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;
188 Many drivers need to communicate with sub-devices. These devices can do all
189 sort of tasks, but most commonly they handle audio and/or video muxing,
190 encoding or decoding. For webcams common sub-devices are sensors and camera
193 Usually these are I2C devices, but not necessarily. In order to provide the
194 driver with a consistent interface to these sub-devices the v4l2_subdev struct
195 (v4l2-subdev.h) was created.
197 Each sub-device driver must have a v4l2_subdev struct. This struct can be
198 stand-alone for simple sub-devices or it might be embedded in a larger struct
199 if more state information needs to be stored. Usually there is a low-level
200 device struct (e.g. i2c_client) that contains the device data as setup
201 by the kernel. It is recommended to store that pointer in the private
202 data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
203 from a v4l2_subdev to the actual low-level bus-specific device data.
205 You also need a way to go from the low-level struct to v4l2_subdev. For the
206 common i2c_client struct the i2c_set_clientdata() call is used to store a
207 v4l2_subdev pointer, for other busses you may have to use other methods.
209 Bridges might also need to store per-subdev private data, such as a pointer to
210 bridge-specific per-subdev private data. The v4l2_subdev structure provides
211 host private data for that purpose that can be accessed with
212 v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata().
214 From the bridge driver perspective you load the sub-device module and somehow
215 obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
216 i2c_get_clientdata(). For other busses something similar needs to be done.
217 Helper functions exists for sub-devices on an I2C bus that do most of this
220 Each v4l2_subdev contains function pointers that sub-device drivers can
221 implement (or leave NULL if it is not applicable). Since sub-devices can do
222 so many different things and you do not want to end up with a huge ops struct
223 of which only a handful of ops are commonly implemented, the function pointers
224 are sorted according to category and each category has its own ops struct.
226 The top-level ops struct contains pointers to the category ops structs, which
227 may be NULL if the subdev driver does not support anything from that category.
231 struct v4l2_subdev_core_ops {
232 int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip);
233 int (*log_status)(struct v4l2_subdev *sd);
234 int (*init)(struct v4l2_subdev *sd, u32 val);
238 struct v4l2_subdev_tuner_ops {
242 struct v4l2_subdev_audio_ops {
246 struct v4l2_subdev_video_ops {
250 struct v4l2_subdev_ops {
251 const struct v4l2_subdev_core_ops *core;
252 const struct v4l2_subdev_tuner_ops *tuner;
253 const struct v4l2_subdev_audio_ops *audio;
254 const struct v4l2_subdev_video_ops *video;
257 The core ops are common to all subdevs, the other categories are implemented
258 depending on the sub-device. E.g. a video device is unlikely to support the
259 audio ops and vice versa.
261 This setup limits the number of function pointers while still making it easy
262 to add new ops and categories.
264 A sub-device driver initializes the v4l2_subdev struct using:
266 v4l2_subdev_init(sd, &ops);
268 Afterwards you need to initialize subdev->name with a unique name and set the
269 module owner. This is done for you if you use the i2c helper functions.
271 A device (bridge) driver needs to register the v4l2_subdev with the
274 int err = v4l2_device_register_subdev(v4l2_dev, sd);
276 This can fail if the subdev module disappeared before it could be registered.
277 After this function was called successfully the subdev->dev field points to
280 You can unregister a sub-device using:
282 v4l2_device_unregister_subdev(sd);
284 Afterwards the subdev module can be unloaded and sd->dev == NULL.
286 You can call an ops function either directly:
288 err = sd->ops->core->g_chip_ident(sd, &chip);
290 but it is better and easier to use this macro:
292 err = v4l2_subdev_call(sd, core, g_chip_ident, &chip);
294 The macro will to the right NULL pointer checks and returns -ENODEV if subdev
295 is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is
296 NULL, or the actual result of the subdev->ops->core->g_chip_ident ops.
298 It is also possible to call all or a subset of the sub-devices:
300 v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip);
302 Any subdev that does not support this ops is skipped and error results are
303 ignored. If you want to check for errors use this:
305 err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip);
307 Any error except -ENOIOCTLCMD will exit the loop with that error. If no
308 errors (except -ENOIOCTLCMD) occured, then 0 is returned.
