linux/Documentation/video4linux/v4l2-framework.txt
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   1Overview of the V4L2 driver framework
   2=====================================
   3
   4This text documents the various structures provided by the V4L2 framework and
   5their relationships.
   6
   7
   8Introduction
   9------------
  10
  11The V4L2 drivers tend to be very complex due to the complexity of the
  12hardware: most devices have multiple ICs, export multiple device nodes in
  13/dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input
  14(IR) devices.
  15
  16Especially the fact that V4L2 drivers have to setup supporting ICs to
  17do audio/video muxing/encoding/decoding makes it more complex than most.
  18Usually these ICs are connected to the main bridge driver through one or
  19more I2C busses, but other busses can also be used. Such devices are
  20called 'sub-devices'.
  21
  22For a long time the framework was limited to the video_device struct for
  23creating V4L device nodes and video_buf for handling the video buffers
  24(note that this document does not discuss the video_buf framework).
  25
  26This meant that all drivers had to do the setup of device instances and
  27connecting to sub-devices themselves. Some of this is quite complicated
  28to do right and many drivers never did do it correctly.
  29
  30There is also a lot of common code that could never be refactored due to
  31the lack of a framework.
  32
  33So this framework sets up the basic building blocks that all drivers
  34need and this same framework should make it much easier to refactor
  35common code into utility functions shared by all drivers.
  36
  37
  38Structure of a driver
  39---------------------
  40
  41All drivers have the following structure:
  42
  431) A struct for each device instance containing the device state.
  44
  452) A way of initializing and commanding sub-devices (if any).
  46
  473) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX)
  48   and keeping track of device-node specific data.
  49
  504) Filehandle-specific structs containing per-filehandle data;
  51
  525) video buffer handling.
  53
  54This is a rough schematic of how it all relates:
  55
  56    device instances
  57      |
  58      +-sub-device instances
  59      |
  60      \-V4L2 device nodes
  61          |
  62          \-filehandle instances
  63
  64
  65Structure of the framework
  66--------------------------
  67
  68The framework closely resembles the driver structure: it has a v4l2_device
  69struct for the device instance data, a v4l2_subdev struct to refer to
  70sub-device instances, the video_device struct stores V4L2 device node data
  71and the v4l2_fh struct keeps track of filehandle instances.
  72
  73The V4L2 framework also optionally integrates with the media framework. If a
  74driver sets the struct v4l2_device mdev field, sub-devices and video nodes
  75will automatically appear in the media framework as entities.
  76
  77
  78struct v4l2_device
  79------------------
  80
  81Each device instance is represented by a struct v4l2_device (v4l2-device.h).
  82Very simple devices can just allocate this struct, but most of the time you
  83would embed this struct inside a larger struct.
  84
  85You must register the device instance:
  86
  87        v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev);
  88
  89Registration will initialize the v4l2_device struct. If the dev->driver_data
  90field is NULL, it will be linked to v4l2_dev.
  91
  92Drivers that want integration with the media device framework need to set
  93dev->driver_data manually to point to the driver-specific device structure
  94that embed the struct v4l2_device instance. This is achieved by a
  95dev_set_drvdata() call before registering the V4L2 device instance. They must
  96also set the struct v4l2_device mdev field to point to a properly initialized
  97and registered media_device instance.
  98
  99If v4l2_dev->name is empty then it will be set to a value derived from dev
 100(driver name followed by the bus_id, to be precise). If you set it up before
 101calling v4l2_device_register then it will be untouched. If dev is NULL, then
 102you *must* setup v4l2_dev->name before calling v4l2_device_register.
 103
 104You can use v4l2_device_set_name() to set the name based on a driver name and
 105a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1,
 106etc. If the name ends with a digit, then it will insert a dash: cx18-0,
 107cx18-1, etc. This function returns the instance number.
 108
 109The first 'dev' argument is normally the struct device pointer of a pci_dev,
 110usb_interface or platform_device. It is rare for dev to be NULL, but it happens
 111with ISA devices or when one device creates multiple PCI devices, thus making
 112it impossible to associate v4l2_dev with a particular parent.
 113
 114You can also supply a notify() callback that can be called by sub-devices to
 115notify you of events. Whether you need to set this depends on the sub-device.
 116Any notifications a sub-device supports must be defined in a header in
 117include/media/<subdevice>.h.
 118
 119You unregister with:
 120
 121        v4l2_device_unregister(struct v4l2_device *v4l2_dev);
 122
 123If the dev->driver_data field points to v4l2_dev, it will be reset to NULL.
