About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / video4linux / v4l2-framework.txt

Custom Search

Based on kernel version 3.13. Page generated on 2014-01-20 22:05 EST.

1	Overview of the V4L2 driver framework
2	=====================================
4	This text documents the various structures provided by the V4L2 framework and
5	their relationships.
8	Introduction
9	------------
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
14	(IR) devices.
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
20	called 'sub-devices'.
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.
38	Structure of a driver
39	---------------------
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:
56	    device instances
57	      |
58	      +-sub-device instances
59	      |
60	      \-V4L2 device nodes
61		  |
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 the v4l2_fh struct keeps track of filehandle instances.
73	The V4L2 framework also optionally integrates with the media framework. If a
74	driver sets the struct v4l2_device mdev field, sub-devices and video nodes
75	will automatically appear in the media framework as entities.
78	struct v4l2_device
79	------------------
81	Each device instance is represented by a struct v4l2_device (v4l2-device.h).
82	Very simple devices can just allocate this struct, but most of the time you
83	would embed this struct inside a larger struct.
85	You must register the device instance:
87		v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev);
89	Registration will initialize the v4l2_device struct. If the dev->driver_data
90	field is NULL, it will be linked to v4l2_dev.
92	Drivers that want integration with the media device framework need to set
93	dev->driver_data manually to point to the driver-specific device structure
94	that embed the struct v4l2_device instance. This is achieved by a
95	dev_set_drvdata() call before registering the V4L2 device instance. They must
96	also set the struct v4l2_device mdev field to point to a properly initialized
97	and registered media_device instance.
99	If v4l2_dev->name is empty then it will be set to a value derived from dev
100	(driver name followed by the bus_id, to be precise). If you set it up before
101	calling v4l2_device_register then it will be untouched. If dev is NULL, then
102	you *must* setup v4l2_dev->name before calling v4l2_device_register.
104	You can use v4l2_device_set_name() to set the name based on a driver name and
105	a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1,
106	etc. If the name ends with a digit, then it will insert a dash: cx18-0,
107	cx18-1, etc. This function returns the instance number.
109	The first 'dev' argument is normally the struct device pointer of a pci_dev,
110	usb_interface or platform_device. It is rare for dev to be NULL, but it happens
111	with ISA devices or when one device creates multiple PCI devices, thus making
112	it impossible to associate v4l2_dev with a particular parent.
114	You can also supply a notify() callback that can be called by sub-devices to
115	notify you of events. Whether you need to set this depends on the sub-device.
116	Any notifications a sub-device supports must be defined in a header in
117	include/media/<subdevice>.h.
119	You unregister with:
121		v4l2_device_unregister(struct v4l2_device *v4l2_dev);
123	If the dev->driver_data field points to v4l2_dev, it will be reset to NULL.
124	Unregistering will also automatically unregister all subdevs from the device.
126	If you have a hotpluggable device (e.g. a USB device), then when a disconnect
127	happens the parent device becomes invalid. Since v4l2_device has a pointer to
128	that parent device it has to be cleared as well to mark that the parent is
129	gone. To do this call:
131		v4l2_device_disconnect(struct v4l2_device *v4l2_dev);
133	This does *not* unregister the subdevs, so you still need to call the
134	v4l2_device_unregister() function for that. If your driver is not hotpluggable,
135	then there is no need to call v4l2_device_disconnect().
137	Sometimes you need to iterate over all devices registered by a specific
138	driver. This is usually the case if multiple device drivers use the same
139	hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
140	hardware. The same is true for alsa drivers for example.
142	You can iterate over all registered devices as follows:
144	static int callback(struct device *dev, void *p)
145	{
146		struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
148		/* test if this device was inited */
149		if (v4l2_dev == NULL)
150			return 0;
151		...
152		return 0;
153	}
155	int iterate(void *p)
156	{
157		struct device_driver *drv;
158		int err;
160		/* Find driver 'ivtv' on the PCI bus.
161		   pci_bus_type is a global. For USB busses use usb_bus_type. */
162		drv = driver_find("ivtv", &pci_bus_type);
163		/* iterate over all ivtv device instances */
164		err = driver_for_each_device(drv, NULL, p, callback);
165		put_driver(drv);
166		return err;
167	}
169	Sometimes you need to keep a running counter of the device instance. This is
170	commonly used to map a device instance to an index of a module option array.
172	The recommended approach is as follows:
174	static atomic_t drv_instance = ATOMIC_INIT(0);
176	static int drv_probe(struct pci_dev *pdev, const struct pci_device_id *pci_id)
177	{
178		...
179		state->instance = atomic_inc_return(&drv_instance) - 1;
180	}
182	If you have multiple device nodes then it can be difficult to know when it is
183	safe to unregister v4l2_device for hotpluggable devices. For this purpose
184	v4l2_device has refcounting support. The refcount is increased whenever
185	video_register_device is called and it is decreased whenever that device node
186	is released. When the refcount reaches zero, then the v4l2_device release()
187	callback is called. You can do your final cleanup there.
