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Hans Verkuil2a1fcdf2008-11-29 21:36:58 -03001Overview of the V4L2 driver framework
2=====================================
3
4This text documents the various structures provided by the V4L2 framework and
5their relationships.
6
7
8Introduction
9------------
10
11The V4L2 drivers tend to be very complex due to the complexity of the
12hardware: most devices have multiple ICs, export multiple device nodes in
13/dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input
14(IR) devices.
15
16Especially the fact that V4L2 drivers have to setup supporting ICs to
17do audio/video muxing/encoding/decoding makes it more complex than most.
18Usually these ICs are connected to the main bridge driver through one or
19more I2C busses, but other busses can also be used. Such devices are
20called 'sub-devices'.
21
22For a long time the framework was limited to the video_device struct for
23creating V4L device nodes and video_buf for handling the video buffers
24(note that this document does not discuss the video_buf framework).
25
26This meant that all drivers had to do the setup of device instances and
27connecting to sub-devices themselves. Some of this is quite complicated
28to do right and many drivers never did do it correctly.
29
30There is also a lot of common code that could never be refactored due to
31the lack of a framework.
32
33So this framework sets up the basic building blocks that all drivers
34need and this same framework should make it much easier to refactor
35common code into utility functions shared by all drivers.
36
37
38Structure of a driver
39---------------------
40
41All drivers have the following structure:
42
431) A struct for each device instance containing the device state.
44
452) A way of initializing and commanding sub-devices (if any).
46
Hans Verkuilf44026d2010-08-06 12:52:43 -0300473) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX)
48 and keeping track of device-node specific data.
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -030049
Mauro Carvalho Chehab44061c02009-02-14 07:29:07 -0300504) Filehandle-specific structs containing per-filehandle data;
51
525) video buffer handling.
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -030053
54This is a rough schematic of how it all relates:
55
56 device instances
57 |
58 +-sub-device instances
59 |
60 \-V4L2 device nodes
61 |
62 \-filehandle instances
63
64
65Structure of the framework
66--------------------------
67
68The framework closely resembles the driver structure: it has a v4l2_device
69struct for the device instance data, a v4l2_subdev struct to refer to
70sub-device instances, the video_device struct stores V4L2 device node data
71and in the future a v4l2_fh struct will keep track of filehandle instances
72(this is not yet implemented).
73
Laurent Pinchart2c0ab672009-12-09 08:40:10 -030074The V4L2 framework also optionally integrates with the media framework. If a
75driver sets the struct v4l2_device mdev field, sub-devices and video nodes
76will automatically appear in the media framework as entities.
77
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -030078
79struct v4l2_device
80------------------
81
82Each device instance is represented by a struct v4l2_device (v4l2-device.h).
83Very simple devices can just allocate this struct, but most of the time you
84would embed this struct inside a larger struct.
85
86You must register the device instance:
87
88 v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev);
89
Laurent Pinchart95db3a62009-12-09 08:40:05 -030090Registration will initialize the v4l2_device struct. If the dev->driver_data
Laurent Pinchart2c0ab672009-12-09 08:40:10 -030091field is NULL, it will be linked to v4l2_dev.
92
93Drivers that want integration with the media device framework need to set
Laurent Pinchart95db3a62009-12-09 08:40:05 -030094dev->driver_data manually to point to the driver-specific device structure
95that embed the struct v4l2_device instance. This is achieved by a
Laurent Pinchart2c0ab672009-12-09 08:40:10 -030096dev_set_drvdata() call before registering the V4L2 device instance. They must
97also set the struct v4l2_device mdev field to point to a properly initialized
98and registered media_device instance.
Laurent Pinchart95db3a62009-12-09 08:40:05 -030099
100If v4l2_dev->name is empty then it will be set to a value derived from dev
101(driver name followed by the bus_id, to be precise). If you set it up before
102calling v4l2_device_register then it will be untouched. If dev is NULL, then
103you *must* setup v4l2_dev->name before calling v4l2_device_register.
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300104
Hans Verkuil102e7812009-05-02 10:12:50 -0300105You can use v4l2_device_set_name() to set the name based on a driver name and
106a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1,
107etc. If the name ends with a digit, then it will insert a dash: cx18-0,
108cx18-1, etc. This function returns the instance number.
109
Hans Verkuila47ddf12008-12-19 10:20:22 -0300110The first 'dev' argument is normally the struct device pointer of a pci_dev,
Janne Grunau073d6962009-04-01 08:30:06 -0300111usb_interface or platform_device. It is rare for dev to be NULL, but it happens
Hans Verkuil00575962009-03-13 10:03:04 -0300112with ISA devices or when one device creates multiple PCI devices, thus making
113it impossible to associate v4l2_dev with a particular parent.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300114
Hans Verkuil98ec6332009-03-08 17:02:10 -0300115You can also supply a notify() callback that can be called by sub-devices to
116notify you of events. Whether you need to set this depends on the sub-device.
117Any notifications a sub-device supports must be defined in a header in
118include/media/<subdevice>.h.
119
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300120You unregister with:
121
122 v4l2_device_unregister(struct v4l2_device *v4l2_dev);
123
Laurent Pinchart95db3a62009-12-09 08:40:05 -0300124If the dev->driver_data field points to v4l2_dev, it will be reset to NULL.