310 The second argument to both calls is a group ID. If 0, then all subdevs are
311 called. If non-zero, then only those whose group ID match that value will
312 be called. Before a bridge driver registers a subdev it can set sd->grp_id
313 to whatever value it wants (it's 0 by default). This value is owned by the
314 bridge driver and the sub-device driver will never modify or use it.
316 The group ID gives the bridge driver more control how callbacks are called.
317 For example, there may be multiple audio chips on a board, each capable of
318 changing the volume. But usually only one will actually be used when the
319 user want to change the volume. You can set the group ID for that subdev to
320 e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
321 v4l2_device_call_all(). That ensures that it will only go to the subdev
324 If the sub-device needs to notify its v4l2_device parent of an event, then
325 it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
326 whether there is a notify() callback defined and returns -ENODEV if not.
327 Otherwise the result of the notify() call is returned.
329 The advantage of using v4l2_subdev is that it is a generic struct and does
330 not contain any knowledge about the underlying hardware. So a driver might
331 contain several subdevs that use an I2C bus, but also a subdev that is
332 controlled through GPIO pins. This distinction is only relevant when setting
333 up the device, but once the subdev is registered it is completely transparent.
336 V4L2 sub-device userspace API
337 -----------------------------
339 Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2
340 sub-devices can also be controlled directly by userspace applications.
342 Device nodes named v4l-subdevX can be created in /dev to access sub-devices
343 directly. If a sub-device supports direct userspace configuration it must set
344 the V4L2_SUBDEV_FL_HAS_DEVNODE flag before being registered.
346 After registering sub-devices, the v4l2_device driver can create device nodes
347 for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling
348 v4l2_device_register_subdev_nodes(). Those device nodes will be automatically
349 removed when sub-devices are unregistered.
351 The device node handles a subset of the V4L2 API.
361 The controls ioctls are identical to the ones defined in V4L2. They
362 behave identically, with the only exception that they deal only with
363 controls implemented in the sub-device. Depending on the driver, those
364 controls can be also be accessed through one (or several) V4L2 device
368 VIDIOC_SUBSCRIBE_EVENT
369 VIDIOC_UNSUBSCRIBE_EVENT
371 The events ioctls are identical to the ones defined in V4L2. They
372 behave identically, with the only exception that they deal only with
373 events generated by the sub-device. Depending on the driver, those
374 events can also be reported by one (or several) V4L2 device nodes.
376 Sub-device drivers that want to use events need to set the
377 V4L2_SUBDEV_USES_EVENTS v4l2_subdev::flags and initialize
378 v4l2_subdev::nevents to events queue depth before registering the
379 sub-device. After registration events can be queued as usual on the
380 v4l2_subdev::devnode device node.
382 To properly support events, the poll() file operation is also
386 I2C sub-device drivers
387 ----------------------
389 Since these drivers are so common, special helper functions are available to
390 ease the use of these drivers (v4l2-common.h).
392 The recommended method of adding v4l2_subdev support to an I2C driver is to
393 embed the v4l2_subdev struct into the state struct that is created for each
394 I2C device instance. Very simple devices have no state struct and in that case
395 you can just create a v4l2_subdev directly.
397 A typical state struct would look like this (where 'chipname' is replaced by
398 the name of the chip):
400 struct chipname_state {
401 struct v4l2_subdev sd;
402 ... /* additional state fields */
405 Initialize the v4l2_subdev struct as follows:
407 v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
409 This function will fill in all the fields of v4l2_subdev and ensure that the
410 v4l2_subdev and i2c_client both point to one another.
412 You should also add a helper inline function to go from a v4l2_subdev pointer
413 to a chipname_state struct:
415 static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
417 return container_of(sd, struct chipname_state, sd);
420 Use this to go from the v4l2_subdev struct to the i2c_client struct:
422 struct i2c_client *client = v4l2_get_subdevdata(sd);
424 And this to go from an i2c_client to a v4l2_subdev struct:
426 struct v4l2_subdev *sd = i2c_get_clientdata(client);
428 Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
429 is called. This will unregister the sub-device from the bridge driver. It is
430 safe to call this even if the sub-device was never registered.
