1 Overview of the V4L2 driver framework
2 =====================================
4 This text documents the various structures provided by the V4L2 framework and
11 The V4L2 drivers tend to be very complex due to the complexity of the
12 hardware: 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
16 Especially the fact that V4L2 drivers have to setup supporting ICs to
17 do audio/video muxing/encoding/decoding makes it more complex than most.
18 Usually these ICs are connected to the main bridge driver through one or
19 more I2C busses, but other busses can also be used. Such devices are
22 For a long time the framework was limited to the video_device struct for
23 creating V4L device nodes and video_buf for handling the video buffers
24 (note that this document does not discuss the video_buf framework).
26 This meant that all drivers had to do the setup of device instances and
27 connecting to sub-devices themselves. Some of this is quite complicated
28 to do right and many drivers never did do it correctly.
30 There is also a lot of common code that could never be refactored due to
31 the lack of a framework.
33 So this framework sets up the basic building blocks that all drivers
34 need and this same framework should make it much easier to refactor
35 common code into utility functions shared by all drivers.
41 All drivers have the following structure:
43 1) A struct for each device instance containing the device state.
45 2) A way of initializing and commanding sub-devices (if any).
47 3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX)
48 and keeping track of device-node specific data.
50 4) Filehandle-specific structs containing per-filehandle data;
52 5) video buffer handling.
54 This is a rough schematic of how it all relates:
58 +-sub-device instances
62 \-filehandle instances
65 Structure of the framework
66 --------------------------
68 The framework closely resembles the driver structure: it has a v4l2_device
69 struct for the device instance data, a v4l2_subdev struct to refer to
70 sub-device instances, the video_device struct stores V4L2 device node data
71 and in the future a v4l2_fh struct will keep track of filehandle instances
72 (this is not yet implemented).
74 The V4L2 framework also optionally integrates with the media framework. If a
75 driver sets the struct v4l2_device mdev field, sub-devices and video nodes
76 will automatically appear in the media framework as entities.
82 Each device instance is represented by a struct v4l2_device (v4l2-device.h).
83 Very simple devices can just allocate this struct, but most of the time you
84 would embed this struct inside a larger struct.
86 You must register the device instance:
88 v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev);
90 Registration will initialize the v4l2_device struct. If the dev->driver_data
91 field is NULL, it will be linked to v4l2_dev.
93 Drivers that want integration with the media device framework need to set
94 dev->driver_data manually to point to the driver-specific device structure
95 that embed the struct v4l2_device instance. This is achieved by a
96 dev_set_drvdata() call before registering the V4L2 device instance. They must
97 also set the struct v4l2_device mdev field to point to a properly initialized
98 and registered media_device instance.
100 If 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
102 calling v4l2_device_register then it will be untouched. If dev is NULL, then
103 you *must* setup v4l2_dev->name before calling v4l2_device_register.
105 You can use v4l2_device_set_name() to set the name based on a driver name and
106 a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1,
107 etc. If the name ends with a digit, then it will insert a dash: cx18-0,
108 cx18-1, etc. This function returns the instance number.
110 The first 'dev' argument is normally the struct device pointer of a pci_dev,
111 usb_interface or platform_device. It is rare for dev to be NULL, but it happens
112 with ISA devices or when one device creates multiple PCI devices, thus making
113 it impossible to associate v4l2_dev with a particular parent.
115 You can also supply a notify() callback that can be called by sub-devices to
116 notify you of events. Whether you need to set this depends on the sub-device.
117 Any notifications a sub-device supports must be defined in a header in
118 include/media/<subdevice>.h.
122 v4l2_device_unregister(struct v4l2_device *v4l2_dev);
124 If the dev->driver_data field points to v4l2_dev, it will be reset to NULL.
125 Unregistering will also automatically unregister all subdevs from the device.
127 If you have a hotpluggable device (e.g. a USB device), then when a disconnect
128 happens the parent device becomes invalid. Since v4l2_device has a pointer to
129 that parent device it has to be cleared as well to mark that the parent is
130 gone. To do this call:
132 v4l2_device_disconnect(struct v4l2_device *v4l2_dev);
134 This does *not* unregister the subdevs, so you still need to call the
135 v4l2_device_unregister() function for that. If your driver is not hotpluggable,
136 then there is no need to call v4l2_device_disconnect().
