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.
37 A good example to look at as a reference is the v4l2-pci-skeleton.c
38 source that is available in samples/v4l/. It is a skeleton driver for
39 a PCI capture card, and demonstrates how to use the V4L2 driver
40 framework. It can be used as a template for real PCI video capture driver.
45 All drivers have the following structure:
47 1) A struct for each device instance containing the device state.
49 2) A way of initializing and commanding sub-devices (if any).
51 3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX)
52 and keeping track of device-node specific data.
54 4) Filehandle-specific structs containing per-filehandle data;
56 5) video buffer handling.
58 This is a rough schematic of how it all relates:
64 +-sub-device instances
68 \-filehandle instances
71 Structure of the framework
72 --------------------------
74 The framework closely resembles the driver structure: it has a v4l2_device
75 struct for the device instance data, a v4l2_subdev struct to refer to
76 sub-device instances, the video_device struct stores V4L2 device node data
77 and the v4l2_fh struct keeps track of filehandle instances.
79 The V4L2 framework also optionally integrates with the media framework. If a
80 driver sets the struct v4l2_device mdev field, sub-devices and video nodes
81 will automatically appear in the media framework as entities.
87 The actual device nodes in the /dev directory are created using the
88 video_device struct (v4l2-dev.h). This struct can either be allocated
89 dynamically or embedded in a larger struct.
91 To allocate it dynamically use:
95 struct video_device *vdev = video_device_alloc();
100 vdev->release = video_device_release;
102 If you embed it in a larger struct, then you must set the release()
103 callback to your own function:
107 struct video_device *vdev = &my_vdev->vdev;
109 vdev->release = my_vdev_release;
111 The release callback must be set and it is called when the last user
112 of the video device exits.
114 The default video_device_release() callback just calls kfree to free the
117 There is also a video_device_release_empty() function that does nothing
118 (is empty) and can be used if the struct is embedded and there is nothing
119 to do when it is released.
121 You should also set these fields:
123 - v4l2_dev: must be set to the v4l2_device parent device.
125 - name: set to something descriptive and unique.
127 - vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0,
128 so this is normally already the default), set to VFL_DIR_TX for output
129 devices and VFL_DIR_M2M for mem2mem (codec) devices.
131 - fops: set to the v4l2_file_operations struct.
133 - ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
134 (highly recommended to use this and it might become compulsory in the
135 future!), then set this to your v4l2_ioctl_ops struct. The vfl_type and
136 vfl_dir fields are used to disable ops that do not match the type/dir
137 combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops
138 are disabled for a capture device. This makes it possible to provide
139 just one v4l2_ioctl_ops struct for both vbi and video nodes.
141 - lock: leave to NULL if you want to do all the locking in the driver.
142 Otherwise you give it a pointer to a struct mutex_lock and before the
143 unlocked_ioctl file operation is called this lock will be taken by the
144 core and released afterwards. See the next section for more details.
146 - queue: a pointer to the struct vb2_queue associated with this device node.
147 If queue is non-NULL, and queue->lock is non-NULL, then queue->lock is
148 used for the queuing ioctls (VIDIOC_REQBUFS, CREATE_BUFS, QBUF, DQBUF,
149 QUERYBUF, PREPARE_BUF, STREAMON and STREAMOFF) instead of the lock above.
150 That way the vb2 queuing framework does not have to wait for other ioctls.
151 This queue pointer is also used by the vb2 helper functions to check for
152 queuing ownership (i.e. is the filehandle calling it allowed to do the
155 - prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
156 If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
157 If you want to have a separate priority state per (group of) device node(s),
158 then you can point it to your own struct v4l2_prio_state.
160 - dev_parent: you only set this if v4l2_device was registered with NULL as
161 the parent device struct. This only happens in cases where one hardware
162 device has multiple PCI devices that all share the same v4l2_device core.
