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. A good example to look at as a reference is the v4l2-pci-skeleton.c source that is available in samples/v4l/. It is a skeleton driver for a PCI capture card, and demonstrates how to use the V4L2 driver framework. It can be used as a template for real PCI video capture driver. 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: .. code-block:: none 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 the v4l2_fh struct keeps track of filehandle instances. 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. 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): .. code-block:: none struct chipname_state { struct v4l2_subdev sd; ... /* additional state fields */ }; Initialize the v4l2_subdev struct as follows: .. code-block:: none 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: .. code-block:: none 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: .. code-block:: none struct i2c_client *client = v4l2_get_subdevdata(sd); And this to go from an i2c_client to a v4l2_subdev struct: .. code-block:: none 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: .. code-block:: none 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: .. code-block:: none 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: .. code-block:: none 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: .. code-block:: none 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. There is also a video_device_release_empty() function that does nothing (is empty) and can be used if the struct is embedded and there is nothing to do when it is released. You should also set these fields: - v4l2_dev: must be set to the v4l2_device parent device. - name: set to something descriptive and unique. - vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0, so this is normally already the default), set to VFL_DIR_TX for output devices and VFL_DIR_M2M for mem2mem (codec) devices. - 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. The vfl_type and vfl_dir fields are used to disable ops that do not match the type/dir combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops are disabled for a capture device. This makes it possible to provide just one v4l2_ioctl_ops struct for both vbi and video nodes. - 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 the unlocked_ioctl file operation is called this lock will be taken by the core and released afterwards. See the next section for more details. - queue: a pointer to the struct vb2_queue associated with this device node. If queue is non-NULL, and queue->lock is non-NULL, then queue->lock is used for the queuing ioctls (VIDIOC_REQBUFS, CREATE_BUFS, QBUF, DQBUF, QUERYBUF, PREPARE_BUF, STREAMON and STREAMOFF) instead of the lock above. That way the vb2 queuing framework does not have to wait for other ioctls. This queue pointer is also used by the vb2 helper functions to check for queuing ownership (i.e. is the filehandle calling it allowed to do the operation). - 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. - dev_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 a raw video PCI device (cx8800) and a MPEG PCI device (cx8802). Since the v4l2_device cannot be associated with two PCI devices at the same time it is setup without a parent device. But when the struct video_device is initialized you *do* know which parent PCI device to use and so you set dev_device to the correct PCI device. 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. In some cases you want to tell the core that a function you had specified in your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this function before video_device_register is called: .. code-block:: none void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd); This tends to be needed if based on external factors (e.g. which card is being used) you want to turns off certain features in v4l2_ioctl_ops without having to make a new struct. 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_pads_init(): .. code-block:: none struct media_pad *pad = &my_vdev->pad; int err; err = media_entity_pads_init(&vdev->entity, 1, pad); 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. ioctls and locking ------------------ The V4L core provides optional locking services. The main service is the lock field in struct video_device, which is a pointer to a mutex. If you set this pointer, then that will be used by unlocked_ioctl to serialize all ioctls. If you are using the videobuf2 framework, then there is a second lock that you can set: video_device->queue->lock. If set, then this lock will be used instead of video_device->lock to serialize all queuing ioctls (see the previous section for the full list of those ioctls). The advantage of using a different lock for the queuing ioctls is that for some drivers (particularly USB drivers) certain commands such as setting controls can take a long time, so you want to use a separate lock for the buffer queuing ioctls. That way your VIDIOC_DQBUF doesn't stall because the driver is busy changing the e.g. exposure of the webcam. Of course, you can always do all the locking yourself by leaving both lock pointers at NULL. If you use the old videobuf then you must pass the video_device 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. In the case of videobuf2 you will need to implement the wait_prepare and wait_finish callbacks to unlock/lock if applicable. If you use the queue->lock pointer, then you can use the helper functions vb2_ops_wait_prepare/finish. The implementation of a hotplug disconnect should also take the lock from video_device before calling v4l2_device_disconnect. If you are also using video_device->queue->lock, then you have to first lock video_device->queue->lock followed by video_device->lock. That way you can be sure no ioctl is running when you call v4l2_device_disconnect. video_device registration ------------------------- Next you register the video device: this will create the character device for you. .. code-block:: none 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 VFL_TYPE_SDR: swradioX for Software Defined 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', 'dev_debug' and 'index' attributes. The 'name' attribute is the 'name' field of the video_device struct. The 'dev_debug' attribute can be used to enable core debugging. See the next section for more detailed information on this. 