138 Sometimes you need to iterate over all devices registered by a specific
139 driver. This is usually the case if multiple device drivers use the same
140 hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
141 hardware. The same is true for alsa drivers for example.
143 You can iterate over all registered devices as follows:
145 static int callback(struct device *dev, void *p)
147 struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
149 /* test if this device was inited */
150 if (v4l2_dev == NULL)
158 struct device_driver *drv;
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);
170 Sometimes you need to keep a running counter of the device instance. This is
171 commonly used to map a device instance to an index of a module option array.
173 The recommended approach is as follows:
175 static atomic_t drv_instance = ATOMIC_INIT(0);
177 static int __devinit drv_probe(struct pci_dev *pdev,
178 const struct pci_device_id *pci_id)
181 state->instance = atomic_inc_return(&drv_instance) - 1;
184 If you have multiple device nodes then it can be difficult to know when it is
185 safe to unregister v4l2_device for hotpluggable devices. For this purpose
186 v4l2_device has refcounting support. The refcount is increased whenever
187 video_register_device is called and it is decreased whenever that device node
188 is released. When the refcount reaches zero, then the v4l2_device release()
189 callback is called. You can do your final cleanup there.
191 If other device nodes (e.g. ALSA) are created, then you can increase and
192 decrease the refcount manually as well by calling:
194 void v4l2_device_get(struct v4l2_device *v4l2_dev);
198 int v4l2_device_put(struct v4l2_device *v4l2_dev);
200 Since the initial refcount is 1 you also need to call v4l2_device_put in the
201 disconnect() callback (for USB devices) or in the remove() callback (for e.g.
202 PCI devices), otherwise the refcount will never reach 0.
207 Many drivers need to communicate with sub-devices. These devices can do all
208 sort of tasks, but most commonly they handle audio and/or video muxing,
209 encoding or decoding. For webcams common sub-devices are sensors and camera
212 Usually these are I2C devices, but not necessarily. In order to provide the
213 driver with a consistent interface to these sub-devices the v4l2_subdev struct
214 (v4l2-subdev.h) was created.
216 Each sub-device driver must have a v4l2_subdev struct. This struct can be
217 stand-alone for simple sub-devices or it might be embedded in a larger struct
218 if more state information needs to be stored. Usually there is a low-level
219 device struct (e.g. i2c_client) that contains the device data as setup
220 by the kernel. It is recommended to store that pointer in the private
221 data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
222 from a v4l2_subdev to the actual low-level bus-specific device data.
224 You also need a way to go from the low-level struct to v4l2_subdev. For the
225 common i2c_client struct the i2c_set_clientdata() call is used to store a
226 v4l2_subdev pointer, for other busses you may have to use other methods.
228 Bridges might also need to store per-subdev private data, such as a pointer to
229 bridge-specific per-subdev private data. The v4l2_subdev structure provides
230 host private data for that purpose that can be accessed with
231 v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata().
233 From the bridge driver perspective you load the sub-device module and somehow
234 obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
235 i2c_get_clientdata(). For other busses something similar needs to be done.
236 Helper functions exists for sub-devices on an I2C bus that do most of this
239 Each v4l2_subdev contains function pointers that sub-device drivers can
240 implement (or leave NULL if it is not applicable). Since sub-devices can do
241 so many different things and you do not want to end up with a huge ops struct
242 of which only a handful of ops are commonly implemented, the function pointers
243 are sorted according to category and each category has its own ops struct.
245 The top-level ops struct contains pointers to the category ops structs, which
246 may be NULL if the subdev driver does not support anything from that category.
250 struct v4l2_subdev_core_ops {
251 int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip);
252 int (*log_status)(struct v4l2_subdev *sd);
253 int (*init)(struct v4l2_subdev *sd, u32 val);
257 struct v4l2_subdev_tuner_ops {
261 struct v4l2_subdev_audio_ops {
265 struct v4l2_subdev_video_ops {
269 struct v4l2_subdev_pad_ops {
273 struct v4l2_subdev_ops {
274 const struct v4l2_subdev_core_ops *core;
275 const struct v4l2_subdev_tuner_ops *tuner;
276 const struct v4l2_subdev_audio_ops *audio;
277 const struct v4l2_subdev_video_ops *video;
278 const struct v4l2_subdev_pad_ops *video;
281 The core ops are common to all subdevs, the other categories are implemented
282 depending on the sub-device. E.g. a video device is unlikely to support the
283 audio ops and vice versa.