189	If other device nodes (e.g. ALSA) are created, then you can increase and
190	decrease the refcount manually as well by calling:
192	void v4l2_device_get(struct v4l2_device *v4l2_dev);
194	or:
196	int v4l2_device_put(struct v4l2_device *v4l2_dev);
198	Since the initial refcount is 1 you also need to call v4l2_device_put in the
199	disconnect() callback (for USB devices) or in the remove() callback (for e.g.
200	PCI devices), otherwise the refcount will never reach 0.
202	struct v4l2_subdev
203	------------------
205	Many drivers need to communicate with sub-devices. These devices can do all
206	sort of tasks, but most commonly they handle audio and/or video muxing,
207	encoding or decoding. For webcams common sub-devices are sensors and camera
208	controllers.
210	Usually these are I2C devices, but not necessarily. In order to provide the
211	driver with a consistent interface to these sub-devices the v4l2_subdev struct
212	(v4l2-subdev.h) was created.
214	Each sub-device driver must have a v4l2_subdev struct. This struct can be
215	stand-alone for simple sub-devices or it might be embedded in a larger struct
216	if more state information needs to be stored. Usually there is a low-level
217	device struct (e.g. i2c_client) that contains the device data as setup
218	by the kernel. It is recommended to store that pointer in the private
219	data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
220	from a v4l2_subdev to the actual low-level bus-specific device data.
222	You also need a way to go from the low-level struct to v4l2_subdev. For the
223	common i2c_client struct the i2c_set_clientdata() call is used to store a
224	v4l2_subdev pointer, for other busses you may have to use other methods.
226	Bridges might also need to store per-subdev private data, such as a pointer to
227	bridge-specific per-subdev private data. The v4l2_subdev structure provides
228	host private data for that purpose that can be accessed with
229	v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata().
231	From the bridge driver perspective you load the sub-device module and somehow
232	obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
233	i2c_get_clientdata(). For other busses something similar needs to be done.
234	Helper functions exists for sub-devices on an I2C bus that do most of this
235	tricky work for you.
237	Each v4l2_subdev contains function pointers that sub-device drivers can
238	implement (or leave NULL if it is not applicable). Since sub-devices can do
239	so many different things and you do not want to end up with a huge ops struct
240	of which only a handful of ops are commonly implemented, the function pointers
241	are sorted according to category and each category has its own ops struct.
243	The top-level ops struct contains pointers to the category ops structs, which
244	may be NULL if the subdev driver does not support anything from that category.
246	It looks like this:
248	struct v4l2_subdev_core_ops {
249		int (*log_status)(struct v4l2_subdev *sd);
250		int (*init)(struct v4l2_subdev *sd, u32 val);
251		...
252	};
254	struct v4l2_subdev_tuner_ops {
255		...
256	};
258	struct v4l2_subdev_audio_ops {
259		...
260	};
262	struct v4l2_subdev_video_ops {
263		...
264	};
266	struct v4l2_subdev_pad_ops {
267		...
268	};
270	struct v4l2_subdev_ops {
271		const struct v4l2_subdev_core_ops  *core;
272		const struct v4l2_subdev_tuner_ops *tuner;
273		const struct v4l2_subdev_audio_ops *audio;
274		const struct v4l2_subdev_video_ops *video;
275		const struct v4l2_subdev_pad_ops *video;
276	};
278	The core ops are common to all subdevs, the other categories are implemented
279	depending on the sub-device. E.g. a video device is unlikely to support the
280	audio ops and vice versa.
282	This setup limits the number of function pointers while still making it easy
283	to add new ops and categories.
285	A sub-device driver initializes the v4l2_subdev struct using:
287		v4l2_subdev_init(sd, &ops);
289	Afterwards you need to initialize subdev->name with a unique name and set the
290	module owner. This is done for you if you use the i2c helper functions.
292	If integration with the media framework is needed, you must initialize the
293	media_entity struct embedded in the v4l2_subdev struct (entity field) by
294	calling media_entity_init():
296		struct media_pad *pads = &my_sd->pads;
297		int err;
299		err = media_entity_init(&sd->entity, npads, pads, 0);
301	The pads array must have been previously initialized. There is no need to
302	manually set the struct media_entity type and name fields, but the revision
303	field must be initialized if needed.
305	A reference to the entity will be automatically acquired/released when the
306	subdev device node (if any) is opened/closed.
308	Don't forget to cleanup the media entity before the sub-device is destroyed:
310		media_entity_cleanup(&sd->entity);
312	If the subdev driver intends to process video and integrate with the media
313	framework, it must implement format related functionality using
314	v4l2_subdev_pad_ops instead of v4l2_subdev_video_ops.
316	In that case, the subdev driver may set the link_validate field to provide
317	its own link validation function. The link validation function is called for
318	every link in the pipeline where both of the ends of the links are V4L2
319	sub-devices. The driver is still responsible for validating the correctness
320	of the format configuration between sub-devices and video nodes.