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300125Unregistering will also automatically unregister all subdevs from the device.
126
Hans Verkuilae6cfaa2009-03-14 08:28:45 -0300127If you have a hotpluggable device (e.g. a USB device), then when a disconnect
128happens the parent device becomes invalid. Since v4l2_device has a pointer to
129that parent device it has to be cleared as well to mark that the parent is
130gone. To do this call:
131
132 v4l2_device_disconnect(struct v4l2_device *v4l2_dev);
133
134This does *not* unregister the subdevs, so you still need to call the
135v4l2_device_unregister() function for that. If your driver is not hotpluggable,
136then there is no need to call v4l2_device_disconnect().
137
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300138Sometimes you need to iterate over all devices registered by a specific
139driver. This is usually the case if multiple device drivers use the same
140hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
141hardware. The same is true for alsa drivers for example.
142
143You can iterate over all registered devices as follows:
144
145static int callback(struct device *dev, void *p)
146{
147 struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
148
149 /* test if this device was inited */
150 if (v4l2_dev == NULL)
151 return 0;
152 ...
153 return 0;
154}
155
156int iterate(void *p)
157{
158 struct device_driver *drv;
159 int err;
160
161 /* Find driver 'ivtv' on the PCI bus.
162 pci_bus_type is a global. For USB busses use usb_bus_type. */
163 drv = driver_find("ivtv", &pci_bus_type);
164 /* iterate over all ivtv device instances */
165 err = driver_for_each_device(drv, NULL, p, callback);
166 put_driver(drv);
167 return err;
168}
169
170Sometimes you need to keep a running counter of the device instance. This is
171commonly used to map a device instance to an index of a module option array.
172
173The recommended approach is as follows:
174
175static atomic_t drv_instance = ATOMIC_INIT(0);
176
Hans Verkuil89aec3e2009-02-07 07:07:04 -0300177static int __devinit drv_probe(struct pci_dev *pdev,
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300178 const struct pci_device_id *pci_id)
179{
180 ...
181 state->instance = atomic_inc_return(&drv_instance) - 1;
182}
183
Hans Verkuil2335e2b2011-02-24 06:28:46 -0300184If you have multiple device nodes then it can be difficult to know when it is
185safe to unregister v4l2_device. For this purpose v4l2_device has refcounting
186support. The refcount is increased whenever video_register_device is called and
187it is decreased whenever that device node is released. When the refcount reaches
188zero, then the v4l2_device release() callback is called. You can do your final
189cleanup there.
190
191If other device nodes (e.g. ALSA) are created, then you can increase and
192decrease the refcount manually as well by calling:
193
194void v4l2_device_get(struct v4l2_device *v4l2_dev);
195
196or:
197
198int v4l2_device_put(struct v4l2_device *v4l2_dev);
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300199
200struct v4l2_subdev
201------------------
202
203Many drivers need to communicate with sub-devices. These devices can do all
204sort of tasks, but most commonly they handle audio and/or video muxing,
205encoding or decoding. For webcams common sub-devices are sensors and camera
206controllers.
207
208Usually these are I2C devices, but not necessarily. In order to provide the
209driver with a consistent interface to these sub-devices the v4l2_subdev struct
210(v4l2-subdev.h) was created.
211
212Each sub-device driver must have a v4l2_subdev struct. This struct can be
213stand-alone for simple sub-devices or it might be embedded in a larger struct
214if more state information needs to be stored. Usually there is a low-level
215device struct (e.g. i2c_client) that contains the device data as setup
216by the kernel. It is recommended to store that pointer in the private
217data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
218from a v4l2_subdev to the actual low-level bus-specific device data.
219
220You also need a way to go from the low-level struct to v4l2_subdev. For the
221common i2c_client struct the i2c_set_clientdata() call is used to store a
222v4l2_subdev pointer, for other busses you may have to use other methods.
223
Laurent Pinchart692d55222010-07-30 17:24:55 -0300224Bridges might also need to store per-subdev private data, such as a pointer to
225bridge-specific per-subdev private data. The v4l2_subdev structure provides
226host private data for that purpose that can be accessed with
227v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata().
228
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300229From the bridge driver perspective you load the sub-device module and somehow
230obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
231i2c_get_clientdata(). For other busses something similar needs to be done.
232Helper functions exists for sub-devices on an I2C bus that do most of this
233tricky work for you.
234
235Each v4l2_subdev contains function pointers that sub-device drivers can
236implement (or leave NULL if it is not applicable). Since sub-devices can do
237so many different things and you do not want to end up with a huge ops struct
238of which only a handful of ops are commonly implemented, the function pointers
239are sorted according to category and each category has its own ops struct.
240
241The top-level ops struct contains pointers to the category ops structs, which
242may be NULL if the subdev driver does not support anything from that category.
243
244It looks like this:
245
246struct v4l2_subdev_core_ops {
Hans Verkuilaecde8b52008-12-30 07:14:19 -0300247 int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip);
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300248 int (*log_status)(struct v4l2_subdev *sd);
249 int (*init)(struct v4l2_subdev *sd, u32 val);
250 ...
251};
252
253struct v4l2_subdev_tuner_ops {
254 ...
255};
256
257struct v4l2_subdev_audio_ops {
258 ...
