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_ops {
270 const struct v4l2_subdev_core_ops *core;
271 const struct v4l2_subdev_tuner_ops *tuner;
272 const struct v4l2_subdev_audio_ops *audio;
273 const struct v4l2_subdev_video_ops *video;
276 The core ops are common to all subdevs, the other categories are implemented
277 depending on the sub-device. E.g. a video device is unlikely to support the
278 audio ops and vice versa.
280 This setup limits the number of function pointers while still making it easy
281 to add new ops and categories.
283 A sub-device driver initializes the v4l2_subdev struct using:
285 v4l2_subdev_init(sd, &ops);
287 Afterwards you need to initialize subdev->name with a unique name and set the
288 module owner. This is done for you if you use the i2c helper functions.
290 If integration with the media framework is needed, you must initialize the
291 media_entity struct embedded in the v4l2_subdev struct (entity field) by
292 calling media_entity_init():
294 struct media_pad *pads = &my_sd->pads;
297 err = media_entity_init(&sd->entity, npads, pads, 0);
299 The pads array must have been previously initialized. There is no need to
300 manually set the struct media_entity type and name fields, but the revision
301 field must be initialized if needed.
303 A reference to the entity will be automatically acquired/released when the
304 subdev device node (if any) is opened/closed.
306 Don't forget to cleanup the media entity before the sub-device is destroyed:
308 media_entity_cleanup(&sd->entity);
310 A device (bridge) driver needs to register the v4l2_subdev with the
313 int err = v4l2_device_register_subdev(v4l2_dev, sd);
315 This can fail if the subdev module disappeared before it could be registered.
316 After this function was called successfully the subdev->dev field points to
319 If the v4l2_device parent device has a non-NULL mdev field, the sub-device
320 entity will be automatically registered with the media device.
322 You can unregister a sub-device using:
324 v4l2_device_unregister_subdev(sd);
326 Afterwards the subdev module can be unloaded and sd->dev == NULL.
328 You can call an ops function either directly:
330 err = sd->ops->core->g_chip_ident(sd, &chip);
332 but it is better and easier to use this macro:
334 err = v4l2_subdev_call(sd, core, g_chip_ident, &chip);
336 The macro will to the right NULL pointer checks and returns -ENODEV if subdev
337 is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is
338 NULL, or the actual result of the subdev->ops->core->g_chip_ident ops.
340 It is also possible to call all or a subset of the sub-devices:
342 v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip);
344 Any subdev that does not support this ops is skipped and error results are
345 ignored. If you want to check for errors use this:
347 err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip);
349 Any error except -ENOIOCTLCMD will exit the loop with that error. If no
350 errors (except -ENOIOCTLCMD) occurred, then 0 is returned.
352 The second argument to both calls is a group ID. If 0, then all subdevs are
353 called. If non-zero, then only those whose group ID match that value will
354 be called. Before a bridge driver registers a subdev it can set sd->grp_id
355 to whatever value it wants (it's 0 by default). This value is owned by the
356 bridge driver and the sub-device driver will never modify or use it.
358 The group ID gives the bridge driver more control how callbacks are called.
359 For example, there may be multiple audio chips on a board, each capable of
360 changing the volume. But usually only one will actually be used when the
361 user want to change the volume. You can set the group ID for that subdev to
362 e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
363 v4l2_device_call_all(). That ensures that it will only go to the subdev
366 If the sub-device needs to notify its v4l2_device parent of an event, then
367 it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
368 whether there is a notify() callback defined and returns -ENODEV if not.
369 Otherwise the result of the notify() call is returned.
371 The advantage of using v4l2_subdev is that it is a generic struct and does
372 not contain any knowledge about the underlying hardware. So a driver might
373 contain several subdevs that use an I2C bus, but also a subdev that is
374 controlled through GPIO pins. This distinction is only relevant when setting
375 up the device, but once the subdev is registered it is completely transparent.