322	If link_validate op is not set, the default function
323	v4l2_subdev_link_validate_default() is used instead. This function ensures
324	that width, height and the media bus pixel code are equal on both source and
325	sink of the link. Subdev drivers are also free to use this function to
326	perform the checks mentioned above in addition to their own checks.
328	There are currently two ways to register subdevices with the V4L2 core. The
329	first (traditional) possibility is to have subdevices registered by bridge
330	drivers. This can be done when the bridge driver has the complete information
331	about subdevices connected to it and knows exactly when to register them. This
332	is typically the case for internal subdevices, like video data processing units
333	within SoCs or complex PCI(e) boards, camera sensors in USB cameras or connected
334	to SoCs, which pass information about them to bridge drivers, usually in their
335	platform data.
337	There are however also situations where subdevices have to be registered
338	asynchronously to bridge devices. An example of such a configuration is a Device
339	Tree based system where information about subdevices is made available to the
340	system independently from the bridge devices, e.g. when subdevices are defined
341	in DT as I2C device nodes. The API used in this second case is described further
342	below.
344	Using one or the other registration method only affects the probing process, the
345	run-time bridge-subdevice interaction is in both cases the same.
347	In the synchronous case a device (bridge) driver needs to register the
348	v4l2_subdev with the v4l2_device:
350		int err = v4l2_device_register_subdev(v4l2_dev, sd);
352	This can fail if the subdev module disappeared before it could be registered.
353	After this function was called successfully the subdev->dev field points to
354	the v4l2_device.
356	If the v4l2_device parent device has a non-NULL mdev field, the sub-device
357	entity will be automatically registered with the media device.
359	You can unregister a sub-device using:
361		v4l2_device_unregister_subdev(sd);
363	Afterwards the subdev module can be unloaded and sd->dev == NULL.
365	You can call an ops function either directly:
367		err = sd->ops->core->g_std(sd, &norm);
369	but it is better and easier to use this macro:
371		err = v4l2_subdev_call(sd, core, g_std, &norm);
373	The macro will to the right NULL pointer checks and returns -ENODEV if subdev
374	is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_std is
375	NULL, or the actual result of the subdev->ops->core->g_std ops.
377	It is also possible to call all or a subset of the sub-devices:
379		v4l2_device_call_all(v4l2_dev, 0, core, g_std, &norm);
381	Any subdev that does not support this ops is skipped and error results are
382	ignored. If you want to check for errors use this:
384		err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_std, &norm);
386	Any error except -ENOIOCTLCMD will exit the loop with that error. If no
387	errors (except -ENOIOCTLCMD) occurred, then 0 is returned.
389	The second argument to both calls is a group ID. If 0, then all subdevs are
390	called. If non-zero, then only those whose group ID match that value will
391	be called. Before a bridge driver registers a subdev it can set sd->grp_id
392	to whatever value it wants (it's 0 by default). This value is owned by the
393	bridge driver and the sub-device driver will never modify or use it.
395	The group ID gives the bridge driver more control how callbacks are called.
396	For example, there may be multiple audio chips on a board, each capable of
397	changing the volume. But usually only one will actually be used when the
398	user want to change the volume. You can set the group ID for that subdev to
399	e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
400	v4l2_device_call_all(). That ensures that it will only go to the subdev
401	that needs it.
403	If the sub-device needs to notify its v4l2_device parent of an event, then
404	it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
405	whether there is a notify() callback defined and returns -ENODEV if not.
406	Otherwise the result of the notify() call is returned.
408	The advantage of using v4l2_subdev is that it is a generic struct and does
409	not contain any knowledge about the underlying hardware. So a driver might
410	contain several subdevs that use an I2C bus, but also a subdev that is
411	controlled through GPIO pins. This distinction is only relevant when setting
412	up the device, but once the subdev is registered it is completely transparent.
415	In the asynchronous case subdevice probing can be invoked independently of the
416	bridge driver availability. The subdevice driver then has to verify whether all
417	the requirements for a successful probing are satisfied. This can include a
418	check for a master clock availability. If any of the conditions aren't satisfied
419	the driver might decide to return -EPROBE_DEFER to request further reprobing
420	attempts. Once all conditions are met the subdevice shall be registered using
421	the v4l2_async_register_subdev() function. Unregistration is performed using
422	the v4l2_async_unregister_subdev() call. Subdevices registered this way are
423	stored in a global list of subdevices, ready to be picked up by bridge drivers.
425	Bridge drivers in turn have to register a notifier object with an array of
426	subdevice descriptors that the bridge device needs for its operation. This is
427	performed using the v4l2_async_notifier_register() call. To unregister the
428	notifier the driver has to call v4l2_async_notifier_unregister(). The former of
429	the two functions takes two arguments: a pointer to struct v4l2_device and a
430	pointer to struct v4l2_async_notifier. The latter contains a pointer to an array
431	of pointers to subdevice descriptors of type struct v4l2_async_subdev type. The
432	V4L2 core will then use these descriptors to match asynchronously registered
433	subdevices to them. If a match is detected the .bound() notifier callback is
434	called. After all subdevices have been located the .complete() callback is
435	called. When a subdevice is removed from the system the .unbind() method is
436	called. All three callbacks are optional.