164 The cx88 driver is an example of this: one core v4l2_device struct, but
165 it is used by both a raw video PCI device (cx8800) and a MPEG PCI device
166 (cx8802). Since the v4l2_device cannot be associated with two PCI devices
167 at the same time it is setup without a parent device. But when the struct
168 video_device is initialized you *do* know which parent PCI device to use and
169 so you set dev_device to the correct PCI device.
171 If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
172 in your v4l2_file_operations struct.
174 Do not use .ioctl! This is deprecated and will go away in the future.
176 In some cases you want to tell the core that a function you had specified in
177 your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this
178 function before video_device_register is called:
182 void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd);
184 This tends to be needed if based on external factors (e.g. which card is
185 being used) you want to turns off certain features in v4l2_ioctl_ops without
186 having to make a new struct.
188 The v4l2_file_operations struct is a subset of file_operations. The main
189 difference is that the inode argument is omitted since it is never used.
191 If integration with the media framework is needed, you must initialize the
192 media_entity struct embedded in the video_device struct (entity field) by
193 calling media_entity_pads_init():
197 struct media_pad *pad = &my_vdev->pad;
200 err = media_entity_pads_init(&vdev->entity, 1, pad);
202 The pads array must have been previously initialized. There is no need to
203 manually set the struct media_entity type and name fields.
205 A reference to the entity will be automatically acquired/released when the
206 video device is opened/closed.
211 The V4L core provides optional locking services. The main service is the
212 lock field in struct video_device, which is a pointer to a mutex. If you set
213 this pointer, then that will be used by unlocked_ioctl to serialize all ioctls.
215 If you are using the videobuf2 framework, then there is a second lock that you
216 can set: video_device->queue->lock. If set, then this lock will be used instead
217 of video_device->lock to serialize all queuing ioctls (see the previous section
218 for the full list of those ioctls).
220 The advantage of using a different lock for the queuing ioctls is that for some
221 drivers (particularly USB drivers) certain commands such as setting controls
222 can take a long time, so you want to use a separate lock for the buffer queuing
223 ioctls. That way your VIDIOC_DQBUF doesn't stall because the driver is busy
224 changing the e.g. exposure of the webcam.
226 Of course, you can always do all the locking yourself by leaving both lock
229 If you use the old videobuf then you must pass the video_device lock to the
230 videobuf queue initialize function: if videobuf has to wait for a frame to
231 arrive, then it will temporarily unlock the lock and relock it afterwards. If
232 your driver also waits in the code, then you should do the same to allow other
233 processes to access the device node while the first process is waiting for
236 In the case of videobuf2 you will need to implement the wait_prepare and
237 wait_finish callbacks to unlock/lock if applicable. If you use the queue->lock
238 pointer, then you can use the helper functions vb2_ops_wait_prepare/finish.
240 The implementation of a hotplug disconnect should also take the lock from
241 video_device before calling v4l2_device_disconnect. If you are also using
242 video_device->queue->lock, then you have to first lock video_device->queue->lock
243 followed by video_device->lock. That way you can be sure no ioctl is running
244 when you call v4l2_device_disconnect.
246 video_device registration
247 -------------------------
249 Next you register the video device: this will create the character device
254 err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
256 video_device_release(vdev); /* or kfree(my_vdev); */
260 If the v4l2_device parent device has a non-NULL mdev field, the video device
261 entity will be automatically registered with the media device.
263 Which device is registered depends on the type argument. The following
266 VFL_TYPE_GRABBER: videoX for video input/output devices
267 VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
268 VFL_TYPE_RADIO: radioX for radio tuners
269 VFL_TYPE_SDR: swradioX for Software Defined Radio tuners
271 The last argument gives you a certain amount of control over the device
272 device node number used (i.e. the X in videoX). Normally you will pass -1
273 to let the v4l2 framework pick the first free number. But sometimes users
274 want to select a specific node number. It is common that drivers allow
275 the user to select a specific device node number through a driver module
276 option. That number is then passed to this function and video_register_device
277 will attempt to select that device node number. If that number was already
278 in use, then the next free device node number will be selected and it
279 will send a warning to the kernel log.
