1 .. SPDX-License-Identifier: GPL-2.0
2
3 Video device' s internal representation
4 =======================================
5
6 The actual device nodes in the ``/dev`` directory are created using the
7 :c:type:`video_device` struct (``v4l2-dev.h``). This struct can either be
8 allocated dynamically or embedded in a larger struct.
9
10 To allocate it dynamically use :c:func:`video_device_alloc`:
11
12 .. code-block:: c
13
14 struct video_device *vdev = video_device_alloc();
15
16 if (vdev == NULL)
17 return -ENOMEM;
18
19 vdev->release = video_device_release;
20
21 If you embed it in a larger struct, then you must set the ``release()``
22 callback to your own function:
23
24 .. code-block:: c
25
26 struct video_device *vdev = &my_vdev->vdev;
27
28 vdev->release = my_vdev_release;
29
30 The ``release()`` callback must be set and it is called when the last user
31 of the video device exits.
32
33 The default :c:func:`video_device_release` callback currently
34 just calls ``kfree`` to free the allocated memory.
35
36 There is also a :c:func:`video_device_release_empty` function that does
37 nothing (is empty) and should be used if the struct is embedded and there
38 is nothing to do when it is released.
39
40 You should also set these fields of :c:type:`video_device`:
41
42 - :c:type:`video_device`->v4l2_dev: must be set to the :c:type:`v4l2_device`
43 parent device.
44
45 - :c:type:`video_device`->name: set to something descriptive and unique.
46
47 - :c:type:`video_device`->vfl_dir: set this to ``VFL_DIR_RX`` for capture
48 devices (``VFL_DIR_RX`` has value 0, so this is normally already the
49 default), set to ``VFL_DIR_TX`` for output devices and ``VFL_DIR_M2M`` for mem2mem (codec) devices.
50
51 - :c:type:`video_device`->fops: set to the :c:type:`v4l2_file_operations`
52 struct.
53
54 - :c:type:`video_device`->ioctl_ops: if you use the :c:type:`v4l2_ioctl_ops`
55 to simplify ioctl maintenance (highly recommended to use this and it might
56 become compulsory in the future!), then set this to your
57 :c:type:`v4l2_ioctl_ops` struct. The :c:type:`video_device`->vfl_type and
58 :c:type:`video_device`->vfl_dir fields are used to disable ops that do not
59 match the type/dir combination. E.g. VBI ops are disabled for non-VBI nodes,
60 and output ops are disabled for a capture device. This makes it possible to
61 provide just one :c:type:`v4l2_ioctl_ops` struct for both vbi and
62 video nodes.
63
64 - :c:type:`video_device`->lock: leave to ``NULL`` if you want to do all the
65 locking in the driver. Otherwise you give it a pointer to a struct
66 ``mutex_lock`` and before the :c:type:`video_device`->unlocked_ioctl
67 file operation is called this lock will be taken by the core and released
68 afterwards. See the next section for more details.
69
70 - :c:type:`video_device`->queue: a pointer to the struct vb2_queue
71 associated with this device node.
72 If queue is not ``NULL``, and queue->lock is not ``NULL``, then queue->lock
73 is used for the queuing ioctls (``VIDIOC_REQBUFS``, ``CREATE_BUFS``,
74 ``QBUF``, ``DQBUF``, ``QUERYBUF``, ``PREPARE_BUF``, ``STREAMON`` and
75 ``STREAMOFF``) instead of the lock above.
76 That way the :ref:`vb2 <vb2_framework>` queuing framework does not have
77 to wait for other ioctls. This queue pointer is also used by the
78 :ref:`vb2 <vb2_framework>` helper functions to check for
79 queuing ownership (i.e. is the filehandle calling it allowed to do the
80 operation).
81
82 - :c:type:`video_device`->prio: keeps track of the priorities. Used to
83 implement ``VIDIOC_G_PRIORITY`` and ``VIDIOC_S_PRIORITY``.