439	V4L2 sub-device userspace API
440	-----------------------------
442	Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2
443	sub-devices can also be controlled directly by userspace applications.
445	Device nodes named v4l-subdevX can be created in /dev to access sub-devices
446	directly. If a sub-device supports direct userspace configuration it must set
447	the V4L2_SUBDEV_FL_HAS_DEVNODE flag before being registered.
449	After registering sub-devices, the v4l2_device driver can create device nodes
450	for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling
451	v4l2_device_register_subdev_nodes(). Those device nodes will be automatically
452	removed when sub-devices are unregistered.
454	The device node handles a subset of the V4L2 API.
464		The controls ioctls are identical to the ones defined in V4L2. They
465		behave identically, with the only exception that they deal only with
466		controls implemented in the sub-device. Depending on the driver, those
467		controls can be also be accessed through one (or several) V4L2 device
468		nodes.
474		The events ioctls are identical to the ones defined in V4L2. They
475		behave identically, with the only exception that they deal only with
476		events generated by the sub-device. Depending on the driver, those
477		events can also be reported by one (or several) V4L2 device nodes.
479		Sub-device drivers that want to use events need to set the
480		V4L2_SUBDEV_USES_EVENTS v4l2_subdev::flags and initialize
481		v4l2_subdev::nevents to events queue depth before registering the
482		sub-device. After registration events can be queued as usual on the
483		v4l2_subdev::devnode device node.
485		To properly support events, the poll() file operation is also
486		implemented.
488	Private ioctls
490		All ioctls not in the above list are passed directly to the sub-device
491		driver through the core::ioctl operation.
494	I2C sub-device drivers
495	----------------------
497	Since these drivers are so common, special helper functions are available to
498	ease the use of these drivers (v4l2-common.h).
500	The recommended method of adding v4l2_subdev support to an I2C driver is to
501	embed the v4l2_subdev struct into the state struct that is created for each
502	I2C device instance. Very simple devices have no state struct and in that case
503	you can just create a v4l2_subdev directly.
505	A typical state struct would look like this (where 'chipname' is replaced by
506	the name of the chip):
508	struct chipname_state {
509		struct v4l2_subdev sd;
510		...  /* additional state fields */
511	};
513	Initialize the v4l2_subdev struct as follows:
515		v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
517	This function will fill in all the fields of v4l2_subdev and ensure that the
518	v4l2_subdev and i2c_client both point to one another.
520	You should also add a helper inline function to go from a v4l2_subdev pointer
521	to a chipname_state struct:
523	static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
524	{
525		return container_of(sd, struct chipname_state, sd);
526	}
528	Use this to go from the v4l2_subdev struct to the i2c_client struct:
530		struct i2c_client *client = v4l2_get_subdevdata(sd);
532	And this to go from an i2c_client to a v4l2_subdev struct:
534		struct v4l2_subdev *sd = i2c_get_clientdata(client);
536	Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
537	is called. This will unregister the sub-device from the bridge driver. It is
538	safe to call this even if the sub-device was never registered.
540	You need to do this because when the bridge driver destroys the i2c adapter
541	the remove() callbacks are called of the i2c devices on that adapter.
542	After that the corresponding v4l2_subdev structures are invalid, so they
543	have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
544	from the remove() callback ensures that this is always done correctly.
547	The bridge driver also has some helper functions it can use:
549	struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
550		       "module_foo", "chipid", 0x36, NULL);
552	This loads the given module (can be NULL if no module needs to be loaded) and
553	calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
554	If all goes well, then it registers the subdev with the v4l2_device.
556	You can also use the last argument of v4l2_i2c_new_subdev() to pass an array
557	of possible I2C addresses that it should probe. These probe addresses are
558	only used if the previous argument is 0. A non-zero argument means that you
559	know the exact i2c address so in that case no probing will take place.
561	Both functions return NULL if something went wrong.
563	Note that the chipid you pass to v4l2_i2c_new_subdev() is usually
564	the same as the module name. It allows you to specify a chip variant, e.g.
565	"saa7114" or "saa7115". In general though the i2c driver autodetects this.
566	The use of chipid is something that needs to be looked at more closely at a
567	later date. It differs between i2c drivers and as such can be confusing.
568	To see which chip variants are supported you can look in the i2c driver code
569	for the i2c_device_id table. This lists all the possibilities.
571	There are two more helper functions:
573	v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data
574	arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not
575	0 then that will be used (non-probing variant), otherwise the probed_addrs
576	are probed.
