Documentation / video4linux / v4l2-framework.txt

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Based on kernel version 3.2. Page generated on 2012-01-05 23:29 EST.

	Overview of the V4L2 driver framework
	=====================================
	
	This text documents the various structures provided by the V4L2 framework and
	their relationships.
	
	
	Introduction
	------------
	
	The V4L2 drivers tend to be very complex due to the complexity of the
	hardware: most devices have multiple ICs, export multiple device nodes in
	/dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input
	(IR) devices.
	
	Especially the fact that V4L2 drivers have to setup supporting ICs to
	do audio/video muxing/encoding/decoding makes it more complex than most.
	Usually these ICs are connected to the main bridge driver through one or
	more I2C busses, but other busses can also be used. Such devices are
	called 'sub-devices'.
	
	For a long time the framework was limited to the video_device struct for
	creating V4L device nodes and video_buf for handling the video buffers
	(note that this document does not discuss the video_buf framework).
	
	This meant that all drivers had to do the setup of device instances and
	connecting to sub-devices themselves. Some of this is quite complicated
	to do right and many drivers never did do it correctly.
	
	There is also a lot of common code that could never be refactored due to
	the lack of a framework.
	
	So this framework sets up the basic building blocks that all drivers
	need and this same framework should make it much easier to refactor
	common code into utility functions shared by all drivers.
	
	
	Structure of a driver
	---------------------
	
	All drivers have the following structure:
	
	1) A struct for each device instance containing the device state.
	
	2) A way of initializing and commanding sub-devices (if any).
	
	3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX)
	   and keeping track of device-node specific data.
	
	4) Filehandle-specific structs containing per-filehandle data;
	
	5) video buffer handling.
	
	This is a rough schematic of how it all relates:
	
	    device instances
	      |
	      +-sub-device instances
	      |
	      \-V4L2 device nodes
		  |
		  \-filehandle instances
	
	
	Structure of the framework
	--------------------------
	
	The framework closely resembles the driver structure: it has a v4l2_device
	struct for the device instance data, a v4l2_subdev struct to refer to
	sub-device instances, the video_device struct stores V4L2 device node data
	and in the future a v4l2_fh struct will keep track of filehandle instances
	(this is not yet implemented).
	
	The V4L2 framework also optionally integrates with the media framework. If a
	driver sets the struct v4l2_device mdev field, sub-devices and video nodes
	will automatically appear in the media framework as entities.
	
	
	struct v4l2_device
	------------------
	
	Each device instance is represented by a struct v4l2_device (v4l2-device.h).
	Very simple devices can just allocate this struct, but most of the time you
	would embed this struct inside a larger struct.
	
	You must register the device instance:
	
		v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev);
	
	Registration will initialize the v4l2_device struct. If the dev->driver_data
	field is NULL, it will be linked to v4l2_dev.
	
	Drivers that want integration with the media device framework need to set
	dev->driver_data manually to point to the driver-specific device structure
	that embed the struct v4l2_device instance. This is achieved by a
	dev_set_drvdata() call before registering the V4L2 device instance. They must
	also set the struct v4l2_device mdev field to point to a properly initialized
	and registered media_device instance.
	
	If v4l2_dev->name is empty then it will be set to a value derived from dev
	(driver name followed by the bus_id, to be precise). If you set it up before
	calling v4l2_device_register then it will be untouched. If dev is NULL, then
	you *must* setup v4l2_dev->name before calling v4l2_device_register.
	
	You can use v4l2_device_set_name() to set the name based on a driver name and
	a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1,
	etc. If the name ends with a digit, then it will insert a dash: cx18-0,
	cx18-1, etc. This function returns the instance number.
	
	The first 'dev' argument is normally the struct device pointer of a pci_dev,
	usb_interface or platform_device. It is rare for dev to be NULL, but it happens
	with ISA devices or when one device creates multiple PCI devices, thus making
	it impossible to associate v4l2_dev with a particular parent.
	
	You can also supply a notify() callback that can be called by sub-devices to
	notify you of events. Whether you need to set this depends on the sub-device.
	Any notifications a sub-device supports must be defined in a header in
	include/media/<subdevice>.h.
	