 124Unregistering will also automatically unregister all subdevs from the device.
 125
 126If you have a hotpluggable device (e.g. a USB device), then when a disconnect
 127happens the parent device becomes invalid. Since v4l2_device has a pointer to
 128that parent device it has to be cleared as well to mark that the parent is
 129gone. To do this call:
 130
 131        v4l2_device_disconnect(struct v4l2_device *v4l2_dev);
 132
 133This does *not* unregister the subdevs, so you still need to call the
 134v4l2_device_unregister() function for that. If your driver is not hotpluggable,
 135then there is no need to call v4l2_device_disconnect().
 136
 137Sometimes you need to iterate over all devices registered by a specific
 138driver. This is usually the case if multiple device drivers use the same
 139hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
 140hardware. The same is true for alsa drivers for example.
 141
 142You can iterate over all registered devices as follows:
 143
 144static int callback(struct device *dev, void *p)
 145{
 146        struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
 147
 148        /* test if this device was inited */
 149        if (v4l2_dev == NULL)
 150                return 0;
 151        ...
 152        return 0;
 153}
 154
 155int iterate(void *p)
 156{
 157        struct device_driver *drv;
 158        int err;
 159
 160        /* Find driver 'ivtv' on the PCI bus.
 161           pci_bus_type is a global. For USB busses use usb_bus_type. */
 162        drv = driver_find("ivtv", &pci_bus_type);
 163        /* iterate over all ivtv device instances */
 164        err = driver_for_each_device(drv, NULL, p, callback);
 165        put_driver(drv);
 166        return err;
 167}
 168
 169Sometimes you need to keep a running counter of the device instance. This is
 170commonly used to map a device instance to an index of a module option array.
 171
 172The recommended approach is as follows:
 173
 174static atomic_t drv_instance = ATOMIC_INIT(0);
 175
 176static int drv_probe(struct pci_dev *pdev, const struct pci_device_id *pci_id)
 177{
 178        ...
 179        state->instance = atomic_inc_return(&drv_instance) - 1;
 180}
 181
 182If you have multiple device nodes then it can be difficult to know when it is
 183safe to unregister v4l2_device for hotpluggable devices. For this purpose
 184v4l2_device has refcounting support. The refcount is increased whenever
 185video_register_device is called and it is decreased whenever that device node
 186is released. When the refcount reaches zero, then the v4l2_device release()
 187callback is called. You can do your final cleanup there.
 188
 189If other device nodes (e.g. ALSA) are created, then you can increase and
 190decrease the refcount manually as well by calling:
 191
 192void v4l2_device_get(struct v4l2_device *v4l2_dev);
 193
 194or:
 195
 196int v4l2_device_put(struct v4l2_device *v4l2_dev);
 197
 198Since the initial refcount is 1 you also need to call v4l2_device_put in the
 199disconnect() callback (for USB devices) or in the remove() callback (for e.g.
 200PCI devices), otherwise the refcount will never reach 0.
 201
 202struct v4l2_subdev
 203------------------
 204
 205Many drivers need to communicate with sub-devices. These devices can do all
 206sort of tasks, but most commonly they handle audio and/or video muxing,
 207encoding or decoding. For webcams common sub-devices are sensors and camera
 208controllers.
 209
 210Usually these are I2C devices, but not necessarily. In order to provide the
 211driver with a consistent interface to these sub-devices the v4l2_subdev struct
 212(v4l2-subdev.h) was created.
 213
 214Each sub-device driver must have a v4l2_subdev struct. This struct can be
 215stand-alone for simple sub-devices or it might be embedded in a larger struct
 216if more state information needs to be stored. Usually there is a low-level
 217device struct (e.g. i2c_client) that contains the device data as setup
 218by the kernel. It is recommended to store that pointer in the private
 219data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
 220from a v4l2_subdev to the actual low-level bus-specific device data.
 221
 222You also need a way to go from the low-level struct to v4l2_subdev. For the
 223common i2c_client struct the i2c_set_clientdata() call is used to store a
 224v4l2_subdev pointer, for other busses you may have to use other methods.
 225
 226Bridges might also need to store per-subdev private data, such as a pointer to
 227bridge-specific per-subdev private data. The v4l2_subdev structure provides
 228host private data for that purpose that can be accessed with
 229v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata().
 230
 231From the bridge driver perspective you load the sub-device module and somehow
 232obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
 233i2c_get_clientdata(). For other busses something similar needs to be done.
 234Helper functions exists for sub-devices on an I2C bus that do most of this
 235tricky work for you.