285 This setup limits the number of function pointers while still making it easy
286 to add new ops and categories.
288 A sub-device driver initializes the v4l2_subdev struct using:
290 v4l2_subdev_init(sd, &ops);
292 Afterwards you need to initialize subdev->name with a unique name and set the
293 module owner. This is done for you if you use the i2c helper functions.
295 If integration with the media framework is needed, you must initialize the
296 media_entity struct embedded in the v4l2_subdev struct (entity field) by
297 calling media_entity_init():
299 struct media_pad *pads = &my_sd->pads;
302 err = media_entity_init(&sd->entity, npads, pads, 0);
304 The pads array must have been previously initialized. There is no need to
305 manually set the struct media_entity type and name fields, but the revision
306 field must be initialized if needed.
308 A reference to the entity will be automatically acquired/released when the
309 subdev device node (if any) is opened/closed.
311 Don't forget to cleanup the media entity before the sub-device is destroyed:
313 media_entity_cleanup(&sd->entity);
315 If the subdev driver intends to process video and integrate with the media
316 framework, it must implement format related functionality using
317 v4l2_subdev_pad_ops instead of v4l2_subdev_video_ops.
319 In that case, the subdev driver may set the link_validate field to provide
320 its own link validation function. The link validation function is called for
321 every link in the pipeline where both of the ends of the links are V4L2
322 sub-devices. The driver is still responsible for validating the correctness
323 of the format configuration between sub-devices and video nodes.
325 If link_validate op is not set, the default function
326 v4l2_subdev_link_validate_default() is used instead. This function ensures
327 that width, height and the media bus pixel code are equal on both source and
328 sink of the link. Subdev drivers are also free to use this function to
329 perform the checks mentioned above in addition to their own checks.
331 A device (bridge) driver needs to register the v4l2_subdev with the
334 int err = v4l2_device_register_subdev(v4l2_dev, sd);
336 This can fail if the subdev module disappeared before it could be registered.
337 After this function was called successfully the subdev->dev field points to
340 If the v4l2_device parent device has a non-NULL mdev field, the sub-device
341 entity will be automatically registered with the media device.
343 You can unregister a sub-device using:
345 v4l2_device_unregister_subdev(sd);
347 Afterwards the subdev module can be unloaded and sd->dev == NULL.
349 You can call an ops function either directly:
351 err = sd->ops->core->g_chip_ident(sd, &chip);
353 but it is better and easier to use this macro:
355 err = v4l2_subdev_call(sd, core, g_chip_ident, &chip);
357 The macro will to the right NULL pointer checks and returns -ENODEV if subdev
358 is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is
359 NULL, or the actual result of the subdev->ops->core->g_chip_ident ops.
361 It is also possible to call all or a subset of the sub-devices:
363 v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip);
365 Any subdev that does not support this ops is skipped and error results are
366 ignored. If you want to check for errors use this:
368 err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip);
370 Any error except -ENOIOCTLCMD will exit the loop with that error. If no
371 errors (except -ENOIOCTLCMD) occurred, then 0 is returned.
373 The second argument to both calls is a group ID. If 0, then all subdevs are
374 called. If non-zero, then only those whose group ID match that value will
375 be called. Before a bridge driver registers a subdev it can set sd->grp_id
376 to whatever value it wants (it's 0 by default). This value is owned by the
377 bridge driver and the sub-device driver will never modify or use it.
379 The group ID gives the bridge driver more control how callbacks are called.
380 For example, there may be multiple audio chips on a board, each capable of
381 changing the volume. But usually only one will actually be used when the
382 user want to change the volume. You can set the group ID for that subdev to
383 e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
384 v4l2_device_call_all(). That ensures that it will only go to the subdev
387 If the sub-device needs to notify its v4l2_device parent of an event, then
388 it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
389 whether there is a notify() callback defined and returns -ENODEV if not.