259};
260
261struct v4l2_subdev_video_ops {
262 ...
263};
264
265struct v4l2_subdev_ops {
266 const struct v4l2_subdev_core_ops *core;
267 const struct v4l2_subdev_tuner_ops *tuner;
268 const struct v4l2_subdev_audio_ops *audio;
269 const struct v4l2_subdev_video_ops *video;
270};
271
272The core ops are common to all subdevs, the other categories are implemented
273depending on the sub-device. E.g. a video device is unlikely to support the
274audio ops and vice versa.
275
276This setup limits the number of function pointers while still making it easy
277to add new ops and categories.
278
279A sub-device driver initializes the v4l2_subdev struct using:
280
Hans Verkuil89aec3e2009-02-07 07:07:04 -0300281 v4l2_subdev_init(sd, &ops);
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300282
283Afterwards you need to initialize subdev->name with a unique name and set the
284module owner. This is done for you if you use the i2c helper functions.
285
Laurent Pinchart61f5db52009-12-09 08:40:08 -0300286If integration with the media framework is needed, you must initialize the
287media_entity struct embedded in the v4l2_subdev struct (entity field) by
288calling media_entity_init():
289
290 struct media_pad *pads = &my_sd->pads;
291 int err;
292
293 err = media_entity_init(&sd->entity, npads, pads, 0);
294
295The pads array must have been previously initialized. There is no need to
296manually set the struct media_entity type and name fields, but the revision
297field must be initialized if needed.
298
299A reference to the entity will be automatically acquired/released when the
300subdev device node (if any) is opened/closed.
301
302Don't forget to cleanup the media entity before the sub-device is destroyed:
303
304 media_entity_cleanup(&sd->entity);
305
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300306A device (bridge) driver needs to register the v4l2_subdev with the
307v4l2_device:
308
Hans Verkuil89aec3e2009-02-07 07:07:04 -0300309 int err = v4l2_device_register_subdev(v4l2_dev, sd);
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300310
311This can fail if the subdev module disappeared before it could be registered.
312After this function was called successfully the subdev->dev field points to
313the v4l2_device.
314
Laurent Pinchart61f5db52009-12-09 08:40:08 -0300315If the v4l2_device parent device has a non-NULL mdev field, the sub-device
316entity will be automatically registered with the media device.
317
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300318You can unregister a sub-device using:
319
Hans Verkuil89aec3e2009-02-07 07:07:04 -0300320 v4l2_device_unregister_subdev(sd);
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300321
Hans Verkuil89aec3e2009-02-07 07:07:04 -0300322Afterwards the subdev module can be unloaded and sd->dev == NULL.
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300323
324You can call an ops function either directly:
325
Hans Verkuil89aec3e2009-02-07 07:07:04 -0300326 err = sd->ops->core->g_chip_ident(sd, &chip);
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300327
328but it is better and easier to use this macro:
329
Hans Verkuil89aec3e2009-02-07 07:07:04 -0300330 err = v4l2_subdev_call(sd, core, g_chip_ident, &chip);
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300331
332The macro will to the right NULL pointer checks and returns -ENODEV if subdev
333is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is
334NULL, or the actual result of the subdev->ops->core->g_chip_ident ops.
335
336It is also possible to call all or a subset of the sub-devices:
337
Hans Verkuil89aec3e2009-02-07 07:07:04 -0300338 v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip);
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300339
340Any subdev that does not support this ops is skipped and error results are
341ignored. If you want to check for errors use this:
342
Hans Verkuil89aec3e2009-02-07 07:07:04 -0300343 err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip);
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300344
345Any error except -ENOIOCTLCMD will exit the loop with that error. If no
Lucas De Marchi25985ed2011-03-30 22:57:33 -0300346errors (except -ENOIOCTLCMD) occurred, then 0 is returned.
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300347
348The second argument to both calls is a group ID. If 0, then all subdevs are
349called. If non-zero, then only those whose group ID match that value will
Hans Verkuilb0167602009-02-14 12:00:53 -0300350be called. Before a bridge driver registers a subdev it can set sd->grp_id
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300351to whatever value it wants (it's 0 by default). This value is owned by the
352bridge driver and the sub-device driver will never modify or use it.
353
354The group ID gives the bridge driver more control how callbacks are called.
355For example, there may be multiple audio chips on a board, each capable of
356changing the volume. But usually only one will actually be used when the
357user want to change the volume. You can set the group ID for that subdev to
358e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
359v4l2_device_call_all(). That ensures that it will only go to the subdev
360that needs it.
361
Hans Verkuil98ec6332009-03-08 17:02:10 -0300362If the sub-device needs to notify its v4l2_device parent of an event, then
363it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
364whether there is a notify() callback defined and returns -ENODEV if not.
365Otherwise the result of the notify() call is returned.
366
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300367The advantage of using v4l2_subdev is that it is a generic struct and does
368not contain any knowledge about the underlying hardware. So a driver might
369contain several subdevs that use an I2C bus, but also a subdev that is
370controlled through GPIO pins. This distinction is only relevant when setting
371up the device, but once the subdev is registered it is completely transparent.
372
373
Laurent Pinchart2096a5d2009-12-09 08:38:49 -0300374V4L2 sub-device userspace API
375-----------------------------
376
377Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2
378sub-devices can also be controlled directly by userspace applications.