432 You need to do this because when the bridge driver destroys the i2c adapter
433 the remove() callbacks are called of the i2c devices on that adapter.
434 After that the corresponding v4l2_subdev structures are invalid, so they
435 have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
436 from the remove() callback ensures that this is always done correctly.
439 The bridge driver also has some helper functions it can use:
441 struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
442 "module_foo", "chipid", 0x36, NULL);
444 This loads the given module (can be NULL if no module needs to be loaded) and
445 calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
446 If all goes well, then it registers the subdev with the v4l2_device.
448 You can also use the last argument of v4l2_i2c_new_subdev() to pass an array
449 of possible I2C addresses that it should probe. These probe addresses are
450 only used if the previous argument is 0. A non-zero argument means that you
451 know the exact i2c address so in that case no probing will take place.
453 Both functions return NULL if something went wrong.
455 Note that the chipid you pass to v4l2_i2c_new_subdev() is usually
456 the same as the module name. It allows you to specify a chip variant, e.g.
457 "saa7114" or "saa7115". In general though the i2c driver autodetects this.
458 The use of chipid is something that needs to be looked at more closely at a
459 later date. It differs between i2c drivers and as such can be confusing.
460 To see which chip variants are supported you can look in the i2c driver code
461 for the i2c_device_id table. This lists all the possibilities.
463 There are two more helper functions:
465 v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data
466 arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not
467 0 then that will be used (non-probing variant), otherwise the probed_addrs
470 For example: this will probe for address 0x10:
472 struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter,
473 "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10));
475 v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed
476 to the i2c driver and replaces the irq, platform_data and addr arguments.
478 If the subdev supports the s_config core ops, then that op is called with
479 the irq and platform_data arguments after the subdev was setup. The older
480 v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with
481 irq set to 0 and platform_data set to NULL.
486 The actual device nodes in the /dev directory are created using the
487 video_device struct (v4l2-dev.h). This struct can either be allocated
488 dynamically or embedded in a larger struct.
490 To allocate it dynamically use:
492 struct video_device *vdev = video_device_alloc();
497 vdev->release = video_device_release;
499 If you embed it in a larger struct, then you must set the release()
500 callback to your own function:
502 struct video_device *vdev = &my_vdev->vdev;
504 vdev->release = my_vdev_release;
506 The release callback must be set and it is called when the last user
507 of the video device exits.
509 The default video_device_release() callback just calls kfree to free the
512 You should also set these fields:
514 - v4l2_dev: set to the v4l2_device parent device.
515 - name: set to something descriptive and unique.
516 - fops: set to the v4l2_file_operations struct.
517 - ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
518 (highly recommended to use this and it might become compulsory in the
519 future!), then set this to your v4l2_ioctl_ops struct.
520 - lock: leave to NULL if you want to do all the locking in the driver.
521 Otherwise you give it a pointer to a struct mutex_lock and before any
522 of the v4l2_file_operations is called this lock will be taken by the
523 core and released afterwards.
524 - parent: you only set this if v4l2_device was registered with NULL as
525 the parent device struct. This only happens in cases where one hardware
526 device has multiple PCI devices that all share the same v4l2_device core.
528 The cx88 driver is an example of this: one core v4l2_device struct, but
529 it is used by both an raw video PCI device (cx8800) and a MPEG PCI device
530 (cx8802). Since the v4l2_device cannot be associated with a particular
531 PCI device it is setup without a parent device. But when the struct
532 video_device is setup you do know which parent PCI device to use.
534 If you use v4l2_ioctl_ops, then you should set either .unlocked_ioctl or
535 .ioctl to video_ioctl2 in your v4l2_file_operations struct.
537 The v4l2_file_operations struct is a subset of file_operations. The main
538 difference is that the inode argument is omitted since it is never used.
540 If integration with the media framework is needed, you must initialize the
541 media_entity struct embedded in the video_device struct (entity field) by
542 calling media_entity_init():
544 struct media_pad *pad = &my_vdev->pad;
547 err = media_entity_init(&vdev->entity, 1, pad, 0);
549 The pads array must have been previously initialized. There is no need to
550 manually set the struct media_entity type and name fields.