390 Otherwise the result of the notify() call is returned.
392 The advantage of using v4l2_subdev is that it is a generic struct and does
393 not contain any knowledge about the underlying hardware. So a driver might
394 contain several subdevs that use an I2C bus, but also a subdev that is
395 controlled through GPIO pins. This distinction is only relevant when setting
396 up the device, but once the subdev is registered it is completely transparent.
399 V4L2 sub-device userspace API
400 -----------------------------
402 Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2
403 sub-devices can also be controlled directly by userspace applications.
405 Device nodes named v4l-subdevX can be created in /dev to access sub-devices
406 directly. If a sub-device supports direct userspace configuration it must set
407 the V4L2_SUBDEV_FL_HAS_DEVNODE flag before being registered.
409 After registering sub-devices, the v4l2_device driver can create device nodes
410 for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling
411 v4l2_device_register_subdev_nodes(). Those device nodes will be automatically
412 removed when sub-devices are unregistered.
414 The device node handles a subset of the V4L2 API.
424 The controls ioctls are identical to the ones defined in V4L2. They
425 behave identically, with the only exception that they deal only with
426 controls implemented in the sub-device. Depending on the driver, those
427 controls can be also be accessed through one (or several) V4L2 device
431 VIDIOC_SUBSCRIBE_EVENT
432 VIDIOC_UNSUBSCRIBE_EVENT
434 The events ioctls are identical to the ones defined in V4L2. They
435 behave identically, with the only exception that they deal only with
436 events generated by the sub-device. Depending on the driver, those
437 events can also be reported by one (or several) V4L2 device nodes.
439 Sub-device drivers that want to use events need to set the
440 V4L2_SUBDEV_USES_EVENTS v4l2_subdev::flags and initialize
441 v4l2_subdev::nevents to events queue depth before registering the
442 sub-device. After registration events can be queued as usual on the
443 v4l2_subdev::devnode device node.
445 To properly support events, the poll() file operation is also
450 All ioctls not in the above list are passed directly to the sub-device
451 driver through the core::ioctl operation.
454 I2C sub-device drivers
455 ----------------------
457 Since these drivers are so common, special helper functions are available to
458 ease the use of these drivers (v4l2-common.h).
460 The recommended method of adding v4l2_subdev support to an I2C driver is to
461 embed the v4l2_subdev struct into the state struct that is created for each
462 I2C device instance. Very simple devices have no state struct and in that case
463 you can just create a v4l2_subdev directly.
465 A typical state struct would look like this (where 'chipname' is replaced by
466 the name of the chip):
468 struct chipname_state {
469 struct v4l2_subdev sd;
470 ... /* additional state fields */
473 Initialize the v4l2_subdev struct as follows:
475 v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
477 This function will fill in all the fields of v4l2_subdev and ensure that the
478 v4l2_subdev and i2c_client both point to one another.
480 You should also add a helper inline function to go from a v4l2_subdev pointer
481 to a chipname_state struct:
483 static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
485 return container_of(sd, struct chipname_state, sd);
488 Use this to go from the v4l2_subdev struct to the i2c_client struct:
490 struct i2c_client *client = v4l2_get_subdevdata(sd);
492 And this to go from an i2c_client to a v4l2_subdev struct:
494 struct v4l2_subdev *sd = i2c_get_clientdata(client);
496 Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
497 is called. This will unregister the sub-device from the bridge driver. It is
498 safe to call this even if the sub-device was never registered.
500 You need to do this because when the bridge driver destroys the i2c adapter
501 the remove() callbacks are called of the i2c devices on that adapter.
502 After that the corresponding v4l2_subdev structures are invalid, so they
503 have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
504 from the remove() callback ensures that this is always done correctly.
507 The bridge driver also has some helper functions it can use:
509 struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
510 "module_foo", "chipid", 0x36, NULL);
512 This loads the given module (can be NULL if no module needs to be loaded) and
513 calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
514 If all goes well, then it registers the subdev with the v4l2_device.
516 You can also use the last argument of v4l2_i2c_new_subdev() to pass an array
517 of possible I2C addresses that it should probe. These probe addresses are
518 only used if the previous argument is 0. A non-zero argument means that you
519 know the exact i2c address so in that case no probing will take place.