379
380Device nodes named v4l-subdevX can be created in /dev to access sub-devices
381directly. If a sub-device supports direct userspace configuration it must set
382the V4L2_SUBDEV_FL_HAS_DEVNODE flag before being registered.
383
384After registering sub-devices, the v4l2_device driver can create device nodes
385for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling
386v4l2_device_register_subdev_nodes(). Those device nodes will be automatically
387removed when sub-devices are unregistered.
388
Laurent Pinchartea8aa432009-12-09 08:39:54 -0300389The device node handles a subset of the V4L2 API.
390
391VIDIOC_QUERYCTRL
392VIDIOC_QUERYMENU
393VIDIOC_G_CTRL
394VIDIOC_S_CTRL
395VIDIOC_G_EXT_CTRLS
396VIDIOC_S_EXT_CTRLS
397VIDIOC_TRY_EXT_CTRLS
398
399 The controls ioctls are identical to the ones defined in V4L2. They
400 behave identically, with the only exception that they deal only with
401 controls implemented in the sub-device. Depending on the driver, those
402 controls can be also be accessed through one (or several) V4L2 device
403 nodes.
404
Sakari Ailus02adb1c2010-03-03 12:49:38 -0300405VIDIOC_DQEVENT
406VIDIOC_SUBSCRIBE_EVENT
407VIDIOC_UNSUBSCRIBE_EVENT
408
409 The events ioctls are identical to the ones defined in V4L2. They
410 behave identically, with the only exception that they deal only with
411 events generated by the sub-device. Depending on the driver, those
412 events can also be reported by one (or several) V4L2 device nodes.
413
414 Sub-device drivers that want to use events need to set the
415 V4L2_SUBDEV_USES_EVENTS v4l2_subdev::flags and initialize
416 v4l2_subdev::nevents to events queue depth before registering the
417 sub-device. After registration events can be queued as usual on the
418 v4l2_subdev::devnode device node.
419
420 To properly support events, the poll() file operation is also
421 implemented.
422
Laurent Pinchartc30b46e2010-02-26 12:23:10 -0300423Private ioctls
424
425 All ioctls not in the above list are passed directly to the sub-device
426 driver through the core::ioctl operation.
427
Laurent Pinchart2096a5d2009-12-09 08:38:49 -0300428
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300429I2C sub-device drivers
430----------------------
431
432Since these drivers are so common, special helper functions are available to
433ease the use of these drivers (v4l2-common.h).
434
435The recommended method of adding v4l2_subdev support to an I2C driver is to
436embed the v4l2_subdev struct into the state struct that is created for each
437I2C device instance. Very simple devices have no state struct and in that case
438you can just create a v4l2_subdev directly.
439
440A typical state struct would look like this (where 'chipname' is replaced by
441the name of the chip):
442
443struct chipname_state {
444 struct v4l2_subdev sd;
445 ... /* additional state fields */
446};
447
448Initialize the v4l2_subdev struct as follows:
449
450 v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
451
452This function will fill in all the fields of v4l2_subdev and ensure that the
453v4l2_subdev and i2c_client both point to one another.
454
455You should also add a helper inline function to go from a v4l2_subdev pointer
456to a chipname_state struct:
457
458static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
459{
460 return container_of(sd, struct chipname_state, sd);
461}
462
463Use this to go from the v4l2_subdev struct to the i2c_client struct:
464
465 struct i2c_client *client = v4l2_get_subdevdata(sd);
466
467And this to go from an i2c_client to a v4l2_subdev struct:
468
469 struct v4l2_subdev *sd = i2c_get_clientdata(client);
470
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300471Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
472is called. This will unregister the sub-device from the bridge driver. It is
473safe to call this even if the sub-device was never registered.
474
Hans Verkuilf5360bd2009-01-15 06:09:05 -0300475You need to do this because when the bridge driver destroys the i2c adapter
476the remove() callbacks are called of the i2c devices on that adapter.
477After that the corresponding v4l2_subdev structures are invalid, so they
478have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
479from the remove() callback ensures that this is always done correctly.
480
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300481
482The bridge driver also has some helper functions it can use:
483
Hans Verkuile6574f22009-04-01 03:57:53 -0300484struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
Hans Verkuil53dacb12009-08-10 02:49:08 -0300485 "module_foo", "chipid", 0x36, NULL);
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300486
487This loads the given module (can be NULL if no module needs to be loaded) and
488calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
Hans Verkuile6574f22009-04-01 03:57:53 -0300489If all goes well, then it registers the subdev with the v4l2_device.
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300490
Hans Verkuil53dacb12009-08-10 02:49:08 -0300491You can also use the last argument of v4l2_i2c_new_subdev() to pass an array
492of possible I2C addresses that it should probe. These probe addresses are
493only used if the previous argument is 0. A non-zero argument means that you
494know the exact i2c address so in that case no probing will take place.
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300495
496Both functions return NULL if something went wrong.
497
Hans Verkuil53dacb12009-08-10 02:49:08 -0300498Note that the chipid you pass to v4l2_i2c_new_subdev() is usually
Hans Verkuil2c792522009-03-12 18:34:19 -0300499the same as the module name. It allows you to specify a chip variant, e.g.