552 A reference to the entity will be automatically acquired/released when the
553 video device is opened/closed.
555 v4l2_file_operations and locking
556 --------------------------------
558 You can set a pointer to a mutex_lock in struct video_device. Usually this
559 will be either a top-level mutex or a mutex per device node. If you want
560 finer-grained locking then you have to set it to NULL and do you own locking.
562 If a lock is specified then all file operations will be serialized on that
563 lock. If you use videobuf then you must pass the same lock to the videobuf
564 queue initialize function: if videobuf has to wait for a frame to arrive, then
565 it will temporarily unlock the lock and relock it afterwards. If your driver
566 also waits in the code, then you should do the same to allow other processes
567 to access the device node while the first process is waiting for something.
569 The implementation of a hotplug disconnect should also take the lock before
570 calling v4l2_device_disconnect.
572 video_device registration
573 -------------------------
575 Next you register the video device: this will create the character device
578 err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
580 video_device_release(vdev); /* or kfree(my_vdev); */
584 If the v4l2_device parent device has a non-NULL mdev field, the video device
585 entity will be automatically registered with the media device.
587 Which device is registered depends on the type argument. The following
590 VFL_TYPE_GRABBER: videoX for video input/output devices
591 VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
592 VFL_TYPE_RADIO: radioX for radio tuners
594 The last argument gives you a certain amount of control over the device
595 device node number used (i.e. the X in videoX). Normally you will pass -1
596 to let the v4l2 framework pick the first free number. But sometimes users
597 want to select a specific node number. It is common that drivers allow
598 the user to select a specific device node number through a driver module
599 option. That number is then passed to this function and video_register_device
600 will attempt to select that device node number. If that number was already
601 in use, then the next free device node number will be selected and it
602 will send a warning to the kernel log.
604 Another use-case is if a driver creates many devices. In that case it can
605 be useful to place different video devices in separate ranges. For example,
606 video capture devices start at 0, video output devices start at 16.
607 So you can use the last argument to specify a minimum device node number
608 and the v4l2 framework will try to pick the first free number that is equal
609 or higher to what you passed. If that fails, then it will just pick the
612 Since in this case you do not care about a warning about not being able
613 to select the specified device node number, you can call the function
614 video_register_device_no_warn() instead.
616 Whenever a device node is created some attributes are also created for you.
617 If you look in /sys/class/video4linux you see the devices. Go into e.g.
618 video0 and you will see 'name' and 'index' attributes. The 'name' attribute
619 is the 'name' field of the video_device struct.
621 The 'index' attribute is the index of the device node: for each call to
622 video_register_device() the index is just increased by 1. The first video
623 device node you register always starts with index 0.
625 Users can setup udev rules that utilize the index attribute to make fancy
626 device names (e.g. 'mpegX' for MPEG video capture device nodes).
628 After the device was successfully registered, then you can use these fields:
630 - vfl_type: the device type passed to video_register_device.
631 - minor: the assigned device minor number.
632 - num: the device node number (i.e. the X in videoX).
633 - index: the device index number.
635 If the registration failed, then you need to call video_device_release()
636 to free the allocated video_device struct, or free your own struct if the
637 video_device was embedded in it. The vdev->release() callback will never
638 be called if the registration failed, nor should you ever attempt to
639 unregister the device if the registration failed.
645 When the video device nodes have to be removed, either during the unload
646 of the driver or because the USB device was disconnected, then you should
649 video_unregister_device(vdev);
651 This will remove the device nodes from sysfs (causing udev to remove them
654 After video_unregister_device() returns no new opens can be done. However,
655 in the case of USB devices some application might still have one of these
656 device nodes open. So after the unregister all file operations (except
657 release, of course) will return an error as well.
659 When the last user of the video device node exits, then the vdev->release()
660 callback is called and you can do the final cleanup there.
662 Don't forget to cleanup the media entity associated with the video device if
663 it has been initialized:
665 media_entity_cleanup(&vdev->entity);
667 This can be done from the release callback.
670 video_device helper functions
671 -----------------------------
673 There are a few useful helper functions:
675 - file/video_device private data
677 You can set/get driver private data in the video_device struct using:
679 void *video_get_drvdata(struct video_device *vdev);
680 void video_set_drvdata(struct video_device *vdev, void *data);
682 Note that you can safely call video_set_drvdata() before calling
683 video_register_device().