	You unregister with:
	
		v4l2_device_unregister(struct v4l2_device *v4l2_dev);
	
	If the dev->driver_data field points to v4l2_dev, it will be reset to NULL.
	Unregistering will also automatically unregister all subdevs from the device.
	
	If you have a hotpluggable device (e.g. a USB device), then when a disconnect
	happens the parent device becomes invalid. Since v4l2_device has a pointer to
	that parent device it has to be cleared as well to mark that the parent is
	gone. To do this call:
	
		v4l2_device_disconnect(struct v4l2_device *v4l2_dev);
	
	This does *not* unregister the subdevs, so you still need to call the
	v4l2_device_unregister() function for that. If your driver is not hotpluggable,
	then there is no need to call v4l2_device_disconnect().
	
	Sometimes you need to iterate over all devices registered by a specific
	driver. This is usually the case if multiple device drivers use the same
	hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
	hardware. The same is true for alsa drivers for example.
	
	You can iterate over all registered devices as follows:
	
	static int callback(struct device *dev, void *p)
	{
		struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
	
		/* test if this device was inited */
		if (v4l2_dev == NULL)
			return 0;
		...
		return 0;
	}
	
	int iterate(void *p)
	{
		struct device_driver *drv;
		int err;
	
		/* Find driver 'ivtv' on the PCI bus.
		   pci_bus_type is a global. For USB busses use usb_bus_type. */
		drv = driver_find("ivtv", &pci_bus_type);
		/* iterate over all ivtv device instances */
		err = driver_for_each_device(drv, NULL, p, callback);
		put_driver(drv);
		return err;
	}
	
	Sometimes you need to keep a running counter of the device instance. This is
	commonly used to map a device instance to an index of a module option array.
	
	The recommended approach is as follows:
	
	static atomic_t drv_instance = ATOMIC_INIT(0);
	
	static int __devinit drv_probe(struct pci_dev *pdev,
					const struct pci_device_id *pci_id)
	{
		...
		state->instance = atomic_inc_return(&drv_instance) - 1;
	}
	
	If you have multiple device nodes then it can be difficult to know when it is
	safe to unregister v4l2_device. For this purpose v4l2_device has refcounting
	support. The refcount is increased whenever video_register_device is called and
	it is decreased whenever that device node is released. When the refcount reaches
	zero, then the v4l2_device release() callback is called. You can do your final
	cleanup there.
	
	If other device nodes (e.g. ALSA) are created, then you can increase and
	decrease the refcount manually as well by calling:
	
	void v4l2_device_get(struct v4l2_device *v4l2_dev);
	
	or:
	
	int v4l2_device_put(struct v4l2_device *v4l2_dev);
	
	struct v4l2_subdev
	------------------
	
	Many drivers need to communicate with sub-devices. These devices can do all
	sort of tasks, but most commonly they handle audio and/or video muxing,
	encoding or decoding. For webcams common sub-devices are sensors and camera
	controllers.
	
	Usually these are I2C devices, but not necessarily. In order to provide the
	driver with a consistent interface to these sub-devices the v4l2_subdev struct
	(v4l2-subdev.h) was created.
	
	Each sub-device driver must have a v4l2_subdev struct. This struct can be
	stand-alone for simple sub-devices or it might be embedded in a larger struct
	if more state information needs to be stored. Usually there is a low-level
	device struct (e.g. i2c_client) that contains the device data as setup
	by the kernel. It is recommended to store that pointer in the private
	data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
	from a v4l2_subdev to the actual low-level bus-specific device data.
	
	You also need a way to go from the low-level struct to v4l2_subdev. For the
	common i2c_client struct the i2c_set_clientdata() call is used to store a
	v4l2_subdev pointer, for other busses you may have to use other methods.
	
	Bridges might also need to store per-subdev private data, such as a pointer to
	bridge-specific per-subdev private data. The v4l2_subdev structure provides
	host private data for that purpose that can be accessed with
	v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata().
	
	From the bridge driver perspective you load the sub-device module and somehow
	obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
	i2c_get_clientdata(). For other busses something similar needs to be done.
	Helper functions exists for sub-devices on an I2C bus that do most of this
	tricky work for you.
	