521 Both functions return NULL if something went wrong.
523 Note that the chipid you pass to v4l2_i2c_new_subdev() is usually
524 the same as the module name. It allows you to specify a chip variant, e.g.
525 "saa7114" or "saa7115". In general though the i2c driver autodetects this.
526 The use of chipid is something that needs to be looked at more closely at a
527 later date. It differs between i2c drivers and as such can be confusing.
528 To see which chip variants are supported you can look in the i2c driver code
529 for the i2c_device_id table. This lists all the possibilities.
531 There are two more helper functions:
533 v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data
534 arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not
535 0 then that will be used (non-probing variant), otherwise the probed_addrs
538 For example: this will probe for address 0x10:
540 struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter,
541 "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10));
543 v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed
544 to the i2c driver and replaces the irq, platform_data and addr arguments.
546 If the subdev supports the s_config core ops, then that op is called with
547 the irq and platform_data arguments after the subdev was setup. The older
548 v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with
549 irq set to 0 and platform_data set to NULL.
554 The actual device nodes in the /dev directory are created using the
555 video_device struct (v4l2-dev.h). This struct can either be allocated
556 dynamically or embedded in a larger struct.
558 To allocate it dynamically use:
560 struct video_device *vdev = video_device_alloc();
565 vdev->release = video_device_release;
567 If you embed it in a larger struct, then you must set the release()
568 callback to your own function:
570 struct video_device *vdev = &my_vdev->vdev;
572 vdev->release = my_vdev_release;
574 The release callback must be set and it is called when the last user
575 of the video device exits.
577 The default video_device_release() callback just calls kfree to free the
580 You should also set these fields:
582 - v4l2_dev: set to the v4l2_device parent device.
584 - name: set to something descriptive and unique.
586 - fops: set to the v4l2_file_operations struct.
588 - ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
589 (highly recommended to use this and it might become compulsory in the
590 future!), then set this to your v4l2_ioctl_ops struct.
592 - lock: leave to NULL if you want to do all the locking in the driver.
593 Otherwise you give it a pointer to a struct mutex_lock and before the
594 unlocked_ioctl file operation is called this lock will be taken by the
595 core and released afterwards. See the next section for more details.
597 - prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
598 If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
599 If you want to have a separate priority state per (group of) device node(s),
600 then you can point it to your own struct v4l2_prio_state.
602 - parent: you only set this if v4l2_device was registered with NULL as
603 the parent device struct. This only happens in cases where one hardware
604 device has multiple PCI devices that all share the same v4l2_device core.
606 The cx88 driver is an example of this: one core v4l2_device struct, but
607 it is used by both an raw video PCI device (cx8800) and a MPEG PCI device
608 (cx8802). Since the v4l2_device cannot be associated with a particular
609 PCI device it is setup without a parent device. But when the struct
610 video_device is setup you do know which parent PCI device to use.
612 - flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework
613 handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct
614 v4l2_fh. Eventually this flag will disappear once all drivers use the core
615 priority handling. But for now it has to be set explicitly.
617 If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
618 in your v4l2_file_operations struct.
620 Do not use .ioctl! This is deprecated and will go away in the future.
622 In some cases you want to tell the core that a function you had specified in
623 your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this
624 function before video_device_register is called:
626 void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd);
628 This tends to be needed if based on external factors (e.g. which card is
629 being used) you want to turns off certain features in v4l2_ioctl_ops without
630 having to make a new struct.
632 The v4l2_file_operations struct is a subset of file_operations. The main
633 difference is that the inode argument is omitted since it is never used.
635 If integration with the media framework is needed, you must initialize the
636 media_entity struct embedded in the video_device struct (entity field) by
637 calling media_entity_init():
639 struct media_pad *pad = &my_vdev->pad;
642 err = media_entity_init(&vdev->entity, 1, pad, 0);
644 The pads array must have been previously initialized. There is no need to
645 manually set the struct media_entity type and name fields.
647 A reference to the entity will be automatically acquired/released when the
648 video device is opened/closed.