 236
 237Each v4l2_subdev contains function pointers that sub-device drivers can
 238implement (or leave NULL if it is not applicable). Since sub-devices can do
 239so many different things and you do not want to end up with a huge ops struct
 240of which only a handful of ops are commonly implemented, the function pointers
 241are sorted according to category and each category has its own ops struct.
 242
 243The top-level ops struct contains pointers to the category ops structs, which
 244may be NULL if the subdev driver does not support anything from that category.
 245
 246It looks like this:
 247
 248struct v4l2_subdev_core_ops {
 249        int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip);
 250        int (*log_status)(struct v4l2_subdev *sd);
 251        int (*init)(struct v4l2_subdev *sd, u32 val);
 252        ...
 253};
 254
 255struct v4l2_subdev_tuner_ops {
 256        ...
 257};
 258
 259struct v4l2_subdev_audio_ops {
 260        ...
 261};
 262
 263struct v4l2_subdev_video_ops {
 264        ...
 265};
 266
 267struct v4l2_subdev_pad_ops {
 268        ...
 269};
 270
 271struct v4l2_subdev_ops {
 272        const struct v4l2_subdev_core_ops  *core;
 273        const struct v4l2_subdev_tuner_ops *tuner;
 274        const struct v4l2_subdev_audio_ops *audio;
 275        const struct v4l2_subdev_video_ops *video;
 276        const struct v4l2_subdev_pad_ops *video;
 277};
 278
 279The core ops are common to all subdevs, the other categories are implemented
 280depending on the sub-device. E.g. a video device is unlikely to support the
 281audio ops and vice versa.
 282
 283This setup limits the number of function pointers while still making it easy
 284to add new ops and categories.
 285
 286A sub-device driver initializes the v4l2_subdev struct using:
 287
 288        v4l2_subdev_init(sd, &ops);
 289
 290Afterwards you need to initialize subdev->name with a unique name and set the
 291module owner. This is done for you if you use the i2c helper functions.
 292
 293If integration with the media framework is needed, you must initialize the
 294media_entity struct embedded in the v4l2_subdev struct (entity field) by
 295calling media_entity_init():
 296
 297        struct media_pad *pads = &my_sd->pads;
 298        int err;
 299
 300        err = media_entity_init(&sd->entity, npads, pads, 0);
 301
 302The pads array must have been previously initialized. There is no need to
 303manually set the struct media_entity type and name fields, but the revision
 304field must be initialized if needed.
 305
 306A reference to the entity will be automatically acquired/released when the
 307subdev device node (if any) is opened/closed.
 308
 309Don't forget to cleanup the media entity before the sub-device is destroyed:
 310
 311        media_entity_cleanup(&sd->entity);
 312
 313If the subdev driver intends to process video and integrate with the media
 314framework, it must implement format related functionality using
 315v4l2_subdev_pad_ops instead of v4l2_subdev_video_ops.
 316
 317In that case, the subdev driver may set the link_validate field to provide
 318its own link validation function. The link validation function is called for
 319every link in the pipeline where both of the ends of the links are V4L2
 320sub-devices. The driver is still responsible for validating the correctness
 321of the format configuration between sub-devices and video nodes.
 322
 323If link_validate op is not set, the default function
 324v4l2_subdev_link_validate_default() is used instead. This function ensures
 325that width, height and the media bus pixel code are equal on both source and
 326sink of the link. Subdev drivers are also free to use this function to
 327perform the checks mentioned above in addition to their own checks.
 328
 329A device (bridge) driver needs to register the v4l2_subdev with the
 330v4l2_device:
 331
 332        int err = v4l2_device_register_subdev(v4l2_dev, sd);
 333
 334This can fail if the subdev module disappeared before it could be registered.
 335After this function was called successfully the subdev->dev field points to
 336the v4l2_device.
 337
 338If the v4l2_device parent device has a non-NULL mdev field, the sub-device
 339entity will be automatically registered with the media device.
 340
 341You can unregister a sub-device using:
 342
 343        v4l2_device_unregister_subdev(sd);
 344
 345Afterwards the subdev module can be unloaded and sd->dev == NULL.
 346
 347You can call an ops function either directly:
 348
 349        err = sd->ops->core->g_chip_ident(sd, &chip);
 350
 351but it is better and easier to use this macro:
 352
 353        err = v4l2_subdev_call(sd, core, g_chip_ident, &chip);
 354
 355The macro will to the right NULL pointer checks and returns -ENODEV if subdev
 356is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is
 357NULL, or the actual result of the subdev->ops->core->g_chip_ident ops.