578	For example: this will probe for address 0x10:
580	struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter,
581		       "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10));
583	v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed
584	to the i2c driver and replaces the irq, platform_data and addr arguments.
586	If the subdev supports the s_config core ops, then that op is called with
587	the irq and platform_data arguments after the subdev was setup. The older
588	v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with
589	irq set to 0 and platform_data set to NULL.
591	struct video_device
592	-------------------
594	The actual device nodes in the /dev directory are created using the
595	video_device struct (v4l2-dev.h). This struct can either be allocated
596	dynamically or embedded in a larger struct.
598	To allocate it dynamically use:
600		struct video_device *vdev = video_device_alloc();
602		if (vdev == NULL)
603			return -ENOMEM;
605		vdev->release = video_device_release;
607	If you embed it in a larger struct, then you must set the release()
608	callback to your own function:
610		struct video_device *vdev = &my_vdev->vdev;
612		vdev->release = my_vdev_release;
614	The release callback must be set and it is called when the last user
615	of the video device exits.
617	The default video_device_release() callback just calls kfree to free the
618	allocated memory.
620	There is also a video_device_release_empty() function that does nothing
621	(is empty) and can be used if the struct is embedded and there is nothing
622	to do when it is released.
624	You should also set these fields:
626	- v4l2_dev: must be set to the v4l2_device parent device.
628	- name: set to something descriptive and unique.
630	- vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0,
631	  so this is normally already the default), set to VFL_DIR_TX for output
632	  devices and VFL_DIR_M2M for mem2mem (codec) devices.
634	- fops: set to the v4l2_file_operations struct.
636	- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
637	  (highly recommended to use this and it might become compulsory in the
638	  future!), then set this to your v4l2_ioctl_ops struct. The vfl_type and
639	  vfl_dir fields are used to disable ops that do not match the type/dir
640	  combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops
641	  are disabled for a capture device. This makes it possible to provide
642	  just one v4l2_ioctl_ops struct for both vbi and video nodes.
644	- lock: leave to NULL if you want to do all the locking in the driver.
645	  Otherwise you give it a pointer to a struct mutex_lock and before the
646	  unlocked_ioctl file operation is called this lock will be taken by the
647	  core and released afterwards. See the next section for more details.
649	- queue: a pointer to the struct vb2_queue associated with this device node.
650	  If queue is non-NULL, and queue->lock is non-NULL, then queue->lock is
651	  used for the queuing ioctls (VIDIOC_REQBUFS, CREATE_BUFS, QBUF, DQBUF,
652	  QUERYBUF, PREPARE_BUF, STREAMON and STREAMOFF) instead of the lock above.
653	  That way the vb2 queuing framework does not have to wait for other ioctls.
654	  This queue pointer is also used by the vb2 helper functions to check for
655	  queuing ownership (i.e. is the filehandle calling it allowed to do the
656	  operation).
658	- prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
659	  If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
660	  If you want to have a separate priority state per (group of) device node(s),
661	  then you can point it to your own struct v4l2_prio_state.
663	- dev_parent: you only set this if v4l2_device was registered with NULL as
664	  the parent device struct. This only happens in cases where one hardware
665	  device has multiple PCI devices that all share the same v4l2_device core.
667	  The cx88 driver is an example of this: one core v4l2_device struct, but
668	  it is used by both a raw video PCI device (cx8800) and a MPEG PCI device
669	  (cx8802). Since the v4l2_device cannot be associated with two PCI devices
670	  at the same time it is setup without a parent device. But when the struct
671	  video_device is initialized you *do* know which parent PCI device to use and
672	  so you set dev_device to the correct PCI device.
674	- flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework
675	  handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct
676	  v4l2_fh. Eventually this flag will disappear once all drivers use the core
677	  priority handling. But for now it has to be set explicitly.
679	If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
680	in your v4l2_file_operations struct.
682	Do not use .ioctl! This is deprecated and will go away in the future.
684	In some cases you want to tell the core that a function you had specified in
685	your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this
686	function before video_device_register is called:
688	void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd);
690	This tends to be needed if based on external factors (e.g. which card is
691	being used) you want to turns off certain features in v4l2_ioctl_ops without
692	having to make a new struct.
694	The v4l2_file_operations struct is a subset of file_operations. The main
695	difference is that the inode argument is omitted since it is never used.
697	If integration with the media framework is needed, you must initialize the
698	media_entity struct embedded in the video_device struct (entity field) by
699	calling media_entity_init():
701		struct media_pad *pad = &my_vdev->pad;
702		int err;
704		err = media_entity_init(&vdev->entity, 1, pad, 0);
706	The pads array must have been previously initialized. There is no need to
707	manually set the struct media_entity type and name fields.
709	A reference to the entity will be automatically acquired/released when the
710	video device is opened/closed.
712	ioctls and locking
713	------------------
715	The V4L core provides optional locking services. The main service is the
716	lock field in struct video_device, which is a pointer to a mutex. If you set
717	this pointer, then that will be used by unlocked_ioctl to serialize all ioctls.