650 v4l2_file_operations and locking
651 --------------------------------
653 You can set a pointer to a mutex_lock in struct video_device. Usually this
654 will be either a top-level mutex or a mutex per device node. By default this
655 lock will be used for unlocked_ioctl, but you can disable locking for
656 selected ioctls by calling:
658 void v4l2_disable_ioctl_locking(struct video_device *vdev, unsigned int cmd);
660 E.g.: v4l2_disable_ioctl_locking(vdev, VIDIOC_DQBUF);
662 You have to call this before you register the video_device.
664 Particularly with USB drivers where certain commands such as setting controls
665 can take a long time you may want to do your own locking for the buffer queuing
668 If you want still finer-grained locking then you have to set mutex_lock to NULL
669 and do you own locking completely.
671 It is up to the driver developer to decide which method to use. However, if
672 your driver has high-latency operations (for example, changing the exposure
673 of a USB webcam might take a long time), then you might be better off with
674 doing your own locking if you want to allow the user to do other things with
675 the device while waiting for the high-latency command to finish.
677 If a lock is specified then all ioctl commands will be serialized on that
678 lock. If you use videobuf then you must pass the same lock to the videobuf
679 queue initialize function: if videobuf has to wait for a frame to arrive, then
680 it will temporarily unlock the lock and relock it afterwards. If your driver
681 also waits in the code, then you should do the same to allow other processes
682 to access the device node while the first process is waiting for something.
684 In the case of videobuf2 you will need to implement the wait_prepare and
685 wait_finish callbacks to unlock/lock if applicable. In particular, if you use
686 the lock in struct video_device then you must unlock/lock this mutex in
687 wait_prepare and wait_finish.
689 The implementation of a hotplug disconnect should also take the lock before
690 calling v4l2_device_disconnect.
692 video_device registration
693 -------------------------
695 Next you register the video device: this will create the character device
698 err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
700 video_device_release(vdev); /* or kfree(my_vdev); */
704 If the v4l2_device parent device has a non-NULL mdev field, the video device
705 entity will be automatically registered with the media device.
707 Which device is registered depends on the type argument. The following
710 VFL_TYPE_GRABBER: videoX for video input/output devices
711 VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
712 VFL_TYPE_RADIO: radioX for radio tuners
714 The last argument gives you a certain amount of control over the device
715 device node number used (i.e. the X in videoX). Normally you will pass -1
716 to let the v4l2 framework pick the first free number. But sometimes users
717 want to select a specific node number. It is common that drivers allow
718 the user to select a specific device node number through a driver module
719 option. That number is then passed to this function and video_register_device
720 will attempt to select that device node number. If that number was already
721 in use, then the next free device node number will be selected and it
722 will send a warning to the kernel log.
724 Another use-case is if a driver creates many devices. In that case it can
725 be useful to place different video devices in separate ranges. For example,
726 video capture devices start at 0, video output devices start at 16.
727 So you can use the last argument to specify a minimum device node number
728 and the v4l2 framework will try to pick the first free number that is equal
729 or higher to what you passed. If that fails, then it will just pick the
732 Since in this case you do not care about a warning about not being able
733 to select the specified device node number, you can call the function
734 video_register_device_no_warn() instead.
736 Whenever a device node is created some attributes are also created for you.
737 If you look in /sys/class/video4linux you see the devices. Go into e.g.
738 video0 and you will see 'name' and 'index' attributes. The 'name' attribute
739 is the 'name' field of the video_device struct.
741 The 'index' attribute is the index of the device node: for each call to
742 video_register_device() the index is just increased by 1. The first video
743 device node you register always starts with index 0.
745 Users can setup udev rules that utilize the index attribute to make fancy
746 device names (e.g. 'mpegX' for MPEG video capture device nodes).
748 After the device was successfully registered, then you can use these fields:
750 - vfl_type: the device type passed to video_register_device.
751 - minor: the assigned device minor number.
752 - num: the device node number (i.e. the X in videoX).
753 - index: the device index number.
755 If the registration failed, then you need to call video_device_release()
756 to free the allocated video_device struct, or free your own struct if the
757 video_device was embedded in it. The vdev->release() callback will never
758 be called if the registration failed, nor should you ever attempt to
759 unregister the device if the registration failed.