500"saa7114" or "saa7115". In general though the i2c driver autodetects this.
501The use of chipid is something that needs to be looked at more closely at a
502later date. It differs between i2c drivers and as such can be confusing.
503To see which chip variants are supported you can look in the i2c driver code
504for the i2c_device_id table. This lists all the possibilities.
505
Hans Verkuil2c0b19a2009-06-09 17:29:29 -0300506There are two more helper functions:
507
508v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data
509arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not
5100 then that will be used (non-probing variant), otherwise the probed_addrs
511are probed.
512
513For example: this will probe for address 0x10:
514
515struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter,
516 "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10));
517
518v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed
519to the i2c driver and replaces the irq, platform_data and addr arguments.
520
521If the subdev supports the s_config core ops, then that op is called with
522the irq and platform_data arguments after the subdev was setup. The older
523v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with
524irq set to 0 and platform_data set to NULL.
525
Hans Verkuil2a1fcdf2008-11-29 21:36:58 -0300526struct video_device
527-------------------
528
Hans Verkuila47ddf12008-12-19 10:20:22 -0300529The actual device nodes in the /dev directory are created using the
530video_device struct (v4l2-dev.h). This struct can either be allocated
531dynamically or embedded in a larger struct.
532
533To allocate it dynamically use:
534
535 struct video_device *vdev = video_device_alloc();
536
537 if (vdev == NULL)
538 return -ENOMEM;
539
540 vdev->release = video_device_release;
541
542If you embed it in a larger struct, then you must set the release()
543callback to your own function:
544
545 struct video_device *vdev = &my_vdev->vdev;
546
547 vdev->release = my_vdev_release;
548
549The release callback must be set and it is called when the last user
550of the video device exits.
551
552The default video_device_release() callback just calls kfree to free the
553allocated memory.
554
555You should also set these fields:
556
Hans Verkuildfa9a5a2008-12-23 12:17:23 -0300557- v4l2_dev: set to the v4l2_device parent device.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300558- name: set to something descriptive and unique.
Hans Verkuilc7dd09d2008-12-23 13:42:25 -0300559- fops: set to the v4l2_file_operations struct.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300560- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
561 (highly recommended to use this and it might become compulsory in the
562 future!), then set this to your v4l2_ioctl_ops struct.
Hans Verkuilee6869a2010-09-26 08:47:38 -0300563- lock: leave to NULL if you want to do all the locking in the driver.
564 Otherwise you give it a pointer to a struct mutex_lock and before any
565 of the v4l2_file_operations is called this lock will be taken by the
566 core and released afterwards.
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300567- prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
568 If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
569 If you want to have a separate priority state per (group of) device node(s),
570 then you can point it to your own struct v4l2_prio_state.
Hans Verkuil00575962009-03-13 10:03:04 -0300571- parent: you only set this if v4l2_device was registered with NULL as
572 the parent device struct. This only happens in cases where one hardware
573 device has multiple PCI devices that all share the same v4l2_device core.
574
575 The cx88 driver is an example of this: one core v4l2_device struct, but
576 it is used by both an raw video PCI device (cx8800) and a MPEG PCI device
577 (cx8802). Since the v4l2_device cannot be associated with a particular
578 PCI device it is setup without a parent device. But when the struct
579 video_device is setup you do know which parent PCI device to use.
Hans Verkuilb1a873a2011-03-22 10:14:07 -0300580- flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework
581 handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct
582 v4l2_fh. Eventually this flag will disappear once all drivers use the core
583 priority handling. But for now it has to be set explicitly.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300584
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300585If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
586in your v4l2_file_operations struct.
587
588Do not use .ioctl! This is deprecated and will go away in the future.
Hans Verkuilc7dd09d2008-12-23 13:42:25 -0300589
590The v4l2_file_operations struct is a subset of file_operations. The main
591difference is that the inode argument is omitted since it is never used.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300592
Laurent Pinchart2c0ab672009-12-09 08:40:10 -0300593If integration with the media framework is needed, you must initialize the
594media_entity struct embedded in the video_device struct (entity field) by
595calling media_entity_init():
596
597 struct media_pad *pad = &my_vdev->pad;
598 int err;
599
600 err = media_entity_init(&vdev->entity, 1, pad, 0);
601
602The pads array must have been previously initialized. There is no need to
603manually set the struct media_entity type and name fields.
604
605A reference to the entity will be automatically acquired/released when the
606video device is opened/closed.
607
Hans Verkuilee6869a2010-09-26 08:47:38 -0300608v4l2_file_operations and locking
609--------------------------------
610
611You can set a pointer to a mutex_lock in struct video_device. Usually this
612will be either a top-level mutex or a mutex per device node. If you want
613finer-grained locking then you have to set it to NULL and do you own locking.
614
Hans Verkuil43599f32011-11-07 12:44:28 -0300615It is up to the driver developer to decide which method to use. However, if
616your driver has high-latency operations (for example, changing the exposure
617of a USB webcam might take a long time), then you might be better off with
618doing your own locking if you want to allow the user to do other things with
619the device while waiting for the high-latency command to finish.