719	If you are using the videobuf2 framework, then there is a second lock that you
720	can set: video_device->queue->lock. If set, then this lock will be used instead
721	of video_device->lock to serialize all queuing ioctls (see the previous section
722	for the full list of those ioctls).
724	The advantage of using a different lock for the queuing ioctls is that for some
725	drivers (particularly USB drivers) certain commands such as setting controls
726	can take a long time, so you want to use a separate lock for the buffer queuing
727	ioctls. That way your VIDIOC_DQBUF doesn't stall because the driver is busy
728	changing the e.g. exposure of the webcam.
730	Of course, you can always do all the locking yourself by leaving both lock
731	pointers at NULL.
733	If you use the old videobuf then you must pass the video_device lock to the
734	videobuf queue initialize function: if videobuf has to wait for a frame to
735	arrive, then it will temporarily unlock the lock and relock it afterwards. If
736	your driver also waits in the code, then you should do the same to allow other
737	processes to access the device node while the first process is waiting for
738	something.
740	In the case of videobuf2 you will need to implement the wait_prepare and
741	wait_finish callbacks to unlock/lock if applicable. If you use the queue->lock
742	pointer, then you can use the helper functions vb2_ops_wait_prepare/finish.
744	The implementation of a hotplug disconnect should also take the lock from
745	video_device before calling v4l2_device_disconnect. If you are also using
746	video_device->queue->lock, then you have to first lock video_device->queue->lock
747	followed by video_device->lock. That way you can be sure no ioctl is running
748	when you call v4l2_device_disconnect.
750	video_device registration
751	-------------------------
753	Next you register the video device: this will create the character device
754	for you.
756		err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
757		if (err) {
758			video_device_release(vdev); /* or kfree(my_vdev); */
759			return err;
760		}
762	If the v4l2_device parent device has a non-NULL mdev field, the video device
763	entity will be automatically registered with the media device.
765	Which device is registered depends on the type argument. The following
766	types exist:
768	VFL_TYPE_GRABBER: videoX for video input/output devices
769	VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
770	VFL_TYPE_RADIO: radioX for radio tuners
772	The last argument gives you a certain amount of control over the device
773	device node number used (i.e. the X in videoX). Normally you will pass -1
774	to let the v4l2 framework pick the first free number. But sometimes users
775	want to select a specific node number. It is common that drivers allow
776	the user to select a specific device node number through a driver module
777	option. That number is then passed to this function and video_register_device
778	will attempt to select that device node number. If that number was already
779	in use, then the next free device node number will be selected and it
780	will send a warning to the kernel log.
782	Another use-case is if a driver creates many devices. In that case it can
783	be useful to place different video devices in separate ranges. For example,
784	video capture devices start at 0, video output devices start at 16.
785	So you can use the last argument to specify a minimum device node number
786	and the v4l2 framework will try to pick the first free number that is equal
787	or higher to what you passed. If that fails, then it will just pick the
788	first free number.
790	Since in this case you do not care about a warning about not being able
791	to select the specified device node number, you can call the function
792	video_register_device_no_warn() instead.
794	Whenever a device node is created some attributes are also created for you.
795	If you look in /sys/class/video4linux you see the devices. Go into e.g.
796	video0 and you will see 'name' and 'index' attributes. The 'name' attribute
797	is the 'name' field of the video_device struct.
799	The 'index' attribute is the index of the device node: for each call to
800	video_register_device() the index is just increased by 1. The first video
801	device node you register always starts with index 0.
803	Users can setup udev rules that utilize the index attribute to make fancy
804	device names (e.g. 'mpegX' for MPEG video capture device nodes).
806	After the device was successfully registered, then you can use these fields:
808	- vfl_type: the device type passed to video_register_device.
809	- minor: the assigned device minor number.
810	- num: the device node number (i.e. the X in videoX).
811	- index: the device index number.
813	If the registration failed, then you need to call video_device_release()
814	to free the allocated video_device struct, or free your own struct if the
815	video_device was embedded in it. The vdev->release() callback will never
816	be called if the registration failed, nor should you ever attempt to
817	unregister the device if the registration failed.
820	video_device cleanup
821	--------------------
823	When the video device nodes have to be removed, either during the unload
824	of the driver or because the USB device was disconnected, then you should
825	unregister them:
827		video_unregister_device(vdev);
829	This will remove the device nodes from sysfs (causing udev to remove them
830	from /dev).
832	After video_unregister_device() returns no new opens can be done. However,
833	in the case of USB devices some application might still have one of these
834	device nodes open. So after the unregister all file operations (except
835	release, of course) will return an error as well.
837	When the last user of the video device node exits, then the vdev->release()
838	callback is called and you can do the final cleanup there.