281 Another use-case is if a driver creates many devices. In that case it can
282 be useful to place different video devices in separate ranges. For example,
283 video capture devices start at 0, video output devices start at 16.
284 So you can use the last argument to specify a minimum device node number
285 and the v4l2 framework will try to pick the first free number that is equal
286 or higher to what you passed. If that fails, then it will just pick the
289 Since in this case you do not care about a warning about not being able
290 to select the specified device node number, you can call the function
291 video_register_device_no_warn() instead.
293 Whenever a device node is created some attributes are also created for you.
294 If you look in /sys/class/video4linux you see the devices. Go into e.g.
295 video0 and you will see 'name', 'dev_debug' and 'index' attributes. The 'name'
296 attribute is the 'name' field of the video_device struct. The 'dev_debug' attribute
297 can be used to enable core debugging. See the next section for more detailed
300 The 'index' attribute is the index of the device node: for each call to
301 video_register_device() the index is just increased by 1. The first video
302 device node you register always starts with index 0.
304 Users can setup udev rules that utilize the index attribute to make fancy
305 device names (e.g. 'mpegX' for MPEG video capture device nodes).
307 After the device was successfully registered, then you can use these fields:
309 - vfl_type: the device type passed to video_register_device.
310 - minor: the assigned device minor number.
311 - num: the device node number (i.e. the X in videoX).
312 - index: the device index number.
314 If the registration failed, then you need to call video_device_release()
315 to free the allocated video_device struct, or free your own struct if the
316 video_device was embedded in it. The vdev->release() callback will never
317 be called if the registration failed, nor should you ever attempt to
318 unregister the device if the registration failed.
320 video device debugging
321 ----------------------
323 The 'dev_debug' attribute that is created for each video, vbi, radio or swradio
324 device in /sys/class/video4linux/<devX>/ allows you to enable logging of
327 It is a bitmask and the following bits can be set:
331 0x01: Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are only logged
332 if bit 0x08 is also set.
333 0x02: Log the ioctl name arguments and error code. VIDIOC_(D)QBUF ioctls are
334 only logged if bit 0x08 is also set.
335 0x04: Log the file operations open, release, read, write, mmap and
336 get_unmapped_area. The read and write operations are only logged if
337 bit 0x08 is also set.
338 0x08: Log the read and write file operations and the VIDIOC_QBUF and
340 0x10: Log the poll file operation.
345 When the video device nodes have to be removed, either during the unload
346 of the driver or because the USB device was disconnected, then you should
351 video_unregister_device(vdev);
353 This will remove the device nodes from sysfs (causing udev to remove them
356 After video_unregister_device() returns no new opens can be done. However,
357 in the case of USB devices some application might still have one of these
358 device nodes open. So after the unregister all file operations (except
359 release, of course) will return an error as well.
361 When the last user of the video device node exits, then the vdev->release()
362 callback is called and you can do the final cleanup there.
364 Don't forget to cleanup the media entity associated with the video device if
365 it has been initialized:
369 media_entity_cleanup(&vdev->entity);
371 This can be done from the release callback.
374 video_device helper functions
375 -----------------------------
377 There are a few useful helper functions:
379 - file/video_device private data
381 You can set/get driver private data in the video_device struct using:
385 void *video_get_drvdata(struct video_device *vdev);
386 void video_set_drvdata(struct video_device *vdev, void *data);
388 Note that you can safely call video_set_drvdata() before calling
389 video_register_device().
395 struct video_device *video_devdata(struct file *file);
397 returns the video_device belonging to the file struct.
399 The video_drvdata function combines video_get_drvdata with video_devdata:
403 void *video_drvdata(struct file *file);
405 You can go from a video_device struct to the v4l2_device struct using:
409 struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
413 The video_device node kernel name can be retrieved using
417 const char *video_device_node_name(struct video_device *vdev);
419 The name is used as a hint by userspace tools such as udev. The function
420 should be used where possible instead of accessing the video_device::num and
421 video_device::minor fields.