84 If left to ``NULL``, then it will use the struct v4l2_prio_state
85 in :c:type:`v4l2_device`. If you want to have a separate priority state per
86 (group of) device node(s), then you can point it to your own struct
87 :c:type:`v4l2_prio_state`.
88
89 - :c:type:`video_device`->dev_parent: you only set this if v4l2_device was
90 registered with ``NULL`` as the parent ``device`` struct. This only happens
91 in cases where one hardware device has multiple PCI devices that all share
92 the same :c:type:`v4l2_device` core.
93
94 The cx88 driver is an example of this: one core :c:type:`v4l2_device` struct,
95 but it is used by both a raw video PCI device (cx8800) and a MPEG PCI device
96 (cx8802). Since the :c:type:`v4l2_device` cannot be associated with two PCI
97 devices at the same time it is setup without a parent device. But when the
98 struct video_device is initialized you **do** know which parent
99 PCI device to use and so you set ``dev_device`` to the correct PCI device.
100
101 If you use :c:type:`v4l2_ioctl_ops`, then you should set
102 :c:type:`video_device`->unlocked_ioctl to :c:func:`video_ioctl2` in your
103 :c:type:`v4l2_file_operations` struct.
104
105 In some cases you want to tell the core that a function you had specified in
106 your :c:type:`v4l2_ioctl_ops` should be ignored. You can mark such ioctls by
107 calling this function before :c:func:`video_register_device` is called:
108
109 :c:func:`v4l2_disable_ioctl <v4l2_disable_ioctl>`
110 (:c:type:`vdev <video_device>`, cmd).
111
112 This tends to be needed if based on external factors (e.g. which card is
113 being used) you want to turns off certain features in :c:type:`v4l2_ioctl_ops`
114 without having to make a new struct.
115
116 The :c:type:`v4l2_file_operations` struct is a subset of file_operations.
117 The main difference is that the inode argument is omitted since it is never
118 used.
119
120 If integration with the media framework is needed, you must initialize the
121 :c:type:`media_entity` struct embedded in the :c:type:`video_device` struct
122 (entity field) by calling :c:func:`media_entity_pads_init`:
123
124 .. code-block:: c
125
126 struct media_pad *pad = &my_vdev->pad;
127 int err;
128
129 err = media_entity_pads_init(&vdev->entity, 1, pad);
130
131 The pads array must have been previously initialized. There is no need to
132 manually set the struct media_entity type and name fields.
133
134 A reference to the entity will be automatically acquired/released when the
135 video device is opened/closed.
136
137 ioctls and locking
138 ------------------
139
140 The V4L core provides optional locking services. The main service is the
141 lock field in struct video_device, which is a pointer to a mutex.
142 If you set this pointer, then that will be used by unlocked_ioctl to
143 serialize all ioctls.
144
145 If you are using the :ref:`videobuf2 framework <vb2_framework>`, then there
146 is a second lock that you can set: :c:type:`video_device`->queue->lock. If
147 set, then this lock will be used instead of :c:type:`video_device`->lock
148 to serialize all queuing ioctls (see the previous section
149 for the full list of those ioctls).
150
151 The advantage of using a different lock for the queuing ioctls is that for some
152 drivers (particularly USB drivers) certain commands such as setting controls
153 can take a long time, so you want to use a separate lock for the buffer queuing
154 ioctls. That way your ``VIDIOC_DQBUF`` doesn't stall because the driver is busy
155 changing the e.g. exposure of the webcam.
156
157 Of course, you can always do all the locking yourself by leaving both lock
158 pointers at ``NULL``.
159
160 In the case of :ref:`videobuf2 <vb2_framework>` you will need to implement the
161 ``wait_prepare()`` and ``wait_finish()`` callbacks to unlock/lock if applicable.
162 If you use the ``queue->lock`` pointer, then you can use the helper functions
163 :c:func:`vb2_ops_wait_prepare` and :c:func:`vb2_ops_wait_finish`.