	Each v4l2_subdev contains function pointers that sub-device drivers can
	implement (or leave NULL if it is not applicable). Since sub-devices can do
	so many different things and you do not want to end up with a huge ops struct
	of which only a handful of ops are commonly implemented, the function pointers
	are sorted according to category and each category has its own ops struct.
	
	The top-level ops struct contains pointers to the category ops structs, which
	may be NULL if the subdev driver does not support anything from that category.
	
	It looks like this:
	
	struct v4l2_subdev_core_ops {
		int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip);
		int (*log_status)(struct v4l2_subdev *sd);
		int (*init)(struct v4l2_subdev *sd, u32 val);
		...
	};
	
	struct v4l2_subdev_tuner_ops {
		...
	};
	
	struct v4l2_subdev_audio_ops {
		...
	};
	
	struct v4l2_subdev_video_ops {
		...
	};
	
	struct v4l2_subdev_ops {
		const struct v4l2_subdev_core_ops  *core;
		const struct v4l2_subdev_tuner_ops *tuner;
		const struct v4l2_subdev_audio_ops *audio;
		const struct v4l2_subdev_video_ops *video;
	};
	
	The core ops are common to all subdevs, the other categories are implemented
	depending on the sub-device. E.g. a video device is unlikely to support the
	audio ops and vice versa.
	
	This setup limits the number of function pointers while still making it easy
	to add new ops and categories.
	
	A sub-device driver initializes the v4l2_subdev struct using:
	
		v4l2_subdev_init(sd, &ops);
	
	Afterwards you need to initialize subdev->name with a unique name and set the
	module owner. This is done for you if you use the i2c helper functions.
	
	If integration with the media framework is needed, you must initialize the
	media_entity struct embedded in the v4l2_subdev struct (entity field) by
	calling media_entity_init():
	
		struct media_pad *pads = &my_sd->pads;
		int err;
	
		err = media_entity_init(&sd->entity, npads, pads, 0);
	
	The pads array must have been previously initialized. There is no need to
	manually set the struct media_entity type and name fields, but the revision
	field must be initialized if needed.
	
	A reference to the entity will be automatically acquired/released when the
	subdev device node (if any) is opened/closed.
	
	Don't forget to cleanup the media entity before the sub-device is destroyed:
	
		media_entity_cleanup(&sd->entity);
	
	A device (bridge) driver needs to register the v4l2_subdev with the
	v4l2_device:
	
		int err = v4l2_device_register_subdev(v4l2_dev, sd);
	
	This can fail if the subdev module disappeared before it could be registered.
	After this function was called successfully the subdev->dev field points to
	the v4l2_device.
	
	If the v4l2_device parent device has a non-NULL mdev field, the sub-device
	entity will be automatically registered with the media device.
	
	You can unregister a sub-device using:
	
		v4l2_device_unregister_subdev(sd);
	
	Afterwards the subdev module can be unloaded and sd->dev == NULL.
	
	You can call an ops function either directly:
	
		err = sd->ops->core->g_chip_ident(sd, &chip);
	
	but it is better and easier to use this macro:
	
		err = v4l2_subdev_call(sd, core, g_chip_ident, &chip);
	
	The macro will to the right NULL pointer checks and returns -ENODEV if subdev
	is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is
	NULL, or the actual result of the subdev->ops->core->g_chip_ident ops.
	
	It is also possible to call all or a subset of the sub-devices:
	
		v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip);
	
	Any subdev that does not support this ops is skipped and error results are
	ignored. If you want to check for errors use this:
	
		err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip);
	
	Any error except -ENOIOCTLCMD will exit the loop with that error. If no
	errors (except -ENOIOCTLCMD) occurred, then 0 is returned.
	
	The second argument to both calls is a group ID. If 0, then all subdevs are
	called. If non-zero, then only those whose group ID match that value will
	be called. Before a bridge driver registers a subdev it can set sd->grp_id
	to whatever value it wants (it's 0 by default). This value is owned by the
	bridge driver and the sub-device driver will never modify or use it.
	