687 struct video_device *video_devdata(struct file *file);
689 returns the video_device belonging to the file struct.
691 The video_drvdata function combines video_get_drvdata with video_devdata:
693 void *video_drvdata(struct file *file);
695 You can go from a video_device struct to the v4l2_device struct using:
697 struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
701 The video_device node kernel name can be retrieved using
703 const char *video_device_node_name(struct video_device *vdev);
705 The name is used as a hint by userspace tools such as udev. The function
706 should be used where possible instead of accessing the video_device::num and
707 video_device::minor fields.
710 video buffer helper functions
711 -----------------------------
713 The v4l2 core API provides a set of standard methods (called "videobuf")
714 for dealing with video buffers. Those methods allow a driver to implement
715 read(), mmap() and overlay() in a consistent way. There are currently
716 methods for using video buffers on devices that supports DMA with
717 scatter/gather method (videobuf-dma-sg), DMA with linear access
718 (videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
721 Please see Documentation/video4linux/videobuf for more information on how
722 to use the videobuf layer.
727 struct v4l2_fh provides a way to easily keep file handle specific data
728 that is used by the V4L2 framework. Using v4l2_fh is optional for
731 The users of v4l2_fh (in the V4L2 framework, not the driver) know
732 whether a driver uses v4l2_fh as its file->private_data pointer by
733 testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags.
739 Initialise the file handle. This *MUST* be performed in the driver's
740 v4l2_file_operations->open() handler.
744 Add a v4l2_fh to video_device file handle list. May be called after
745 initialising the file handle.
749 Unassociate the file handle from video_device(). The file handle
750 exit function may now be called.
754 Uninitialise the file handle. After uninitialisation the v4l2_fh
757 struct v4l2_fh is allocated as a part of the driver's own file handle
758 structure and is set to file->private_data in the driver's open
759 function by the driver. Drivers can extract their own file handle
760 structure by using the container_of macro. Example:
769 int my_open(struct file *file)
772 struct video_device *vfd;
777 ret = v4l2_fh_init(&my_fh->fh, vfd);
781 v4l2_fh_add(&my_fh->fh);
783 file->private_data = &my_fh->fh;
788 int my_release(struct file *file)
790 struct v4l2_fh *fh = file->private_data;
791 struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
799 The V4L2 events provide a generic way to pass events to user space.
800 The driver must use v4l2_fh to be able to support V4L2 events.
806 To use events, the driver must allocate events for the file handle. By
807 calling the function more than once, the driver may assure that at least n
808 events in total have been allocated. The function may not be called in
813 Queue events to video device. The driver's only responsibility is to fill
814 in the type and the data fields. The other fields will be filled in by
817 - v4l2_event_subscribe()
819 The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
820 is able to produce events with specified event id. Then it calls
821 v4l2_event_subscribe() to subscribe the event.
823 - v4l2_event_unsubscribe()
825 vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
826 v4l2_event_unsubscribe() directly unless it wants to be involved in
827 unsubscription process.
829 The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The
830 drivers may want to handle this in a special way.
832 - v4l2_event_pending()
834 Returns the number of pending events. Useful when implementing poll.
836 Drivers do not initialise events directly. The events are initialised
837 through v4l2_fh_init() if video_device->ioctl_ops->vidioc_subscribe_event is
838 non-NULL. This *MUST* be performed in the driver's
839 v4l2_file_operations->open() handler.
841 Events are delivered to user space through the poll system call. The driver
842 can use v4l2_fh->events->wait wait_queue_head_t as the argument for
845 There are standard and private events. New standard events must use the
846 smallest available event type. The drivers must allocate their events from
847 their own class starting from class base. Class base is
848 V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number.
849 The first event type in the class is reserved for future use, so the first
850 available event type is 'class base + 1'.
852 An example on how the V4L2 events may be used can be found in the OMAP
853 3 ISP driver available at <URL:http://gitorious.org/omap3camera> as of