164
165 The implementation of a hotplug disconnect should also take the lock from
166 :c:type:`video_device` before calling v4l2_device_disconnect. If you are also
167 using :c:type:`video_device`->queue->lock, then you have to first lock
168 :c:type:`video_device`->queue->lock followed by :c:type:`video_device`->lock.
169 That way you can be sure no ioctl is running when you call
170 :c:func:`v4l2_device_disconnect`.
171
172 Video device registration
173 -------------------------
174
175 Next you register the video device with :c:func:`video_register_device`.
176 This will create the character device for you.
177
178 .. code-block:: c
179
180 err = video_register_device(vdev, VFL_TYPE_VIDEO, -1);
181 if (err) {
182 video_device_release(vdev); /* or kfree(my_vdev); */
183 return err;
184 }
185
186 If the :c:type:`v4l2_device` parent device has a not ``NULL`` mdev field,
187 the video device entity will be automatically registered with the media
188 device.
189
190 Which device is registered depends on the type argument. The following
191 types exist:
192
193 ========================== ==================== ==============================
194 :c:type:`vfl_devnode_type` Device name Usage
195 ========================== ==================== ==============================
196 ``VFL_TYPE_VIDEO`` ``/dev/videoX`` for video input/output devices
197 ``VFL_TYPE_VBI`` ``/dev/vbiX`` for vertical blank data (i.e.
198 closed captions, teletext)
199 ``VFL_TYPE_RADIO`` ``/dev/radioX`` for radio tuners
200 ``VFL_TYPE_SUBDEV`` ``/dev/v4l-subdevX`` for V4L2 subdevices
201 ``VFL_TYPE_SDR`` ``/dev/swradioX`` for Software Defined Radio
202 (SDR) tuners
203 ``VFL_TYPE_TOUCH`` ``/dev/v4l-touchX`` for touch sensors
204 ========================== ==================== ==============================
205
206 The last argument gives you a certain amount of control over the device
207 node number used (i.e. the X in ``videoX``). Normally you will pass -1
208 to let the v4l2 framework pick the first free number. But sometimes users
209 want to select a specific node number. It is common that drivers allow
210 the user to select a specific device node number through a driver module
211 option. That number is then passed to this function and video_register_device
212 will attempt to select that device node number. If that number was already
213 in use, then the next free device node number will be selected and it
214 will send a warning to the kernel log.
215
216 Another use-case is if a driver creates many devices. In that case it can
217 be useful to place different video devices in separate ranges. For example,
218 video capture devices start at 0, video output devices start at 16.
219 So you can use the last argument to specify a minimum device node number
220 and the v4l2 framework will try to pick the first free number that is equal
221 or higher to what you passed. If that fails, then it will just pick the
222 first free number.
223
224 Since in this case you do not care about a warning about not being able
225 to select the specified device node number, you can call the function
226 :c:func:`video_register_device_no_warn` instead.
227
228 Whenever a device node is created some attributes are also created for you.
229 If you look in ``/sys/class/video4linux`` you see the devices. Go into e.g.
230 ``video0`` and you will see 'name', 'dev_debug' and 'index' attributes. The
231 'name' attribute is the 'name' field of the video_device struct. The
232 'dev_debug' attribute can be used to enable core debugging. See the next
233 section for more detailed information on this.
234
235 The 'index' attribute is the index of the device node: for each call to
236 :c:func:`video_register_device()` the index is just increased by 1. The
237 first video device node you register always starts with index 0.
238
239 Users can setup udev rules that utilize the index attribute to make fancy
240 device names (e.g. '``mpegX``' for MPEG video capture device nodes).
241
242 After the device was successfully registered, then you can use these fields:
243
244 - :c:type:`video_device`->vfl_type: the device type passed to
245 :c:func:`video_register_device`.
246 - :c:type:`video_device`->minor: the assigned device minor number.
247 - :c:type:`video_device`->num: the device node number (i.e. the X in
248 ``videoX``).
249 - :c:type:`video_device`->index: the device index number.