	The group ID gives the bridge driver more control how callbacks are called.
	For example, there may be multiple audio chips on a board, each capable of
	changing the volume. But usually only one will actually be used when the
	user want to change the volume. You can set the group ID for that subdev to
	e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
	v4l2_device_call_all(). That ensures that it will only go to the subdev
	that needs it.
	
	If the sub-device needs to notify its v4l2_device parent of an event, then
	it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
	whether there is a notify() callback defined and returns -ENODEV if not.
	Otherwise the result of the notify() call is returned.
	
	The advantage of using v4l2_subdev is that it is a generic struct and does
	not contain any knowledge about the underlying hardware. So a driver might
	contain several subdevs that use an I2C bus, but also a subdev that is
	controlled through GPIO pins. This distinction is only relevant when setting
	up the device, but once the subdev is registered it is completely transparent.
	
	
	V4L2 sub-device userspace API
	-----------------------------
	
	Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2
	sub-devices can also be controlled directly by userspace applications.
	
	Device nodes named v4l-subdevX can be created in /dev to access sub-devices
	directly. If a sub-device supports direct userspace configuration it must set
	the V4L2_SUBDEV_FL_HAS_DEVNODE flag before being registered.
	
	After registering sub-devices, the v4l2_device driver can create device nodes
	for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling
	v4l2_device_register_subdev_nodes(). Those device nodes will be automatically
	removed when sub-devices are unregistered.
	
	The device node handles a subset of the V4L2 API.
	
	VIDIOC_QUERYCTRL
	VIDIOC_QUERYMENU
	VIDIOC_G_CTRL
	VIDIOC_S_CTRL
	VIDIOC_G_EXT_CTRLS
	VIDIOC_S_EXT_CTRLS
	VIDIOC_TRY_EXT_CTRLS
	
		The controls ioctls are identical to the ones defined in V4L2. They
		behave identically, with the only exception that they deal only with
		controls implemented in the sub-device. Depending on the driver, those
		controls can be also be accessed through one (or several) V4L2 device
		nodes.
	
	VIDIOC_DQEVENT
	VIDIOC_SUBSCRIBE_EVENT
	VIDIOC_UNSUBSCRIBE_EVENT
	
		The events ioctls are identical to the ones defined in V4L2. They
		behave identically, with the only exception that they deal only with
		events generated by the sub-device. Depending on the driver, those
		events can also be reported by one (or several) V4L2 device nodes.
	
		Sub-device drivers that want to use events need to set the
		V4L2_SUBDEV_USES_EVENTS v4l2_subdev::flags and initialize
		v4l2_subdev::nevents to events queue depth before registering the
		sub-device. After registration events can be queued as usual on the
		v4l2_subdev::devnode device node.
	
		To properly support events, the poll() file operation is also
		implemented.
	
	Private ioctls
	
		All ioctls not in the above list are passed directly to the sub-device
		driver through the core::ioctl operation.
	
	
	I2C sub-device drivers
	----------------------
	
	Since these drivers are so common, special helper functions are available to
	ease the use of these drivers (v4l2-common.h).
	
	The recommended method of adding v4l2_subdev support to an I2C driver is to
	embed the v4l2_subdev struct into the state struct that is created for each
	I2C device instance. Very simple devices have no state struct and in that case
	you can just create a v4l2_subdev directly.
	
	A typical state struct would look like this (where 'chipname' is replaced by
	the name of the chip):
	
	struct chipname_state {
		struct v4l2_subdev sd;
		...  /* additional state fields */
	};
	
	Initialize the v4l2_subdev struct as follows:
	
		v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
	
	This function will fill in all the fields of v4l2_subdev and ensure that the
	v4l2_subdev and i2c_client both point to one another.
	
	You should also add a helper inline function to go from a v4l2_subdev pointer
	to a chipname_state struct:
	
	static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
	{
		return container_of(sd, struct chipname_state, sd);
	}
	
	Use this to go from the v4l2_subdev struct to the i2c_client struct:
	
		struct i2c_client *client = v4l2_get_subdevdata(sd);
	
	And this to go from an i2c_client to a v4l2_subdev struct:
	
		struct v4l2_subdev *sd = i2c_get_clientdata(client);
	
	Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
	is called. This will unregister the sub-device from the bridge driver. It is
	safe to call this even if the sub-device was never registered.
	