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 debugging ---------------------- The 'dev_debug' attribute that is created for each video, vbi, radio or swradio device in /sys/class/video4linux// allows you to enable logging of file operations. It is a bitmask and the following bits can be set: .. code-block:: none 0x01: Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are only logged if bit 0x08 is also set. 0x02: Log the ioctl name arguments and error code. VIDIOC_(D)QBUF ioctls are only logged if bit 0x08 is also set. 0x04: Log the file operations open, release, read, write, mmap and get_unmapped_area. The read and write operations are only logged if bit 0x08 is also set. 0x08: Log the read and write file operations and the VIDIOC_QBUF and VIDIOC_DQBUF ioctls. 0x10: Log the poll file operation. 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: .. code-block:: none 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: .. code-block:: none 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: .. code-block:: none 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: .. code-block:: none 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: .. code-block:: none void *video_drvdata(struct file *file); You can go from a video_device struct to the v4l2_device struct using: .. code-block:: none struct v4l2_device *v4l2_dev = vdev->v4l2_dev; - Device node name The video_device node kernel name can be retrieved using .. code-block:: none 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). 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: .. code-block:: none 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: .. code-block:: none 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. .. code-block:: none 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. .. code-block:: none void v4l2_fh_del(struct v4l2_fh *fh) Unassociate the file handle from video_device(). The file handle exit function may now be called. .. code-block:: none 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: .. code-block:: none 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. .. code-block:: none 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: .. code-block:: none int v4l2_fh_is_singular(struct v4l2_fh *fh) Returns 1 if the file handle is the only open file handle, else 0. .. code-block:: none 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: .. code-block:: none void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev) 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. .. code-block:: none int v4l2_event_subscribe(struct v4l2_fh *fh, struct v4l2_event_subscription *sub, unsigned elems, const struct v4l2_subscribed_event_ops *ops) 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 elems 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). The ops argument allows the driver to specify a number of callbacks: * add: called when a new listener gets added (subscribing to the same event twice will only cause this callback to get called once) * del: called when a listener stops listening * replace: replace event 'old' with event 'new'. * merge: merge event 'old' into event 'new'. All 4 callbacks are optional, if you don't want to specify any callbacks the ops argument itself maybe NULL. .. code-block:: none int v4l2_event_unsubscribe(struct v4l2_fh *fh, struct v4l2_event_subscription *sub) 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. .. code-block:: none int v4l2_event_pending(struct v4l2_fh *fh) 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/platform/omap3isp). A subdev can directly send an event to the v4l2_device notify function with V4L2_DEVICE_NOTIFY_EVENT. This allows the bridge to map the subdev that sends the event to the video node(s) associated with the subdev that need to be informed about such an event. V4L2 clocks ----------- Many subdevices, like camera sensors, TV decoders and encoders, need a clock signal to be supplied by the system. Often this clock is supplied by the respective bridge device. The Linux kernel provides a Common Clock Framework for this purpose. However, it is not (yet) available on all architectures. Besides, the nature of the multi-functional (clock, data + synchronisation, I2C control) connection of subdevices to the system might impose special requirements on the clock API usage. E.g. V4L2 has to support clock provider driver unregistration while a subdevice driver is holding a reference to the clock. For these reasons a V4L2 clock helper API has been developed and is provided to bridge and subdevice drivers. The API consists of two parts: two functions to register and unregister a V4L2 clock source: v4l2_clk_register() and v4l2_clk_unregister() and calls to control a clock object, similar to the respective generic clock API calls: v4l2_clk_get(), v4l2_clk_put(), v4l2_clk_enable(), v4l2_clk_disable(), v4l2_clk_get_rate(), and v4l2_clk_set_rate(). Clock suppliers have to provide clock operations that will be called when clock users invoke respective API methods. It is expected that once the CCF becomes available on all relevant architectures this API will be removed.