 358
 359It is also possible to call all or a subset of the sub-devices:
 360
 361        v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip);
 362
 363Any subdev that does not support this ops is skipped and error results are
 364ignored. If you want to check for errors use this:
 365
 366        err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip);
 367
 368Any error except -ENOIOCTLCMD will exit the loop with that error. If no
 369errors (except -ENOIOCTLCMD) occurred, then 0 is returned.
 370
 371The second argument to both calls is a group ID. If 0, then all subdevs are
 372called. If non-zero, then only those whose group ID match that value will
 373be called. Before a bridge driver registers a subdev it can set sd->grp_id
 374to whatever value it wants (it's 0 by default). This value is owned by the
 375bridge driver and the sub-device driver will never modify or use it.
 376
 377The group ID gives the bridge driver more control how callbacks are called.
 378For example, there may be multiple audio chips on a board, each capable of
 379changing the volume. But usually only one will actually be used when the
 380user want to change the volume. You can set the group ID for that subdev to
 381e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
 382v4l2_device_call_all(). That ensures that it will only go to the subdev
 383that needs it.
 384
 385If the sub-device needs to notify its v4l2_device parent of an event, then
 386it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
 387whether there is a notify() callback defined and returns -ENODEV if not.
 388Otherwise the result of the notify() call is returned.
 389
 390The advantage of using v4l2_subdev is that it is a generic struct and does
 391not contain any knowledge about the underlying hardware. So a driver might
 392contain several subdevs that use an I2C bus, but also a subdev that is
 393controlled through GPIO pins. This distinction is only relevant when setting
 394up the device, but once the subdev is registered it is completely transparent.
 395
 396
 397V4L2 sub-device userspace API
 398-----------------------------
 399
 400Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2
 401sub-devices can also be controlled directly by userspace applications.
 402
 403Device nodes named v4l-subdevX can be created in /dev to access sub-devices
 404directly. If a sub-device supports direct userspace configuration it must set
 405the V4L2_SUBDEV_FL_HAS_DEVNODE flag before being registered.
 406
 407After registering sub-devices, the v4l2_device driver can create device nodes
 408for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling
 409v4l2_device_register_subdev_nodes(). Those device nodes will be automatically
 410removed when sub-devices are unregistered.
 411
 412The device node handles a subset of the V4L2 API.
 413
 414VIDIOC_QUERYCTRL
 415VIDIOC_QUERYMENU
 416VIDIOC_G_CTRL
 417VIDIOC_S_CTRL
 418VIDIOC_G_EXT_CTRLS
 419VIDIOC_S_EXT_CTRLS
 420VIDIOC_TRY_EXT_CTRLS
 421
 422        The controls ioctls are identical to the ones defined in V4L2. They
 423        behave identically, with the only exception that they deal only with
 424        controls implemented in the sub-device. Depending on the driver, those
 425        controls can be also be accessed through one (or several) V4L2 device
 426        nodes.
 427
 428VIDIOC_DQEVENT
 429VIDIOC_SUBSCRIBE_EVENT
 430VIDIOC_UNSUBSCRIBE_EVENT
 431
 432        The events ioctls are identical to the ones defined in V4L2. They
 433        behave identically, with the only exception that they deal only with
 434        events generated by the sub-device. Depending on the driver, those
 435        events can also be reported by one (or several) V4L2 device nodes.
 436
 437        Sub-device drivers that want to use events need to set the
 438        V4L2_SUBDEV_USES_EVENTS v4l2_subdev::flags and initialize
 439        v4l2_subdev::nevents to events queue depth before registering the
 440        sub-device. After registration events can be queued as usual on the
 441        v4l2_subdev::devnode device node.
 442
 443        To properly support events, the poll() file operation is also
 444        implemented.
 445
 446Private ioctls
 447
 448        All ioctls not in the above list are passed directly to the sub-device
 449        driver through the core::ioctl operation.
 450
 451
 452I2C sub-device drivers
 453----------------------
 454
 455Since these drivers are so common, special helper functions are available to
 456ease the use of these drivers (v4l2-common.h).
 457
 458The recommended method of adding v4l2_subdev support to an I2C driver is to
 459embed the v4l2_subdev struct into the state struct that is created for each
 460I2C device instance. Very simple devices have no state struct and in that case
 461you can just create a v4l2_subdev directly.