378 V4L2 sub-device userspace API
379 -----------------------------
381 Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2
382 sub-devices can also be controlled directly by userspace applications.
384 Device nodes named v4l-subdevX can be created in /dev to access sub-devices
385 directly. If a sub-device supports direct userspace configuration it must set
386 the V4L2_SUBDEV_FL_HAS_DEVNODE flag before being registered.
388 After registering sub-devices, the v4l2_device driver can create device nodes
389 for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling
390 v4l2_device_register_subdev_nodes(). Those device nodes will be automatically
391 removed when sub-devices are unregistered.
393 The device node handles a subset of the V4L2 API.
403 The controls ioctls are identical to the ones defined in V4L2. They
404 behave identically, with the only exception that they deal only with
405 controls implemented in the sub-device. Depending on the driver, those
406 controls can be also be accessed through one (or several) V4L2 device
410 VIDIOC_SUBSCRIBE_EVENT
411 VIDIOC_UNSUBSCRIBE_EVENT
413 The events ioctls are identical to the ones defined in V4L2. They
414 behave identically, with the only exception that they deal only with
415 events generated by the sub-device. Depending on the driver, those
416 events can also be reported by one (or several) V4L2 device nodes.
418 Sub-device drivers that want to use events need to set the
419 V4L2_SUBDEV_USES_EVENTS v4l2_subdev::flags and initialize
420 v4l2_subdev::nevents to events queue depth before registering the
421 sub-device. After registration events can be queued as usual on the
422 v4l2_subdev::devnode device node.
424 To properly support events, the poll() file operation is also
429 All ioctls not in the above list are passed directly to the sub-device
430 driver through the core::ioctl operation.
433 I2C sub-device drivers
434 ----------------------
436 Since these drivers are so common, special helper functions are available to
437 ease the use of these drivers (v4l2-common.h).
439 The recommended method of adding v4l2_subdev support to an I2C driver is to
440 embed the v4l2_subdev struct into the state struct that is created for each
441 I2C device instance. Very simple devices have no state struct and in that case
442 you can just create a v4l2_subdev directly.
444 A typical state struct would look like this (where 'chipname' is replaced by
445 the name of the chip):
447 struct chipname_state {
448 struct v4l2_subdev sd;
449 ... /* additional state fields */
452 Initialize the v4l2_subdev struct as follows:
454 v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
456 This function will fill in all the fields of v4l2_subdev and ensure that the
457 v4l2_subdev and i2c_client both point to one another.
459 You should also add a helper inline function to go from a v4l2_subdev pointer
460 to a chipname_state struct:
462 static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
464 return container_of(sd, struct chipname_state, sd);
467 Use this to go from the v4l2_subdev struct to the i2c_client struct:
469 struct i2c_client *client = v4l2_get_subdevdata(sd);
471 And this to go from an i2c_client to a v4l2_subdev struct:
473 struct v4l2_subdev *sd = i2c_get_clientdata(client);
475 Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
476 is called. This will unregister the sub-device from the bridge driver. It is
477 safe to call this even if the sub-device was never registered.
479 You need to do this because when the bridge driver destroys the i2c adapter
480 the remove() callbacks are called of the i2c devices on that adapter.
481 After that the corresponding v4l2_subdev structures are invalid, so they
482 have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
483 from the remove() callback ensures that this is always done correctly.
486 The bridge driver also has some helper functions it can use:
488 struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
489 "module_foo", "chipid", 0x36, NULL);
491 This loads the given module (can be NULL if no module needs to be loaded) and
492 calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
493 If all goes well, then it registers the subdev with the v4l2_device.
495 You can also use the last argument of v4l2_i2c_new_subdev() to pass an array
496 of possible I2C addresses that it should probe. These probe addresses are
497 only used if the previous argument is 0. A non-zero argument means that you
498 know the exact i2c address so in that case no probing will take place.