765 When the video device nodes have to be removed, either during the unload
766 of the driver or because the USB device was disconnected, then you should
769 video_unregister_device(vdev);
771 This will remove the device nodes from sysfs (causing udev to remove them
774 After video_unregister_device() returns no new opens can be done. However,
775 in the case of USB devices some application might still have one of these
776 device nodes open. So after the unregister all file operations (except
777 release, of course) will return an error as well.
779 When the last user of the video device node exits, then the vdev->release()
780 callback is called and you can do the final cleanup there.
782 Don't forget to cleanup the media entity associated with the video device if
783 it has been initialized:
785 media_entity_cleanup(&vdev->entity);
787 This can be done from the release callback.
790 video_device helper functions
791 -----------------------------
793 There are a few useful helper functions:
795 - file/video_device private data
797 You can set/get driver private data in the video_device struct using:
799 void *video_get_drvdata(struct video_device *vdev);
800 void video_set_drvdata(struct video_device *vdev, void *data);
802 Note that you can safely call video_set_drvdata() before calling
803 video_register_device().
807 struct video_device *video_devdata(struct file *file);
809 returns the video_device belonging to the file struct.
811 The video_drvdata function combines video_get_drvdata with video_devdata:
813 void *video_drvdata(struct file *file);
815 You can go from a video_device struct to the v4l2_device struct using:
817 struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
821 The video_device node kernel name can be retrieved using
823 const char *video_device_node_name(struct video_device *vdev);
825 The name is used as a hint by userspace tools such as udev. The function
826 should be used where possible instead of accessing the video_device::num and
827 video_device::minor fields.
830 video buffer helper functions
831 -----------------------------
833 The v4l2 core API provides a set of standard methods (called "videobuf")
834 for dealing with video buffers. Those methods allow a driver to implement
835 read(), mmap() and overlay() in a consistent way. There are currently
836 methods for using video buffers on devices that supports DMA with
837 scatter/gather method (videobuf-dma-sg), DMA with linear access
838 (videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
841 Please see Documentation/video4linux/videobuf for more information on how
842 to use the videobuf layer.
847 struct v4l2_fh provides a way to easily keep file handle specific data
848 that is used by the V4L2 framework. New drivers must use struct v4l2_fh
849 since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY)
850 if the video_device flag V4L2_FL_USE_FH_PRIO is also set.
852 The users of v4l2_fh (in the V4L2 framework, not the driver) know
853 whether a driver uses v4l2_fh as its file->private_data pointer by
854 testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is
855 set whenever v4l2_fh_init() is called.
857 struct v4l2_fh is allocated as a part of the driver's own file handle
858 structure and file->private_data is set to it in the driver's open
859 function by the driver.
861 In many cases the struct v4l2_fh will be embedded in a larger structure.
862 In that case you should call v4l2_fh_init+v4l2_fh_add in open() and
863 v4l2_fh_del+v4l2_fh_exit in release().
865 Drivers can extract their own file handle structure by using the container_of
875 int my_open(struct file *file)
878 struct video_device *vfd;
883 my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
887 v4l2_fh_init(&my_fh->fh, vfd);
891 file->private_data = &my_fh->fh;
892 v4l2_fh_add(&my_fh->fh);
896 int my_release(struct file *file)
898 struct v4l2_fh *fh = file->private_data;
899 struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
902 v4l2_fh_del(&my_fh->fh);
903 v4l2_fh_exit(&my_fh->fh);
908 Below is a short description of the v4l2_fh functions used:
910 void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
912 Initialise the file handle. This *MUST* be performed in the driver's
913 v4l2_file_operations->open() handler.
915 void v4l2_fh_add(struct v4l2_fh *fh)
917 Add a v4l2_fh to video_device file handle list. Must be called once the
918 file handle is completely initialized.
920 void v4l2_fh_del(struct v4l2_fh *fh)
922 Unassociate the file handle from video_device(). The file handle
923 exit function may now be called.
925 void v4l2_fh_exit(struct v4l2_fh *fh)
927 Uninitialise the file handle. After uninitialisation the v4l2_fh
931 If struct v4l2_fh is not embedded, then you can use these helper functions:
933 int v4l2_fh_open(struct file *filp)
935 This allocates a struct v4l2_fh, initializes it and adds it to the struct
936 video_device associated with the file struct.