620
Hans Verkuilee6869a2010-09-26 08:47:38 -0300621If a lock is specified then all file operations will be serialized on that
622lock. If you use videobuf then you must pass the same lock to the videobuf
623queue initialize function: if videobuf has to wait for a frame to arrive, then
624it will temporarily unlock the lock and relock it afterwards. If your driver
625also waits in the code, then you should do the same to allow other processes
626to access the device node while the first process is waiting for something.
627
Hans Verkuil43599f32011-11-07 12:44:28 -0300628In the case of videobuf2 you will need to implement the wait_prepare and
629wait_finish callbacks to unlock/lock if applicable. In particular, if you use
630the lock in struct video_device then you must unlock/lock this mutex in
631wait_prepare and wait_finish.
632
Hans Verkuilee6869a2010-09-26 08:47:38 -0300633The implementation of a hotplug disconnect should also take the lock before
Hans Verkuil9c84d892010-10-11 12:36:37 -0300634calling v4l2_device_disconnect.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300635
636video_device registration
637-------------------------
638
639Next you register the video device: this will create the character device
640for you.
641
642 err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
643 if (err) {
Hans Verkuil50a2a8b2008-12-22 09:13:11 -0300644 video_device_release(vdev); /* or kfree(my_vdev); */
Hans Verkuila47ddf12008-12-19 10:20:22 -0300645 return err;
646 }
647
Laurent Pinchart2c0ab672009-12-09 08:40:10 -0300648If the v4l2_device parent device has a non-NULL mdev field, the video device
649entity will be automatically registered with the media device.
650
Hans Verkuila47ddf12008-12-19 10:20:22 -0300651Which device is registered depends on the type argument. The following
652types exist:
653
654VFL_TYPE_GRABBER: videoX for video input/output devices
655VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
656VFL_TYPE_RADIO: radioX for radio tuners
Hans Verkuila47ddf12008-12-19 10:20:22 -0300657
658The last argument gives you a certain amount of control over the device
Hans Verkuil6b5270d2009-09-06 07:54:00 -0300659device node number used (i.e. the X in videoX). Normally you will pass -1
660to let the v4l2 framework pick the first free number. But sometimes users
661want to select a specific node number. It is common that drivers allow
662the user to select a specific device node number through a driver module
663option. That number is then passed to this function and video_register_device
664will attempt to select that device node number. If that number was already
665in use, then the next free device node number will be selected and it
666will send a warning to the kernel log.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300667
Hans Verkuil6b5270d2009-09-06 07:54:00 -0300668Another use-case is if a driver creates many devices. In that case it can
669be useful to place different video devices in separate ranges. For example,
670video capture devices start at 0, video output devices start at 16.
Hans Verkuil22e22122009-09-06 07:13:14 -0300671So you can use the last argument to specify a minimum device node number
672and the v4l2 framework will try to pick the first free number that is equal
Hans Verkuila47ddf12008-12-19 10:20:22 -0300673or higher to what you passed. If that fails, then it will just pick the
674first free number.
675
Hans Verkuil6b5270d2009-09-06 07:54:00 -0300676Since in this case you do not care about a warning about not being able
677to select the specified device node number, you can call the function
678video_register_device_no_warn() instead.
679
Hans Verkuila47ddf12008-12-19 10:20:22 -0300680Whenever a device node is created some attributes are also created for you.
681If you look in /sys/class/video4linux you see the devices. Go into e.g.
682video0 and you will see 'name' and 'index' attributes. The 'name' attribute
Hans Verkuil7ae0cd92009-06-19 11:32:56 -0300683is the 'name' field of the video_device struct.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300684
Hans Verkuil7ae0cd92009-06-19 11:32:56 -0300685The 'index' attribute is the index of the device node: for each call to
686video_register_device() the index is just increased by 1. The first video
687device node you register always starts with index 0.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300688
689Users can setup udev rules that utilize the index attribute to make fancy
690device names (e.g. 'mpegX' for MPEG video capture device nodes).
691
692After the device was successfully registered, then you can use these fields:
693
694- vfl_type: the device type passed to video_register_device.
695- minor: the assigned device minor number.
Hans Verkuil22e22122009-09-06 07:13:14 -0300696- num: the device node number (i.e. the X in videoX).
Hans Verkuil7ae0cd92009-06-19 11:32:56 -0300697- index: the device index number.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300698
699If the registration failed, then you need to call video_device_release()
700to free the allocated video_device struct, or free your own struct if the
701video_device was embedded in it. The vdev->release() callback will never
702be called if the registration failed, nor should you ever attempt to
703unregister the device if the registration failed.
704
705
706video_device cleanup
707--------------------
708
709When the video device nodes have to be removed, either during the unload
710of the driver or because the USB device was disconnected, then you should
711unregister them:
712
713 video_unregister_device(vdev);
714
715This will remove the device nodes from sysfs (causing udev to remove them
716from /dev).
717
Hans Verkuildd1ad942010-04-06 11:44:39 -0300718After video_unregister_device() returns no new opens can be done. However,
719in the case of USB devices some application might still have one of these
Hans Verkuild69f2712010-09-26 08:16:56 -0300720device nodes open. So after the unregister all file operations (except
721release, of course) will return an error as well.
Hans Verkuila47ddf12008-12-19 10:20:22 -0300722
723When the last user of the video device node exits, then the vdev->release()
724callback is called and you can do the final cleanup there.