500 Both functions return NULL if something went wrong.
502 Note that the chipid you pass to v4l2_i2c_new_subdev() is usually
503 the same as the module name. It allows you to specify a chip variant, e.g.
504 "saa7114" or "saa7115". In general though the i2c driver autodetects this.
505 The use of chipid is something that needs to be looked at more closely at a
506 later date. It differs between i2c drivers and as such can be confusing.
507 To see which chip variants are supported you can look in the i2c driver code
508 for the i2c_device_id table. This lists all the possibilities.
510 There are two more helper functions:
512 v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data
513 arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not
514 0 then that will be used (non-probing variant), otherwise the probed_addrs
517 For example: this will probe for address 0x10:
519 struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter,
520 "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10));
522 v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed
523 to the i2c driver and replaces the irq, platform_data and addr arguments.
525 If the subdev supports the s_config core ops, then that op is called with
526 the irq and platform_data arguments after the subdev was setup. The older
527 v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with
528 irq set to 0 and platform_data set to NULL.
533 The actual device nodes in the /dev directory are created using the
534 video_device struct (v4l2-dev.h). This struct can either be allocated
535 dynamically or embedded in a larger struct.
537 To allocate it dynamically use:
539 struct video_device *vdev = video_device_alloc();
544 vdev->release = video_device_release;
546 If you embed it in a larger struct, then you must set the release()
547 callback to your own function:
549 struct video_device *vdev = &my_vdev->vdev;
551 vdev->release = my_vdev_release;
553 The release callback must be set and it is called when the last user
554 of the video device exits.
556 The default video_device_release() callback just calls kfree to free the
559 You should also set these fields:
561 - v4l2_dev: set to the v4l2_device parent device.
562 - name: set to something descriptive and unique.
563 - fops: set to the v4l2_file_operations struct.
564 - ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
565 (highly recommended to use this and it might become compulsory in the
566 future!), then set this to your v4l2_ioctl_ops struct.
567 - lock: leave to NULL if you want to do all the locking in the driver.
568 Otherwise you give it a pointer to a struct mutex_lock and before any
569 of the v4l2_file_operations is called this lock will be taken by the
570 core and released afterwards.
571 - prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
572 If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
573 If you want to have a separate priority state per (group of) device node(s),
574 then you can point it to your own struct v4l2_prio_state.
575 - parent: you only set this if v4l2_device was registered with NULL as
576 the parent device struct. This only happens in cases where one hardware
577 device has multiple PCI devices that all share the same v4l2_device core.
579 The cx88 driver is an example of this: one core v4l2_device struct, but
580 it is used by both an raw video PCI device (cx8800) and a MPEG PCI device
581 (cx8802). Since the v4l2_device cannot be associated with a particular
582 PCI device it is setup without a parent device. But when the struct
583 video_device is setup you do know which parent PCI device to use.
584 - flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework
585 handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct
586 v4l2_fh. Eventually this flag will disappear once all drivers use the core
587 priority handling. But for now it has to be set explicitly.
589 If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
590 in your v4l2_file_operations struct.
592 Do not use .ioctl! This is deprecated and will go away in the future.
594 The v4l2_file_operations struct is a subset of file_operations. The main
595 difference is that the inode argument is omitted since it is never used.
597 If integration with the media framework is needed, you must initialize the
598 media_entity struct embedded in the video_device struct (entity field) by
599 calling media_entity_init():
601 struct media_pad *pad = &my_vdev->pad;
604 err = media_entity_init(&vdev->entity, 1, pad, 0);
606 The pads array must have been previously initialized. There is no need to
607 manually set the struct media_entity type and name fields.
609 A reference to the entity will be automatically acquired/released when the
610 video device is opened/closed.
612 v4l2_file_operations and locking
613 --------------------------------
615 You can set a pointer to a mutex_lock in struct video_device. Usually this
616 will be either a top-level mutex or a mutex per device node. If you want
617 finer-grained locking then you have to set it to NULL and do you own locking.