424 video buffer helper functions
425 -----------------------------
427 The v4l2 core API provides a set of standard methods (called "videobuf")
428 for dealing with video buffers. Those methods allow a driver to implement
429 read(), mmap() and overlay() in a consistent way. There are currently
430 methods for using video buffers on devices that supports DMA with
431 scatter/gather method (videobuf-dma-sg), DMA with linear access
432 (videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
435 Please see Documentation/video4linux/videobuf for more information on how
436 to use the videobuf layer.
441 struct v4l2_fh provides a way to easily keep file handle specific data
442 that is used by the V4L2 framework. New drivers must use struct v4l2_fh
443 since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY).
445 The users of v4l2_fh (in the V4L2 framework, not the driver) know
446 whether a driver uses v4l2_fh as its file->private_data pointer by
447 testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is
448 set whenever v4l2_fh_init() is called.
450 struct v4l2_fh is allocated as a part of the driver's own file handle
451 structure and file->private_data is set to it in the driver's open
452 function by the driver.
454 In many cases the struct v4l2_fh will be embedded in a larger structure.
455 In that case you should call v4l2_fh_init+v4l2_fh_add in open() and
456 v4l2_fh_del+v4l2_fh_exit in release().
458 Drivers can extract their own file handle structure by using the container_of
470 int my_open(struct file *file)
473 struct video_device *vfd;
478 my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
482 v4l2_fh_init(&my_fh->fh, vfd);
486 file->private_data = &my_fh->fh;
487 v4l2_fh_add(&my_fh->fh);
491 int my_release(struct file *file)
493 struct v4l2_fh *fh = file->private_data;
494 struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
497 v4l2_fh_del(&my_fh->fh);
498 v4l2_fh_exit(&my_fh->fh);
503 Below is a short description of the v4l2_fh functions used:
507 void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
509 Initialise the file handle. This *MUST* be performed in the driver's
510 v4l2_file_operations->open() handler.
514 void v4l2_fh_add(struct v4l2_fh *fh)
516 Add a v4l2_fh to video_device file handle list. Must be called once the
517 file handle is completely initialized.
521 void v4l2_fh_del(struct v4l2_fh *fh)
523 Unassociate the file handle from video_device(). The file handle
524 exit function may now be called.
528 void v4l2_fh_exit(struct v4l2_fh *fh)
530 Uninitialise the file handle. After uninitialisation the v4l2_fh
534 If struct v4l2_fh is not embedded, then you can use these helper functions:
538 int v4l2_fh_open(struct file *filp)
540 This allocates a struct v4l2_fh, initializes it and adds it to the struct
541 video_device associated with the file struct.
545 int v4l2_fh_release(struct file *filp)
547 This deletes it from the struct video_device associated with the file
548 struct, uninitialised the v4l2_fh and frees it.
550 These two functions can be plugged into the v4l2_file_operation's open() and
554 Several drivers need to do something when the first file handle is opened and
555 when the last file handle closes. Two helper functions were added to check
556 whether the v4l2_fh struct is the only open filehandle of the associated
561 int v4l2_fh_is_singular(struct v4l2_fh *fh)
563 Returns 1 if the file handle is the only open file handle, else 0.
567 int v4l2_fh_is_singular_file(struct file *filp)
569 Same, but it calls v4l2_fh_is_singular with filp->private_data.
575 The V4L2 events provide a generic way to pass events to user space.
576 The driver must use v4l2_fh to be able to support V4L2 events.
578 Events are defined by a type and an optional ID. The ID may refer to a V4L2
579 object such as a control ID. If unused, then the ID is 0.
581 When the user subscribes to an event the driver will allocate a number of
582 kevent structs for that event. So every (type, ID) event tuple will have
583 its own set of kevent structs. This guarantees that if a driver is generating
584 lots of events of one type in a short time, then that will not overwrite
585 events of another type.
587 But if you get more events of one type than the number of kevents that were
588 reserved, then the oldest event will be dropped and the new one added.