	You need to do this because when the bridge driver destroys the i2c adapter
	the remove() callbacks are called of the i2c devices on that adapter.
	After that the corresponding v4l2_subdev structures are invalid, so they
	have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
	from the remove() callback ensures that this is always done correctly.
	
	
	The bridge driver also has some helper functions it can use:
	
	struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
		       "module_foo", "chipid", 0x36, NULL);
	
	This loads the given module (can be NULL if no module needs to be loaded) and
	calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
	If all goes well, then it registers the subdev with the v4l2_device.
	
	You can also use the last argument of v4l2_i2c_new_subdev() to pass an array
	of possible I2C addresses that it should probe. These probe addresses are
	only used if the previous argument is 0. A non-zero argument means that you
	know the exact i2c address so in that case no probing will take place.
	
	Both functions return NULL if something went wrong.
	
	Note that the chipid you pass to v4l2_i2c_new_subdev() is usually
	the same as the module name. It allows you to specify a chip variant, e.g.
	"saa7114" or "saa7115". In general though the i2c driver autodetects this.
	The use of chipid is something that needs to be looked at more closely at a
	later date. It differs between i2c drivers and as such can be confusing.
	To see which chip variants are supported you can look in the i2c driver code
	for the i2c_device_id table. This lists all the possibilities.
	
	There are two more helper functions:
	
	v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data
	arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not
	0 then that will be used (non-probing variant), otherwise the probed_addrs
	are probed.
	
	For example: this will probe for address 0x10:
	
	struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter,
		       "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10));
	
	v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed
	to the i2c driver and replaces the irq, platform_data and addr arguments.
	
	If the subdev supports the s_config core ops, then that op is called with
	the irq and platform_data arguments after the subdev was setup. The older
	v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with
	irq set to 0 and platform_data set to NULL.
	
	struct video_device
	-------------------
	
	The actual device nodes in the /dev directory are created using the
	video_device struct (v4l2-dev.h). This struct can either be allocated
	dynamically or embedded in a larger struct.
	
	To allocate it dynamically use:
	
		struct video_device *vdev = video_device_alloc();
	
		if (vdev == NULL)
			return -ENOMEM;
	
		vdev->release = video_device_release;
	
	If you embed it in a larger struct, then you must set the release()
	callback to your own function:
	
		struct video_device *vdev = &my_vdev->vdev;
	
		vdev->release = my_vdev_release;
	
	The release callback must be set and it is called when the last user
	of the video device exits.
	
	The default video_device_release() callback just calls kfree to free the
	allocated memory.
	
	You should also set these fields:
	
	- v4l2_dev: set to the v4l2_device parent device.
	- name: set to something descriptive and unique.
	- fops: set to the v4l2_file_operations struct.
	- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
	  (highly recommended to use this and it might become compulsory in the
	  future!), then set this to your v4l2_ioctl_ops struct.
	- lock: leave to NULL if you want to do all the locking in the driver.
	  Otherwise you give it a pointer to a struct mutex_lock and before any
	  of the v4l2_file_operations is called this lock will be taken by the
	  core and released afterwards.
	- prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
	  If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
	  If you want to have a separate priority state per (group of) device node(s),
	  then you can point it to your own struct v4l2_prio_state.
	- parent: you only set this if v4l2_device was registered with NULL as
	  the parent device struct. This only happens in cases where one hardware
	  device has multiple PCI devices that all share the same v4l2_device core.
	
	  The cx88 driver is an example of this: one core v4l2_device struct, but
	  it is used by both an raw video PCI device (cx8800) and a MPEG PCI device
	  (cx8802). Since the v4l2_device cannot be associated with a particular
	  PCI device it is setup without a parent device. But when the struct
	  video_device is setup you do know which parent PCI device to use.
	- flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework
	  handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct
	  v4l2_fh. Eventually this flag will disappear once all drivers use the core
	  priority handling. But for now it has to be set explicitly.
	