619 It is up to the driver developer to decide which method to use. However, if
620 your driver has high-latency operations (for example, changing the exposure
621 of a USB webcam might take a long time), then you might be better off with
622 doing your own locking if you want to allow the user to do other things with
623 the device while waiting for the high-latency command to finish.
625 If a lock is specified then all file operations will be serialized on that
626 lock. If you use videobuf then you must pass the same lock to the videobuf
627 queue initialize function: if videobuf has to wait for a frame to arrive, then
628 it will temporarily unlock the lock and relock it afterwards. If your driver
629 also waits in the code, then you should do the same to allow other processes
630 to access the device node while the first process is waiting for something.
632 In the case of videobuf2 you will need to implement the wait_prepare and
633 wait_finish callbacks to unlock/lock if applicable. In particular, if you use
634 the lock in struct video_device then you must unlock/lock this mutex in
635 wait_prepare and wait_finish.
637 The implementation of a hotplug disconnect should also take the lock before
638 calling v4l2_device_disconnect.
640 video_device registration
641 -------------------------
643 Next you register the video device: this will create the character device
646 err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
648 video_device_release(vdev); /* or kfree(my_vdev); */
652 If the v4l2_device parent device has a non-NULL mdev field, the video device
653 entity will be automatically registered with the media device.
655 Which device is registered depends on the type argument. The following
658 VFL_TYPE_GRABBER: videoX for video input/output devices
659 VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
660 VFL_TYPE_RADIO: radioX for radio tuners
662 The last argument gives you a certain amount of control over the device
663 device node number used (i.e. the X in videoX). Normally you will pass -1
664 to let the v4l2 framework pick the first free number. But sometimes users
665 want to select a specific node number. It is common that drivers allow
666 the user to select a specific device node number through a driver module
667 option. That number is then passed to this function and video_register_device
668 will attempt to select that device node number. If that number was already
669 in use, then the next free device node number will be selected and it
670 will send a warning to the kernel log.
672 Another use-case is if a driver creates many devices. In that case it can
673 be useful to place different video devices in separate ranges. For example,
674 video capture devices start at 0, video output devices start at 16.
675 So you can use the last argument to specify a minimum device node number
676 and the v4l2 framework will try to pick the first free number that is equal
677 or higher to what you passed. If that fails, then it will just pick the
680 Since in this case you do not care about a warning about not being able
681 to select the specified device node number, you can call the function
682 video_register_device_no_warn() instead.
684 Whenever a device node is created some attributes are also created for you.
685 If you look in /sys/class/video4linux you see the devices. Go into e.g.
686 video0 and you will see 'name' and 'index' attributes. The 'name' attribute
687 is the 'name' field of the video_device struct.
689 The 'index' attribute is the index of the device node: for each call to
690 video_register_device() the index is just increased by 1. The first video
691 device node you register always starts with index 0.
693 Users can setup udev rules that utilize the index attribute to make fancy
694 device names (e.g. 'mpegX' for MPEG video capture device nodes).
696 After the device was successfully registered, then you can use these fields:
698 - vfl_type: the device type passed to video_register_device.
699 - minor: the assigned device minor number.
700 - num: the device node number (i.e. the X in videoX).
701 - index: the device index number.
703 If the registration failed, then you need to call video_device_release()
704 to free the allocated video_device struct, or free your own struct if the
705 video_device was embedded in it. The vdev->release() callback will never
706 be called if the registration failed, nor should you ever attempt to
707 unregister the device if the registration failed.
713 When the video device nodes have to be removed, either during the unload
714 of the driver or because the USB device was disconnected, then you should
717 video_unregister_device(vdev);
719 This will remove the device nodes from sysfs (causing udev to remove them
722 After video_unregister_device() returns no new opens can be done. However,
723 in the case of USB devices some application might still have one of these
724 device nodes open. So after the unregister all file operations (except
725 release, of course) will return an error as well.
727 When the last user of the video device node exits, then the vdev->release()
728 callback is called and you can do the final cleanup there.