938 int v4l2_fh_release(struct file *filp)
940 This deletes it from the struct video_device associated with the file
941 struct, uninitialised the v4l2_fh and frees it.
943 These two functions can be plugged into the v4l2_file_operation's open() and
947 Several drivers need to do something when the first file handle is opened and
948 when the last file handle closes. Two helper functions were added to check
949 whether the v4l2_fh struct is the only open filehandle of the associated
952 int v4l2_fh_is_singular(struct v4l2_fh *fh)
954 Returns 1 if the file handle is the only open file handle, else 0.
956 int v4l2_fh_is_singular_file(struct file *filp)
958 Same, but it calls v4l2_fh_is_singular with filp->private_data.
964 The V4L2 events provide a generic way to pass events to user space.
965 The driver must use v4l2_fh to be able to support V4L2 events.
967 Events are defined by a type and an optional ID. The ID may refer to a V4L2
968 object such as a control ID. If unused, then the ID is 0.
970 When the user subscribes to an event the driver will allocate a number of
971 kevent structs for that event. So every (type, ID) event tuple will have
972 its own set of kevent structs. This guarantees that if a driver is generating
973 lots of events of one type in a short time, then that will not overwrite
974 events of another type.
976 But if you get more events of one type than the number of kevents that were
977 reserved, then the oldest event will be dropped and the new one added.
979 Furthermore, the internal struct v4l2_subscribed_event has merge() and
980 replace() callbacks which drivers can set. These callbacks are called when
981 a new event is raised and there is no more room. The replace() callback
982 allows you to replace the payload of the old event with that of the new event,
983 merging any relevant data from the old payload into the new payload that
984 replaces it. It is called when this event type has only one kevent struct
985 allocated. The merge() callback allows you to merge the oldest event payload
986 into that of the second-oldest event payload. It is called when there are two
987 or more kevent structs allocated.
989 This way no status information is lost, just the intermediate steps leading
992 A good example of these replace/merge callbacks is in v4l2-event.c:
993 ctrls_replace() and ctrls_merge() callbacks for the control event.
995 Note: these callbacks can be called from interrupt context, so they must be
1000 void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev)
1002 Queue events to video device. The driver's only responsibility is to fill
1003 in the type and the data fields. The other fields will be filled in by
1006 int v4l2_event_subscribe(struct v4l2_fh *fh,
1007 struct v4l2_event_subscription *sub, unsigned elems,
1008 const struct v4l2_subscribed_event_ops *ops)
1010 The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
1011 is able to produce events with specified event id. Then it calls
1012 v4l2_event_subscribe() to subscribe the event.
1014 The elems argument is the size of the event queue for this event. If it is 0,
1015 then the framework will fill in a default value (this depends on the event
1018 The ops argument allows the driver to specify a number of callbacks:
1019 * add: called when a new listener gets added (subscribing to the same
1020 event twice will only cause this callback to get called once)
1021 * del: called when a listener stops listening
1022 * replace: replace event 'old' with event 'new'.
1023 * merge: merge event 'old' into event 'new'.
1024 All 4 callbacks are optional, if you don't want to specify any callbacks
1025 the ops argument itself maybe NULL.
1027 int v4l2_event_unsubscribe(struct v4l2_fh *fh,
1028 struct v4l2_event_subscription *sub)
1030 vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
1031 v4l2_event_unsubscribe() directly unless it wants to be involved in
1032 unsubscription process.
1034 The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The
1035 drivers may want to handle this in a special way.
1037 int v4l2_event_pending(struct v4l2_fh *fh)
1039 Returns the number of pending events. Useful when implementing poll.
1041 Events are delivered to user space through the poll system call. The driver
1042 can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait().
1044 There are standard and private events. New standard events must use the
1045 smallest available event type. The drivers must allocate their events from
1046 their own class starting from class base. Class base is
1047 V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number.
1048 The first event type in the class is reserved for future use, so the first
1049 available event type is 'class base + 1'.
1051 An example on how the V4L2 events may be used can be found in the OMAP
1052 3 ISP driver (drivers/media/video/omap3isp).