840	Don't forget to cleanup the media entity associated with the video device if
841	it has been initialized:
843		media_entity_cleanup(&vdev->entity);
845	This can be done from the release callback.
848	video_device helper functions
849	-----------------------------
851	There are a few useful helper functions:
853	- file/video_device private data
855	You can set/get driver private data in the video_device struct using:
857	void *video_get_drvdata(struct video_device *vdev);
858	void video_set_drvdata(struct video_device *vdev, void *data);
860	Note that you can safely call video_set_drvdata() before calling
861	video_register_device().
863	And this function:
865	struct video_device *video_devdata(struct file *file);
867	returns the video_device belonging to the file struct.
869	The video_drvdata function combines video_get_drvdata with video_devdata:
871	void *video_drvdata(struct file *file);
873	You can go from a video_device struct to the v4l2_device struct using:
875	struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
877	- Device node name
879	The video_device node kernel name can be retrieved using
881	const char *video_device_node_name(struct video_device *vdev);
883	The name is used as a hint by userspace tools such as udev. The function
884	should be used where possible instead of accessing the video_device::num and
885	video_device::minor fields.
888	video buffer helper functions
889	-----------------------------
891	The v4l2 core API provides a set of standard methods (called "videobuf")
892	for dealing with video buffers. Those methods allow a driver to implement
893	read(), mmap() and overlay() in a consistent way.  There are currently
894	methods for using video buffers on devices that supports DMA with
895	scatter/gather method (videobuf-dma-sg), DMA with linear access
896	(videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
897	(videobuf-vmalloc).
899	Please see Documentation/video4linux/videobuf for more information on how
900	to use the videobuf layer.
902	struct v4l2_fh
903	--------------
905	struct v4l2_fh provides a way to easily keep file handle specific data
906	that is used by the V4L2 framework. New drivers must use struct v4l2_fh
907	since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY)
908	if the video_device flag V4L2_FL_USE_FH_PRIO is also set.
910	The users of v4l2_fh (in the V4L2 framework, not the driver) know
911	whether a driver uses v4l2_fh as its file->private_data pointer by
912	testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is
913	set whenever v4l2_fh_init() is called.
915	struct v4l2_fh is allocated as a part of the driver's own file handle
916	structure and file->private_data is set to it in the driver's open
917	function by the driver.
919	In many cases the struct v4l2_fh will be embedded in a larger structure.
920	In that case you should call v4l2_fh_init+v4l2_fh_add in open() and
921	v4l2_fh_del+v4l2_fh_exit in release().
923	Drivers can extract their own file handle structure by using the container_of
924	macro. Example:
926	struct my_fh {
927		int blah;
928		struct v4l2_fh fh;
929	};
931	...
933	int my_open(struct file *file)
934	{
935		struct my_fh *my_fh;
936		struct video_device *vfd;
937		int ret;
939		...
941		my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
943		...
945		v4l2_fh_init(&my_fh->fh, vfd);
947		...
949		file->private_data = &my_fh->fh;
950		v4l2_fh_add(&my_fh->fh);
951		return 0;
952	}
954	int my_release(struct file *file)
955	{
956		struct v4l2_fh *fh = file->private_data;
957		struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
959		...
960		v4l2_fh_del(&my_fh->fh);
961		v4l2_fh_exit(&my_fh->fh);
962		kfree(my_fh);
963		return 0;
964	}
966	Below is a short description of the v4l2_fh functions used:
968	void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
970	  Initialise the file handle. This *MUST* be performed in the driver's
971	  v4l2_file_operations->open() handler.
973	void v4l2_fh_add(struct v4l2_fh *fh)
975	  Add a v4l2_fh to video_device file handle list. Must be called once the
976	  file handle is completely initialized.
978	void v4l2_fh_del(struct v4l2_fh *fh)
980	  Unassociate the file handle from video_device(). The file handle
981	  exit function may now be called.
983	void v4l2_fh_exit(struct v4l2_fh *fh)
985	  Uninitialise the file handle. After uninitialisation the v4l2_fh
986	  memory can be freed.
989	If struct v4l2_fh is not embedded, then you can use these helper functions:
991	int v4l2_fh_open(struct file *filp)
993	  This allocates a struct v4l2_fh, initializes it and adds it to the struct
994	  video_device associated with the file struct.
996	int v4l2_fh_release(struct file *filp)
998	  This deletes it from the struct video_device associated with the file
999	  struct, uninitialised the v4l2_fh and frees it.
1001	These two functions can be plugged into the v4l2_file_operation's open() and
1002	release() ops.
1005	Several drivers need to do something when the first file handle is opened and
1006	when the last file handle closes. Two helper functions were added to check
1007	whether the v4l2_fh struct is the only open filehandle of the associated
1008	device node:
1010	int v4l2_fh_is_singular(struct v4l2_fh *fh)
1012	  Returns 1 if the file handle is the only open file handle, else 0.
1014	int v4l2_fh_is_singular_file(struct file *filp)
1016	  Same, but it calls v4l2_fh_is_singular with filp->private_data.
1019	V4L2 events
1020	-----------
1022	The V4L2 events provide a generic way to pass events to user space.
1023	The driver must use v4l2_fh to be able to support V4L2 events.
1025	Events are defined by a type and an optional ID. The ID may refer to a V4L2
1026	object such as a control ID. If unused, then the ID is 0.