250
251 If the registration failed, then you need to call
252 :c:func:`video_device_release` to free the allocated :c:type:`video_device`
253 struct, or free your own struct if the :c:type:`video_device` was embedded in
254 it. The ``vdev->release()`` callback will never be called if the registration
255 failed, nor should you ever attempt to unregister the device if the
256 registration failed.
257
258 video device debugging
259 ----------------------
260
261 The 'dev_debug' attribute that is created for each video, vbi, radio or swradio
262 device in ``/sys/class/video4linux/<devX>/`` allows you to enable logging of
263 file operations.
264
265 It is a bitmask and the following bits can be set:
266
267 .. tabularcolumns:: |p{5ex}|L|
268
269 ===== ================================================================
270 Mask Description
271 ===== ================================================================
272 0x01 Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are
273 only logged if bit 0x08 is also set.
274 0x02 Log the ioctl name arguments and error code. VIDIOC_(D)QBUF
275 ioctls are
276 only logged if bit 0x08 is also set.
277 0x04 Log the file operations open, release, read, write, mmap and
278 get_unmapped_area. The read and write operations are only
279 logged if bit 0x08 is also set.
280 0x08 Log the read and write file operations and the VIDIOC_QBUF and
281 VIDIOC_DQBUF ioctls.
282 0x10 Log the poll file operation.
283 0x20 Log error and messages in the control operations.
284 ===== ================================================================
285
286 Video device cleanup
287 --------------------
288
289 When the video device nodes have to be removed, either during the unload
290 of the driver or because the USB device was disconnected, then you should
291 unregister them with:
292
293 :c:func:`video_unregister_device`
294 (:c:type:`vdev <video_device>`);
295
296 This will remove the device nodes from sysfs (causing udev to remove them
297 from ``/dev``).
298
299 After :c:func:`video_unregister_device` returns no new opens can be done.
300 However, in the case of USB devices some application might still have one of
301 these device nodes open. So after the unregister all file operations (except
302 release, of course) will return an error as well.
303
304 When the last user of the video device node exits, then the ``vdev->release()``
305 callback is called and you can do the final cleanup there.
306
307 Don't forget to cleanup the media entity associated with the video device if
308 it has been initialized:
309
310 :c:func:`media_entity_cleanup <media_entity_cleanup>`
311 (&vdev->entity);
312
313 This can be done from the release callback.
314
315
316 helper functions
317 ----------------
318
319 There are a few useful helper functions:
320
321 - file and :c:type:`video_device` private data
322
323 You can set/get driver private data in the video_device struct using:
324
325 :c:func:`video_get_drvdata <video_get_drvdata>`
326 (:c:type:`vdev <video_device>`);
327
328 :c:func:`video_set_drvdata <video_set_drvdata>`
329 (:c:type:`vdev <video_device>`);
330
331 Note that you can safely call :c:func:`video_set_drvdata` before calling
332 :c:func:`video_register_device`.
333
334 And this function:
335
336 :c:func:`video_devdata <video_devdata>`
337 (struct file \*file);
338
339 returns the video_device belonging to the file struct.
340
341 The :c:func:`video_devdata` function combines :c:func:`video_get_drvdata`
342 with :c:func:`video_devdata`:
343
344 :c:func:`video_drvdata <video_drvdata>`
345 (struct file \*file);
346
347 You can go from a :c:type:`video_device` struct to the v4l2_device struct using:
348
349 .. code-block:: c
350
351 struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
352
353 - Device node name
354
355 The :c:type:`video_device` node kernel name can be retrieved using:
356
357 :c:func:`video_device_node_name <video_device_node_name>`
358 (:c:type:`vdev <video_device>`);
359
360 The name is used as a hint by userspace tools such as udev. The function
361 should be used where possible instead of accessing the video_device::num and
362 video_device::minor fields.
363
364 video_device functions and data structures
365 ------------------------------------------
366
367 .. kernel-doc:: include/media/v4l2-dev.h
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