	If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
	in your v4l2_file_operations struct.
	
	Do not use .ioctl! This is deprecated and will go away in the future.
	
	The v4l2_file_operations struct is a subset of file_operations. The main
	difference is that the inode argument is omitted since it is never used.
	
	If integration with the media framework is needed, you must initialize the
	media_entity struct embedded in the video_device struct (entity field) by
	calling media_entity_init():
	
		struct media_pad *pad = &my_vdev->pad;
		int err;
	
		err = media_entity_init(&vdev->entity, 1, pad, 0);
	
	The pads array must have been previously initialized. There is no need to
	manually set the struct media_entity type and name fields.
	
	A reference to the entity will be automatically acquired/released when the
	video device is opened/closed.
	
	v4l2_file_operations and locking
	--------------------------------
	
	You can set a pointer to a mutex_lock in struct video_device. Usually this
	will be either a top-level mutex or a mutex per device node. If you want
	finer-grained locking then you have to set it to NULL and do you own locking.
	
	If a lock is specified then all file operations will be serialized on that
	lock. If you use videobuf then you must pass the same lock to the videobuf
	queue initialize function: if videobuf has to wait for a frame to arrive, then
	it will temporarily unlock the lock and relock it afterwards. If your driver
	also waits in the code, then you should do the same to allow other processes
	to access the device node while the first process is waiting for something.
	
	The implementation of a hotplug disconnect should also take the lock before
	calling v4l2_device_disconnect.
	
	video_device registration
	-------------------------
	
	Next you register the video device: this will create the character device
	for you.
	
		err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
		if (err) {
			video_device_release(vdev); /* or kfree(my_vdev); */
			return err;
		}
	
	If the v4l2_device parent device has a non-NULL mdev field, the video device
	entity will be automatically registered with the media device.
	
	Which device is registered depends on the type argument. The following
	types exist:
	
	VFL_TYPE_GRABBER: videoX for video input/output devices
	VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
	VFL_TYPE_RADIO: radioX for radio tuners
	
	The last argument gives you a certain amount of control over the device
	device node number used (i.e. the X in videoX). Normally you will pass -1
	to let the v4l2 framework pick the first free number. But sometimes users
	want to select a specific node number. It is common that drivers allow
	the user to select a specific device node number through a driver module
	option. That number is then passed to this function and video_register_device
	will attempt to select that device node number. If that number was already
	in use, then the next free device node number will be selected and it
	will send a warning to the kernel log.
	
	Another use-case is if a driver creates many devices. In that case it can
	be useful to place different video devices in separate ranges. For example,
	video capture devices start at 0, video output devices start at 16.
	So you can use the last argument to specify a minimum device node number
	and the v4l2 framework will try to pick the first free number that is equal
	or higher to what you passed. If that fails, then it will just pick the
	first free number.
	
	Since in this case you do not care about a warning about not being able
	to select the specified device node number, you can call the function
	video_register_device_no_warn() instead.
	
	Whenever a device node is created some attributes are also created for you.
	If you look in /sys/class/video4linux you see the devices. Go into e.g.
	video0 and you will see 'name' and 'index' attributes. The 'name' attribute
	is the 'name' field of the video_device struct.
	
	The 'index' attribute is the index of the device node: for each call to
	video_register_device() the index is just increased by 1. The first video
	device node you register always starts with index 0.
	
	Users can setup udev rules that utilize the index attribute to make fancy
	device names (e.g. 'mpegX' for MPEG video capture device nodes).
	
	After the device was successfully registered, then you can use these fields:
	
	- vfl_type: the device type passed to video_register_device.
	- minor: the assigned device minor number.
	- num: the device node number (i.e. the X in videoX).
	- index: the device index number.
	
	If the registration failed, then you need to call video_device_release()
	to free the allocated video_device struct, or free your own struct if the
	video_device was embedded in it. The vdev->release() callback will never
	be called if the registration failed, nor should you ever attempt to
	unregister the device if the registration failed.
	
	
	video_device cleanup
	--------------------
	
	When the video device nodes have to be removed, either during the unload
	of the driver or because the USB device was disconnected, then you should
	unregister them:
	
		video_unregister_device(vdev);
	
	This will remove the device nodes from sysfs (causing udev to remove them
	from /dev).
	
