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diff --git a/Documentation/video4linux/v4l2-framework.txt b/Documentation/video4linux/v4l2-framework.txt new file mode 100644 index 00000000..cf21f7aa --- /dev/null +++ b/Documentation/video4linux/v4l2-framework.txt @@ -0,0 +1,956 @@ +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); + + ... + + ret = v4l2_fh_init(&my_fh->fh, vfd); + if (ret) { + kfree(my_fh); + return ret; + } + + ... + + 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: + +int 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. + +Useful functions: + +- v4l2_event_alloc() + + To use events, the driver must allocate events for the file handle. By + calling the function more than once, the driver may assure that at least n + events in total have been allocated. The function may not be called in + atomic context. + +- 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. + +- 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. + +Drivers do not initialise events directly. The events are initialised +through v4l2_fh_init() if video_device->ioctl_ops->vidioc_subscribe_event is +non-NULL. This *MUST* be performed in the driver's +v4l2_file_operations->open() handler. + +Events are delivered to user space through the poll system call. The driver +can use v4l2_fh->events->wait 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 available at <URL:http://gitorious.org/omap3camera> as of +writing this. |