0, sizeof (reqbuf));
48 reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
49 reqbuf.memory = V4L2_MEMORY_DMABUF;
50 reqbuf.count = 1;
51
52 if (ioctl(fd, VIDIOC_REQBUFS, &reqbuf) == -1) {
53 if (errno == EINVAL)
54 printf("Video capturing or DMABUF streaming is not supported\\n");
55 else
56 perror("VIDIOC_REQBUFS");
57
58 exit(EXIT_FAILURE);
59 }
60
61 The buffer (plane) file descriptor is passed on the fly with the
62 :ref:`VIDIOC_QBUF <VIDIOC_QBUF>` ioctl. In case of multiplanar
63 buffers, every plane can be associated with a different DMABUF
64 descriptor. Although buffers are commonly cycled, applications can pass
65 a different DMABUF descriptor at each :ref:`VIDIOC_QBUF <VIDIOC_QBUF>` call.
66
67 Example: Queueing DMABUF using single plane API
68 ===============================================
69
70 .. code-block:: c
71
72 int buffer_queue(int v4lfd, int index, int dmafd)
73 {
74 struct v4l2_buffer buf;
75
76 memset(&buf, 0, sizeof buf);
77 buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
78 buf.memory = V4L2_MEMORY_DMABUF;
79 buf.index = index;
80 buf.m.fd = dmafd;
81
82 if (ioctl(v4lfd, VIDIOC_QBUF, &buf) == -1) {
83 perror("VIDIOC_QBUF");
84 return -1;
85 }
86
87 return 0;
88 }
89
90 Example 3.6. Queueing DMABUF using multi plane API
91 ==================================================
92
93 .. code-block:: c
94
95 int buffer_queue_mp(int v4lfd, int index, int dmafd[], int n_planes)
96 {
97 struct v4l2_buffer buf;
98 struct v4l2_plane planes[VIDEO_MAX_PLANES];
99 int i;
100
101 memset(&buf, 0, sizeof buf);
102 buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
103 buf.memory = V4L2_MEMORY_DMABUF;
104 buf.index = index;
105 buf.m.planes = planes;
106 buf.length = n_planes;
107
108 memset(&planes, 0, sizeof planes);
109
110 for (i = 0; i < n_planes; ++i)
111 buf.m.planes[i].m.fd = dmafd[i];
112
113 if (ioctl(v4lfd, VIDIOC_QBUF, &buf) == -1) {
114 perror("VIDIOC_QBUF");
115 return -1;
116 }
117
118 return 0;
119 }
120
121 Captured or displayed buffers are dequeued with the
122 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. The driver can unlock the
123 buffer at any time between the completion of the DMA and this ioctl. The
124 memory is also unlocked when
125 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` is called,
126 :ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`, or when the device is closed.
127
128 For capturing applications it is customary to enqueue a number of empty
129 buffers, to start capturing and enter the read loop. Here the
130 application waits until a filled buffer can be dequeued, and re-enqueues
131 the buffer when the data is no longer needed. Output applications fill
132 and enqueue buffers, when enough buffers are stacked up output is
133 started. In the write loop, when the application runs out of free
134 buffers it must wait until an empty buffer can be dequeued and reused.
135 Two methods exist to suspend execution of the application until one or
136 more buffers can be dequeued. By default :ref:`VIDIOC_DQBUF
137 <VIDIOC_QBUF>` blocks when no buffer is in the outgoing queue. When the
138 ``O_NONBLOCK`` flag was given to the :c:func:`open()` function,
139 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` returns immediately with an ``EAGAIN``
140 error code when no buffer is available. The
141 :c:func:`select()` and :c:func:`poll()`
142 functions are always available.
143
144 To start and stop capturing or displaying applications call the
145 :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` and
146 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctls.
147
148 .. note::
149
150 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` removes all buffers from
151 both queues and unlocks all buffers as a side effect. Since there is no
152 notion of doing anything "now" on a multitasking system, if an
153 application needs to synchronize with another event it should examine
154 the struct :c:type:`v4l2_buffer` ``timestamp`` of captured or
155 outputted buffers.
156
157 Drivers implementing DMABUF importing I/O must support the
158 :ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`, :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`,
159 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_STREAMON
160 <VIDIOC_STREAMON>` and :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctls,
161 and the :c:func:`select()` and :c:func:`poll()`
162 functions.
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