	After video_unregister_device() returns no new opens can be done. However,
	in the case of USB devices some application might still have one of these
	device nodes open. So after the unregister all file operations (except
	release, of course) will return an error as well.
	
	When the last user of the video device node exits, then the vdev->release()
	callback is called and you can do the final cleanup there.
	
	Don't forget to cleanup the media entity associated with the video device if
	it has been initialized:
	
		media_entity_cleanup(&vdev->entity);
	
	This can be done from the release callback.
	
	
	video_device helper functions
	-----------------------------
	
	There are a few useful helper functions:
	
	- file/video_device private data
	
	You can set/get driver private data in the video_device struct using:
	
	void *video_get_drvdata(struct video_device *vdev);
	void video_set_drvdata(struct video_device *vdev, void *data);
	
	Note that you can safely call video_set_drvdata() before calling
	video_register_device().
	
	And this function:
	
	struct video_device *video_devdata(struct file *file);
	
	returns the video_device belonging to the file struct.
	
	The video_drvdata function combines video_get_drvdata with video_devdata:
	
	void *video_drvdata(struct file *file);
	
	You can go from a video_device struct to the v4l2_device struct using:
	
	struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
	
	- Device node name
	
	The video_device node kernel name can be retrieved using
	
	const char *video_device_node_name(struct video_device *vdev);
	
	The name is used as a hint by userspace tools such as udev. The function
	should be used where possible instead of accessing the video_device::num and
	video_device::minor fields.
	
	
	video buffer helper functions
	-----------------------------
	
	The v4l2 core API provides a set of standard methods (called "videobuf")
	for dealing with video buffers. Those methods allow a driver to implement
	read(), mmap() and overlay() in a consistent way.  There are currently
	methods for using video buffers on devices that supports DMA with
	scatter/gather method (videobuf-dma-sg), DMA with linear access
	(videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
	(videobuf-vmalloc).
	
	Please see Documentation/video4linux/videobuf for more information on how
	to use the videobuf layer.
	
	struct v4l2_fh
	--------------
	
	struct v4l2_fh provides a way to easily keep file handle specific data
	that is used by the V4L2 framework. New drivers must use struct v4l2_fh
	since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY)
	if the video_device flag V4L2_FL_USE_FH_PRIO is also set.
	
	The users of v4l2_fh (in the V4L2 framework, not the driver) know
	whether a driver uses v4l2_fh as its file->private_data pointer by
	testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is
	set whenever v4l2_fh_init() is called.
	
	struct v4l2_fh is allocated as a part of the driver's own file handle
	structure and file->private_data is set to it in the driver's open
	function by the driver.
	
	In many cases the struct v4l2_fh will be embedded in a larger structure.
	In that case you should call v4l2_fh_init+v4l2_fh_add in open() and
	v4l2_fh_del+v4l2_fh_exit in release().
	
	Drivers can extract their own file handle structure by using the container_of
	macro. Example:
	
	struct my_fh {
		int blah;
		struct v4l2_fh fh;
	};
	
	...
	
	int my_open(struct file *file)
	{
		struct my_fh *my_fh;
		struct video_device *vfd;
		int ret;
	
		...
	
		my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
	
		...
	
		v4l2_fh_init(&my_fh->fh, vfd);
	
		...
	
		file->private_data = &my_fh->fh;
		v4l2_fh_add(&my_fh->fh);
		return 0;
	}
	
	int my_release(struct file *file)
	{
		struct v4l2_fh *fh = file->private_data;
		struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
	
		...
		v4l2_fh_del(&my_fh->fh);
		v4l2_fh_exit(&my_fh->fh);
		kfree(my_fh);
		return 0;
	}
	
	Below is a short description of the v4l2_fh functions used:
	
	void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
	
	  Initialise the file handle. This *MUST* be performed in the driver's
	  v4l2_file_operations->open() handler.
	
	void v4l2_fh_add(struct v4l2_fh *fh)
	
	  Add a v4l2_fh to video_device file handle list. Must be called once the
	  file handle is completely initialized.
	
	void v4l2_fh_del(struct v4l2_fh *fh)
	
	  Unassociate the file handle from video_device(). The file handle
	  exit function may now be called.
	
	void v4l2_fh_exit(struct v4l2_fh *fh)
	
	  Uninitialise the file handle. After uninitialisation the v4l2_fh
	  memory can be freed.
	
