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-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 hotpluggable devices. 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);
-
-Since the initial refcount is 1 you also need to call v4l2_device_put in the
-disconnect() callback (for USB devices) or in the remove() callback (for e.g.
-PCI devices), otherwise the refcount will never reach 0.
-
-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_pad_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;
- const struct v4l2_subdev_pad_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);
-
-If the subdev driver intends to process video and integrate with the media
-framework, it must implement format related functionality using
-v4l2_subdev_pad_ops instead of v4l2_subdev_video_ops.
-
-In that case, the subdev driver may set the link_validate field to provide
-its own link validation function. The link validation function is called for
-every link in the pipeline where both of the ends of the links are V4L2
-sub-devices. The driver is still responsible for validating the correctness
-of the format configuration between sub-devices and video nodes.
-
-If link_validate op is not set, the default function
-v4l2_subdev_link_validate_default() is used instead. This function ensures
-that width, height and the media bus pixel code are equal on both source and
-sink of the link. Subdev drivers are also free to use this function to
-perform the checks mentioned above in addition to their own checks.
-
-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 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.
-
-- 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.
-
-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:
-
-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_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. By default this
-lock will be used for unlocked_ioctl, but you can disable locking for
-selected ioctls by calling:
-
- void v4l2_disable_ioctl_locking(struct video_device *vdev, unsigned int cmd);
-
-E.g.: v4l2_disable_ioctl_locking(vdev, VIDIOC_DQBUF);
-
-You have to call this before you register the video_device.
-
-Particularly with USB drivers where certain commands such as setting controls
-can take a long time you may want to do your own locking for the buffer queuing
-ioctls.
-
-If you want still finer-grained locking then you have to set mutex_lock to NULL
-and do you own locking completely.
-
-It is up to the driver developer to decide which method to use. However, if
-your driver has high-latency operations (for example, changing the exposure
-of a USB webcam might take a long time), then you might be better off with
-doing your own locking if you want to allow the user to do other things with
-the device while waiting for the high-latency command to finish.
-
-If a lock is specified then all ioctl commands 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.
-
-In the case of videobuf2 you will need to implement the wait_prepare and
-wait_finish callbacks to unlock/lock if applicable. In particular, if you use
-the lock in struct video_device then you must unlock/lock this mutex in
-wait_prepare and wait_finish.
-
-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:
-
-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.
-
-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.
-
-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.
-
-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/video/omap3isp).