730 Don't forget to cleanup the media entity associated with the video device if
731 it has been initialized:
733 media_entity_cleanup(&vdev->entity);
735 This can be done from the release callback.
738 video_device helper functions
739 -----------------------------
741 There are a few useful helper functions:
743 - file/video_device private data
745 You can set/get driver private data in the video_device struct using:
747 void *video_get_drvdata(struct video_device *vdev);
748 void video_set_drvdata(struct video_device *vdev, void *data);
750 Note that you can safely call video_set_drvdata() before calling
751 video_register_device().
755 struct video_device *video_devdata(struct file *file);
757 returns the video_device belonging to the file struct.
759 The video_drvdata function combines video_get_drvdata with video_devdata:
761 void *video_drvdata(struct file *file);
763 You can go from a video_device struct to the v4l2_device struct using:
765 struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
769 The video_device node kernel name can be retrieved using
771 const char *video_device_node_name(struct video_device *vdev);
773 The name is used as a hint by userspace tools such as udev. The function
774 should be used where possible instead of accessing the video_device::num and
775 video_device::minor fields.
778 video buffer helper functions
779 -----------------------------
781 The v4l2 core API provides a set of standard methods (called "videobuf")
782 for dealing with video buffers. Those methods allow a driver to implement
783 read(), mmap() and overlay() in a consistent way. There are currently
784 methods for using video buffers on devices that supports DMA with
785 scatter/gather method (videobuf-dma-sg), DMA with linear access
786 (videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
789 Please see Documentation/video4linux/videobuf for more information on how
790 to use the videobuf layer.
795 struct v4l2_fh provides a way to easily keep file handle specific data
796 that is used by the V4L2 framework. New drivers must use struct v4l2_fh
797 since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY)
798 if the video_device flag V4L2_FL_USE_FH_PRIO is also set.
800 The users of v4l2_fh (in the V4L2 framework, not the driver) know
801 whether a driver uses v4l2_fh as its file->private_data pointer by
802 testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is
803 set whenever v4l2_fh_init() is called.
805 struct v4l2_fh is allocated as a part of the driver's own file handle
806 structure and file->private_data is set to it in the driver's open
807 function by the driver.
809 In many cases the struct v4l2_fh will be embedded in a larger structure.
810 In that case you should call v4l2_fh_init+v4l2_fh_add in open() and
811 v4l2_fh_del+v4l2_fh_exit in release().
813 Drivers can extract their own file handle structure by using the container_of
823 int my_open(struct file *file)
826 struct video_device *vfd;
831 my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
835 v4l2_fh_init(&my_fh->fh, vfd);
839 file->private_data = &my_fh->fh;
840 v4l2_fh_add(&my_fh->fh);
844 int my_release(struct file *file)
846 struct v4l2_fh *fh = file->private_data;
847 struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
850 v4l2_fh_del(&my_fh->fh);
851 v4l2_fh_exit(&my_fh->fh);
856 Below is a short description of the v4l2_fh functions used:
858 void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
860 Initialise the file handle. This *MUST* be performed in the driver's
861 v4l2_file_operations->open() handler.
863 void v4l2_fh_add(struct v4l2_fh *fh)
865 Add a v4l2_fh to video_device file handle list. Must be called once the
866 file handle is completely initialized.
868 void v4l2_fh_del(struct v4l2_fh *fh)
870 Unassociate the file handle from video_device(). The file handle
871 exit function may now be called.
873 void v4l2_fh_exit(struct v4l2_fh *fh)
875 Uninitialise the file handle. After uninitialisation the v4l2_fh
879 If struct v4l2_fh is not embedded, then you can use these helper functions:
881 int v4l2_fh_open(struct file *filp)
883 This allocates a struct v4l2_fh, initializes it and adds it to the struct
884 video_device associated with the file struct.