	
	If struct v4l2_fh is not embedded, then you can use these helper functions:
	
	int v4l2_fh_open(struct file *filp)
	
	  This allocates a struct v4l2_fh, initializes it and adds it to the struct
	  video_device associated with the file struct.
	
	int v4l2_fh_release(struct file *filp)
	
	  This deletes it from the struct video_device associated with the file
	  struct, uninitialised the v4l2_fh and frees it.
	
	These two functions can be plugged into the v4l2_file_operation's open() and
	release() ops.
	
	
	Several drivers need to do something when the first file handle is opened and
	when the last file handle closes. Two helper functions were added to check
	whether the v4l2_fh struct is the only open filehandle of the associated
	device node:
	
	int v4l2_fh_is_singular(struct v4l2_fh *fh)
	
	  Returns 1 if the file handle is the only open file handle, else 0.
	
	int v4l2_fh_is_singular_file(struct file *filp)
	
	  Same, but it calls v4l2_fh_is_singular with filp->private_data.
	
	
	V4L2 events
	-----------
	
	The V4L2 events provide a generic way to pass events to user space.
	The driver must use v4l2_fh to be able to support V4L2 events.
	
	Events are defined by a type and an optional ID. The ID may refer to a V4L2
	object such as a control ID. If unused, then the ID is 0.
	
	When the user subscribes to an event the driver will allocate a number of
	kevent structs for that event. So every (type, ID) event tuple will have
	its own set of kevent structs. This guarantees that if a driver is generating
	lots of events of one type in a short time, then that will not overwrite
	events of another type.
	
	But if you get more events of one type than the number of kevents that were
	reserved, then the oldest event will be dropped and the new one added.
	
	Furthermore, the internal struct v4l2_subscribed_event has merge() and
	replace() callbacks which drivers can set. These callbacks are called when
	a new event is raised and there is no more room. The replace() callback
	allows you to replace the payload of the old event with that of the new event,
	merging any relevant data from the old payload into the new payload that
	replaces it. It is called when this event type has only one kevent struct
	allocated. The merge() callback allows you to merge the oldest event payload
	into that of the second-oldest event payload. It is called when there are two
	or more kevent structs allocated.
	
	This way no status information is lost, just the intermediate steps leading
	up to that state.
	
	A good example of these replace/merge callbacks is in v4l2-event.c:
	ctrls_replace() and ctrls_merge() callbacks for the control event.
	
	Note: these callbacks can be called from interrupt context, so they must be
	fast.
	
	Useful functions:
	
	- v4l2_event_queue()
	
	  Queue events to video device. The driver's only responsibility is to fill
	  in the type and the data fields. The other fields will be filled in by
	  V4L2.
	
	- v4l2_event_subscribe()
	
	  The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
	  is able to produce events with specified event id. Then it calls
	  v4l2_event_subscribe() to subscribe the event. The last argument is the
	  size of the event queue for this event. If it is 0, then the framework
	  will fill in a default value (this depends on the event type).
	
	- v4l2_event_unsubscribe()
	
	  vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
	  v4l2_event_unsubscribe() directly unless it wants to be involved in
	  unsubscription process.
	
	  The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The
	  drivers may want to handle this in a special way.
	
	- v4l2_event_pending()
	
	  Returns the number of pending events. Useful when implementing poll.
	
	Events are delivered to user space through the poll system call. The driver
	can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait().
	
	There are standard and private events. New standard events must use the
	smallest available event type. The drivers must allocate their events from
	their own class starting from class base. Class base is
	V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number.
	The first event type in the class is reserved for future use, so the first
	available event type is 'class base + 1'.
	
	An example on how the V4L2 events may be used can be found in the OMAP
	3 ISP driver (drivers/media/video/omap3isp).

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