725
Laurent Pinchart2c0ab672009-12-09 08:40:10 -0300726Don't forget to cleanup the media entity associated with the video device if
727it has been initialized:
728
729 media_entity_cleanup(&vdev->entity);
730
731This can be done from the release callback.
732
Hans Verkuila47ddf12008-12-19 10:20:22 -0300733
734video_device helper functions
735-----------------------------
736
737There are a few useful helper functions:
738
Laurent Pincharteac8ea52009-11-27 13:56:50 -0300739- file/video_device private data
740
Hans Verkuila47ddf12008-12-19 10:20:22 -0300741You can set/get driver private data in the video_device struct using:
742
Hans Verkuil89aec3e2009-02-07 07:07:04 -0300743void *video_get_drvdata(struct video_device *vdev);
744void video_set_drvdata(struct video_device *vdev, void *data);
Hans Verkuila47ddf12008-12-19 10:20:22 -0300745
746Note that you can safely call video_set_drvdata() before calling
747video_register_device().
748
749And this function:
750
751struct video_device *video_devdata(struct file *file);
752
753returns the video_device belonging to the file struct.
754
Laurent Pincharteac8ea52009-11-27 13:56:50 -0300755The video_drvdata function combines video_get_drvdata with video_devdata:
Hans Verkuila47ddf12008-12-19 10:20:22 -0300756
757void *video_drvdata(struct file *file);
758
759You can go from a video_device struct to the v4l2_device struct using:
760
Hans Verkuildfa9a5a2008-12-23 12:17:23 -0300761struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
Mauro Carvalho Chehab44061c02009-02-14 07:29:07 -0300762
Laurent Pincharteac8ea52009-11-27 13:56:50 -0300763- Device node name
764
765The video_device node kernel name can be retrieved using
766
767const char *video_device_node_name(struct video_device *vdev);
768
769The name is used as a hint by userspace tools such as udev. The function
770should be used where possible instead of accessing the video_device::num and
771video_device::minor fields.
772
773
Mauro Carvalho Chehab44061c02009-02-14 07:29:07 -0300774video buffer helper functions
775-----------------------------
776
Jonathan Corbet4b586a32010-02-22 17:47:46 -0300777The v4l2 core API provides a set of standard methods (called "videobuf")
778for dealing with video buffers. Those methods allow a driver to implement
779read(), mmap() and overlay() in a consistent way. There are currently
780methods for using video buffers on devices that supports DMA with
781scatter/gather method (videobuf-dma-sg), DMA with linear access
782(videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
783(videobuf-vmalloc).
Mauro Carvalho Chehab44061c02009-02-14 07:29:07 -0300784
Jonathan Corbet4b586a32010-02-22 17:47:46 -0300785Please see Documentation/video4linux/videobuf for more information on how
786to use the videobuf layer.
Sakari Ailus6cd84b72010-03-20 18:28:48 -0300787
788struct v4l2_fh
789--------------
790
791struct v4l2_fh provides a way to easily keep file handle specific data
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300792that is used by the V4L2 framework. New drivers must use struct v4l2_fh
Hans Verkuilb1a873a2011-03-22 10:14:07 -0300793since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY)
794if the video_device flag V4L2_FL_USE_FH_PRIO is also set.
Sakari Ailus6cd84b72010-03-20 18:28:48 -0300795
796The users of v4l2_fh (in the V4L2 framework, not the driver) know
797whether a driver uses v4l2_fh as its file->private_data pointer by
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300798testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is
799set whenever v4l2_fh_init() is called.
Sakari Ailus6cd84b72010-03-20 18:28:48 -0300800
801struct v4l2_fh is allocated as a part of the driver's own file handle
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300802structure and file->private_data is set to it in the driver's open
803function by the driver.
804
805In many cases the struct v4l2_fh will be embedded in a larger structure.
806In that case you should call v4l2_fh_init+v4l2_fh_add in open() and
807v4l2_fh_del+v4l2_fh_exit in release().
808
809Drivers can extract their own file handle structure by using the container_of
810macro. Example:
Sakari Ailus6cd84b72010-03-20 18:28:48 -0300811
812struct my_fh {
813 int blah;
814 struct v4l2_fh fh;
815};
816
817...
818
819int my_open(struct file *file)
820{
821 struct my_fh *my_fh;
822 struct video_device *vfd;
823 int ret;
824
825 ...
826
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300827 my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
Sakari Ailus6cd84b72010-03-20 18:28:48 -0300828
829 ...
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300830
Hans Verkuil98019f52011-06-18 05:13:55 -0300831 v4l2_fh_init(&my_fh->fh, vfd);
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300832
833 ...
834
835 file->private_data = &my_fh->fh;
836 v4l2_fh_add(&my_fh->fh);
837 return 0;
Sakari Ailus6cd84b72010-03-20 18:28:48 -0300838}
839
840int my_release(struct file *file)
841{
842 struct v4l2_fh *fh = file->private_data;
843 struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
844
845 ...