886 int v4l2_fh_release(struct file *filp)
888 This deletes it from the struct video_device associated with the file
889 struct, uninitialised the v4l2_fh and frees it.
891 These two functions can be plugged into the v4l2_file_operation's open() and
895 Several drivers need to do something when the first file handle is opened and
896 when the last file handle closes. Two helper functions were added to check
897 whether the v4l2_fh struct is the only open filehandle of the associated
900 int v4l2_fh_is_singular(struct v4l2_fh *fh)
902 Returns 1 if the file handle is the only open file handle, else 0.
904 int v4l2_fh_is_singular_file(struct file *filp)
906 Same, but it calls v4l2_fh_is_singular with filp->private_data.
912 The V4L2 events provide a generic way to pass events to user space.
913 The driver must use v4l2_fh to be able to support V4L2 events.
915 Events are defined by a type and an optional ID. The ID may refer to a V4L2
916 object such as a control ID. If unused, then the ID is 0.
918 When the user subscribes to an event the driver will allocate a number of
919 kevent structs for that event. So every (type, ID) event tuple will have
920 its own set of kevent structs. This guarantees that if a driver is generating
921 lots of events of one type in a short time, then that will not overwrite
922 events of another type.
924 But if you get more events of one type than the number of kevents that were
925 reserved, then the oldest event will be dropped and the new one added.
927 Furthermore, the internal struct v4l2_subscribed_event has merge() and
928 replace() callbacks which drivers can set. These callbacks are called when
929 a new event is raised and there is no more room. The replace() callback
930 allows you to replace the payload of the old event with that of the new event,
931 merging any relevant data from the old payload into the new payload that
932 replaces it. It is called when this event type has only one kevent struct
933 allocated. The merge() callback allows you to merge the oldest event payload
934 into that of the second-oldest event payload. It is called when there are two
935 or more kevent structs allocated.
937 This way no status information is lost, just the intermediate steps leading
940 A good example of these replace/merge callbacks is in v4l2-event.c:
941 ctrls_replace() and ctrls_merge() callbacks for the control event.
943 Note: these callbacks can be called from interrupt context, so they must be
948 void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev)
950 Queue events to video device. The driver's only responsibility is to fill
951 in the type and the data fields. The other fields will be filled in by
954 int v4l2_event_subscribe(struct v4l2_fh *fh,
955 struct v4l2_event_subscription *sub, unsigned elems,
956 const struct v4l2_subscribed_event_ops *ops)
958 The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
959 is able to produce events with specified event id. Then it calls
960 v4l2_event_subscribe() to subscribe the event.
962 The elems argument is the size of the event queue for this event. If it is 0,
963 then the framework will fill in a default value (this depends on the event
966 The ops argument allows the driver to specify a number of callbacks:
967 * add: called when a new listener gets added (subscribing to the same
968 event twice will only cause this callback to get called once)
969 * del: called when a listener stops listening
970 * replace: replace event 'old' with event 'new'.
971 * merge: merge event 'old' into event 'new'.
972 All 4 callbacks are optional, if you don't want to specify any callbacks
973 the ops argument itself maybe NULL.
975 int v4l2_event_unsubscribe(struct v4l2_fh *fh,
976 struct v4l2_event_subscription *sub)
978 vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
979 v4l2_event_unsubscribe() directly unless it wants to be involved in
980 unsubscription process.
982 The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The
983 drivers may want to handle this in a special way.
985 int v4l2_event_pending(struct v4l2_fh *fh)
987 Returns the number of pending events. Useful when implementing poll.
989 Events are delivered to user space through the poll system call. The driver
990 can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait().
992 There are standard and private events. New standard events must use the
993 smallest available event type. The drivers must allocate their events from
994 their own class starting from class base. Class base is
995 V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number.
996 The first event type in the class is reserved for future use, so the first
997 available event type is 'class base + 1'.
999 An example on how the V4L2 events may be used can be found in the OMAP
1000 3 ISP driver (drivers/media/video/omap3isp).