1028	When the user subscribes to an event the driver will allocate a number of
1029	kevent structs for that event. So every (type, ID) event tuple will have
1030	its own set of kevent structs. This guarantees that if a driver is generating
1031	lots of events of one type in a short time, then that will not overwrite
1032	events of another type.
1034	But if you get more events of one type than the number of kevents that were
1035	reserved, then the oldest event will be dropped and the new one added.
1037	Furthermore, the internal struct v4l2_subscribed_event has merge() and
1038	replace() callbacks which drivers can set. These callbacks are called when
1039	a new event is raised and there is no more room. The replace() callback
1040	allows you to replace the payload of the old event with that of the new event,
1041	merging any relevant data from the old payload into the new payload that
1042	replaces it. It is called when this event type has only one kevent struct
1043	allocated. The merge() callback allows you to merge the oldest event payload
1044	into that of the second-oldest event payload. It is called when there are two
1045	or more kevent structs allocated.
1047	This way no status information is lost, just the intermediate steps leading
1048	up to that state.
1050	A good example of these replace/merge callbacks is in v4l2-event.c:
1051	ctrls_replace() and ctrls_merge() callbacks for the control event.
1053	Note: these callbacks can be called from interrupt context, so they must be
1054	fast.
1056	Useful functions:
1058	void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev)
1060	  Queue events to video device. The driver's only responsibility is to fill
1061	  in the type and the data fields. The other fields will be filled in by
1062	  V4L2.
1064	int v4l2_event_subscribe(struct v4l2_fh *fh,
1065				 struct v4l2_event_subscription *sub, unsigned elems,
1066				 const struct v4l2_subscribed_event_ops *ops)
1068	  The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
1069	  is able to produce events with specified event id. Then it calls
1070	  v4l2_event_subscribe() to subscribe the event.
1072	  The elems argument is the size of the event queue for this event. If it is 0,
1073	  then the framework will fill in a default value (this depends on the event
1074	  type).
1076	  The ops argument allows the driver to specify a number of callbacks:
1077	  * add:     called when a new listener gets added (subscribing to the same
1078	             event twice will only cause this callback to get called once)
1079	  * del:     called when a listener stops listening
1080	  * replace: replace event 'old' with event 'new'.
1081	  * merge:   merge event 'old' into event 'new'.
1082	  All 4 callbacks are optional, if you don't want to specify any callbacks
1083	  the ops argument itself maybe NULL.
1085	int v4l2_event_unsubscribe(struct v4l2_fh *fh,
1086				   struct v4l2_event_subscription *sub)
1088	  vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
1089	  v4l2_event_unsubscribe() directly unless it wants to be involved in
1090	  unsubscription process.
1092	  The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The
1093	  drivers may want to handle this in a special way.
1095	int v4l2_event_pending(struct v4l2_fh *fh)
1097	  Returns the number of pending events. Useful when implementing poll.
1099	Events are delivered to user space through the poll system call. The driver
1100	can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait().
1102	There are standard and private events. New standard events must use the
1103	smallest available event type. The drivers must allocate their events from
1104	their own class starting from class base. Class base is
1105	V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number.
1106	The first event type in the class is reserved for future use, so the first
1107	available event type is 'class base + 1'.
1109	An example on how the V4L2 events may be used can be found in the OMAP
1110	3 ISP driver (drivers/media/platform/omap3isp).
1113	V4L2 clocks
1114	-----------
1116	Many subdevices, like camera sensors, TV decoders and encoders, need a clock
1117	signal to be supplied by the system. Often this clock is supplied by the
1118	respective bridge device. The Linux kernel provides a Common Clock Framework for
1119	this purpose. However, it is not (yet) available on all architectures. Besides,
1120	the nature of the multi-functional (clock, data + synchronisation, I2C control)
1121	connection of subdevices to the system might impose special requirements on the
1122	clock API usage. E.g. V4L2 has to support clock provider driver unregistration
1123	while a subdevice driver is holding a reference to the clock. For these reasons
1124	a V4L2 clock helper API has been developed and is provided to bridge and
1125	subdevice drivers.
1127	The API consists of two parts: two functions to register and unregister a V4L2
1128	clock source: v4l2_clk_register() and v4l2_clk_unregister() and calls to control
1129	a clock object, similar to the respective generic clock API calls:
1130	v4l2_clk_get(), v4l2_clk_put(), v4l2_clk_enable(), v4l2_clk_disable(),
1131	v4l2_clk_get_rate(), and v4l2_clk_set_rate(). Clock suppliers have to provide
1132	clock operations that will be called when clock users invoke respective API
1133	methods.
1135	It is expected that once the CCF becomes available on all relevant
1136	architectures this API will be removed.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.