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300846 v4l2_fh_del(&my_fh->fh);
847 v4l2_fh_exit(&my_fh->fh);
848 kfree(my_fh);
849 return 0;
Sakari Ailus6cd84b72010-03-20 18:28:48 -0300850}
Sakari Ailusdd966082010-03-27 10:58:24 -0300851
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300852Below is a short description of the v4l2_fh functions used:
853
Hans Verkuil98019f52011-06-18 05:13:55 -0300854void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
Hans Verkuil6e29ad52011-02-24 10:58:13 -0300855
856 Initialise the file handle. This *MUST* be performed in the driver's
857 v4l2_file_operations->open() handler.
858
859void v4l2_fh_add(struct v4l2_fh *fh)
860
861 Add a v4l2_fh to video_device file handle list. Must be called once the
862 file handle is completely initialized.
863
864void v4l2_fh_del(struct v4l2_fh *fh)
865
866 Unassociate the file handle from video_device(). The file handle
867 exit function may now be called.
868
869void v4l2_fh_exit(struct v4l2_fh *fh)
870
871 Uninitialise the file handle. After uninitialisation the v4l2_fh
872 memory can be freed.
873
874
875If struct v4l2_fh is not embedded, then you can use these helper functions:
876
877int v4l2_fh_open(struct file *filp)
878
879 This allocates a struct v4l2_fh, initializes it and adds it to the struct
880 video_device associated with the file struct.
881
882int v4l2_fh_release(struct file *filp)
883
884 This deletes it from the struct video_device associated with the file
885 struct, uninitialised the v4l2_fh and frees it.
886
887These two functions can be plugged into the v4l2_file_operation's open() and
888release() ops.
889
890
891Several drivers need to do something when the first file handle is opened and
892when the last file handle closes. Two helper functions were added to check
893whether the v4l2_fh struct is the only open filehandle of the associated
894device node:
895
896int v4l2_fh_is_singular(struct v4l2_fh *fh)
897
898 Returns 1 if the file handle is the only open file handle, else 0.
899
900int v4l2_fh_is_singular_file(struct file *filp)
901
902 Same, but it calls v4l2_fh_is_singular with filp->private_data.
903
904
Sakari Ailusdd966082010-03-27 10:58:24 -0300905V4L2 events
906-----------
907
908The V4L2 events provide a generic way to pass events to user space.
909The driver must use v4l2_fh to be able to support V4L2 events.
910
Hans Verkuil1de73102011-06-18 06:14:42 -0300911Events are defined by a type and an optional ID. The ID may refer to a V4L2
912object such as a control ID. If unused, then the ID is 0.
913
914When the user subscribes to an event the driver will allocate a number of
915kevent structs for that event. So every (type, ID) event tuple will have
916its own set of kevent structs. This guarantees that if a driver is generating
917lots of events of one type in a short time, then that will not overwrite
918events of another type.
919
920But if you get more events of one type than the number of kevents that were
921reserved, then the oldest event will be dropped and the new one added.
922
923Furthermore, the internal struct v4l2_subscribed_event has merge() and
924replace() callbacks which drivers can set. These callbacks are called when
925a new event is raised and there is no more room. The replace() callback
926allows you to replace the payload of the old event with that of the new event,
927merging any relevant data from the old payload into the new payload that
928replaces it. It is called when this event type has only one kevent struct
929allocated. The merge() callback allows you to merge the oldest event payload
930into that of the second-oldest event payload. It is called when there are two
931or more kevent structs allocated.
932
933This way no status information is lost, just the intermediate steps leading
934up to that state.
935
936A good example of these replace/merge callbacks is in v4l2-event.c:
937ctrls_replace() and ctrls_merge() callbacks for the control event.
938
939Note: these callbacks can be called from interrupt context, so they must be
940fast.
941
Sakari Ailusdd966082010-03-27 10:58:24 -0300942Useful functions:
943
Sakari Ailusdd966082010-03-27 10:58:24 -0300944- v4l2_event_queue()
945
946 Queue events to video device. The driver's only responsibility is to fill
947 in the type and the data fields. The other fields will be filled in by
948 V4L2.
949
950- v4l2_event_subscribe()
951
952 The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
953 is able to produce events with specified event id. Then it calls
Hans Verkuil1de73102011-06-18 06:14:42 -0300954 v4l2_event_subscribe() to subscribe the event. The last argument is the
955 size of the event queue for this event. If it is 0, then the framework
956 will fill in a default value (this depends on the event type).
Sakari Ailusdd966082010-03-27 10:58:24 -0300957
958- v4l2_event_unsubscribe()
959
960 vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
961 v4l2_event_unsubscribe() directly unless it wants to be involved in
962 unsubscription process.
963
964 The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The
965 drivers may want to handle this in a special way.
966
967- v4l2_event_pending()
968
969 Returns the number of pending events. Useful when implementing poll.
970
Sakari Ailusdd966082010-03-27 10:58:24 -0300971Events are delivered to user space through the poll system call. The driver
Hans Verkuil1de73102011-06-18 06:14:42 -0300972can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait().
Sakari Ailusdd966082010-03-27 10:58:24 -0300973
974There are standard and private events. New standard events must use the
975smallest available event type. The drivers must allocate their events from
976their own class starting from class base. Class base is
977V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number.
978The first event type in the class is reserved for future use, so the first
979available event type is 'class base + 1'.
980
981An example on how the V4L2 events may be used can be found in the OMAP
Hans Verkuil1de73102011-06-18 06:14:42 -03009823 ISP driver (drivers/media/video/omap3isp).