590 Furthermore, the internal struct v4l2_subscribed_event has merge() and
591 replace() callbacks which drivers can set. These callbacks are called when
592 a new event is raised and there is no more room. The replace() callback
593 allows you to replace the payload of the old event with that of the new event,
594 merging any relevant data from the old payload into the new payload that
595 replaces it. It is called when this event type has only one kevent struct
596 allocated. The merge() callback allows you to merge the oldest event payload
597 into that of the second-oldest event payload. It is called when there are two
598 or more kevent structs allocated.
600 This way no status information is lost, just the intermediate steps leading
603 A good example of these replace/merge callbacks is in v4l2-event.c:
604 ctrls_replace() and ctrls_merge() callbacks for the control event.
606 Note: these callbacks can be called from interrupt context, so they must be
613 void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev)
615 Queue events to video device. The driver's only responsibility is to fill
616 in the type and the data fields. The other fields will be filled in by
621 int v4l2_event_subscribe(struct v4l2_fh *fh,
622 struct v4l2_event_subscription *sub, unsigned elems,
623 const struct v4l2_subscribed_event_ops *ops)
625 The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
626 is able to produce events with specified event id. Then it calls
627 v4l2_event_subscribe() to subscribe the event.
629 The elems argument is the size of the event queue for this event. If it is 0,
630 then the framework will fill in a default value (this depends on the event
633 The ops argument allows the driver to specify a number of callbacks:
634 * add: called when a new listener gets added (subscribing to the same
635 event twice will only cause this callback to get called once)
636 * del: called when a listener stops listening
637 * replace: replace event 'old' with event 'new'.
638 * merge: merge event 'old' into event 'new'.
639 All 4 callbacks are optional, if you don't want to specify any callbacks
640 the ops argument itself maybe NULL.
644 int v4l2_event_unsubscribe(struct v4l2_fh *fh,
645 struct v4l2_event_subscription *sub)
647 vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
648 v4l2_event_unsubscribe() directly unless it wants to be involved in
649 unsubscription process.
651 The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The
652 drivers may want to handle this in a special way.
656 int v4l2_event_pending(struct v4l2_fh *fh)
658 Returns the number of pending events. Useful when implementing poll.
660 Events are delivered to user space through the poll system call. The driver
661 can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait().
663 There are standard and private events. New standard events must use the
664 smallest available event type. The drivers must allocate their events from
665 their own class starting from class base. Class base is
666 V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number.
667 The first event type in the class is reserved for future use, so the first
668 available event type is 'class base + 1'.
670 An example on how the V4L2 events may be used can be found in the OMAP
671 3 ISP driver (drivers/media/platform/omap3isp).
673 A subdev can directly send an event to the v4l2_device notify function with
674 V4L2_DEVICE_NOTIFY_EVENT. This allows the bridge to map the subdev that sends
675 the event to the video node(s) associated with the subdev that need to be
676 informed about such an event.
681 Many subdevices, like camera sensors, TV decoders and encoders, need a clock
682 signal to be supplied by the system. Often this clock is supplied by the
683 respective bridge device. The Linux kernel provides a Common Clock Framework for
684 this purpose. However, it is not (yet) available on all architectures. Besides,
685 the nature of the multi-functional (clock, data + synchronisation, I2C control)
686 connection of subdevices to the system might impose special requirements on the
687 clock API usage. E.g. V4L2 has to support clock provider driver unregistration
688 while a subdevice driver is holding a reference to the clock. For these reasons
689 a V4L2 clock helper API has been developed and is provided to bridge and
692 The API consists of two parts: two functions to register and unregister a V4L2
693 clock source: v4l2_clk_register() and v4l2_clk_unregister() and calls to control
694 a clock object, similar to the respective generic clock API calls:
695 v4l2_clk_get(), v4l2_clk_put(), v4l2_clk_enable(), v4l2_clk_disable(),
696 v4l2_clk_get_rate(), and v4l2_clk_set_rate(). Clock suppliers have to provide
697 clock operations that will be called when clock users invoke respective API
700 It is expected that once the CCF becomes available on all relevant
701 architectures this API will be removed.
706 .. kernel-doc:: include/media/v4l2-dev.h