 462
 463A typical state struct would look like this (where 'chipname' is replaced by
 464the name of the chip):
 465
 466struct chipname_state {
 467        struct v4l2_subdev sd;
 468        ...  /* additional state fields */
 469};
 470
 471Initialize the v4l2_subdev struct as follows:
 472
 473        v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
 474
 475This function will fill in all the fields of v4l2_subdev and ensure that the
 476v4l2_subdev and i2c_client both point to one another.
 477
 478You should also add a helper inline function to go from a v4l2_subdev pointer
 479to a chipname_state struct:
 480
 481static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
 482{
 483        return container_of(sd, struct chipname_state, sd);
 484}
 485
 486Use this to go from the v4l2_subdev struct to the i2c_client struct:
 487
 488        struct i2c_client *client = v4l2_get_subdevdata(sd);
 489
 490And this to go from an i2c_client to a v4l2_subdev struct:
 491
 492        struct v4l2_subdev *sd = i2c_get_clientdata(client);
 493
 494Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
 495is called. This will unregister the sub-device from the bridge driver. It is
 496safe to call this even if the sub-device was never registered.
 497
 498You need to do this because when the bridge driver destroys the i2c adapter
 499the remove() callbacks are called of the i2c devices on that adapter.
 500After that the corresponding v4l2_subdev structures are invalid, so they
 501have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
 502from the remove() callback ensures that this is always done correctly.
 503
 504
 505The bridge driver also has some helper functions it can use:
 506
 507struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
 508               "module_foo", "chipid", 0x36, NULL);
 509
 510This loads the given module (can be NULL if no module needs to be loaded) and
 511calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
 512If all goes well, then it registers the subdev with the v4l2_device.
 513
 514You can also use the last argument of v4l2_i2c_new_subdev() to pass an array
 515of possible I2C addresses that it should probe. These probe addresses are
 516only used if the previous argument is 0. A non-zero argument means that you
 517know the exact i2c address so in that case no probing will take place.
 518
 519Both functions return NULL if something went wrong.
 520
 521Note that the chipid you pass to v4l2_i2c_new_subdev() is usually
 522the same as the module name. It allows you to specify a chip variant, e.g.
 523"saa7114" or "saa7115". In general though the i2c driver autodetects this.
 524The use of chipid is something that needs to be looked at more closely at a
 525later date. It differs between i2c drivers and as such can be confusing.
 526To see which chip variants are supported you can look in the i2c driver code
 527for the i2c_device_id table. This lists all the possibilities.
 528
 529There are two more helper functions:
 530
 531v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data
 532arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not
 5330 then that will be used (non-probing variant), otherwise the probed_addrs
 534are probed.
 535
 536For example: this will probe for address 0x10:
 537
 538struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter,
 539               "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10));
 540
 541v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed
 542to the i2c driver and replaces the irq, platform_data and addr arguments.
 543
 544If the subdev supports the s_config core ops, then that op is called with
 545the irq and platform_data arguments after the subdev was setup. The older
 546v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with
 547irq set to 0 and platform_data set to NULL.
 548
 549struct video_device
 550-------------------
 551
 552The actual device nodes in the /dev directory are created using the
 553video_device struct (v4l2-dev.h). This struct can either be allocated
 554dynamically or embedded in a larger struct.
 555
 556To allocate it dynamically use:
 557
 558        struct video_device *vdev = video_device_alloc();
 559
 560        if (vdev == NULL)
 561                return -ENOMEM;
 562
 563        vdev->release = video_device_release;
 564
 565If you embed it in a larger struct, then you must set the release()
 566callback to your own function:
 567
 568        struct video_device *vdev = &my_vdev->vdev;
 569
 570        vdev->release = my_vdev_release;
 571
 572The release callback must be set and it is called when the last user
 573of the video device exits.
 574
 575The default video_device_release() callback just calls kfree to free the
 576allocated memory.
 577
 578You should also set these fields:
 579
 580- v4l2_dev: set to the v4l2_device parent device.
 581
 582- name: set to something descriptive and unique.
 583
 584- vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0,
 585  so this is normally already the default), set to VFL_DIR_TX for output
 586  devices and VFL_DIR_M2M for mem2mem (codec) devices.
 587
 588- fops: set to the v4l2_file_operations struct.
 589
 590- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
 591  (highly recommended to use this and it might become compulsory in the
 592  future!), then set this to your v4l2_ioctl_ops struct. The vfl_type and
 593  vfl_dir fields are used to disable ops that do not match the type/dir
 594  combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops
 595  are disabled for a capture device. This makes it possible to provide
 596  just one v4l2_ioctl_ops struct for both vbi and video nodes.
 597
 598- lock: leave to NULL if you want to do all the locking in the driver.
 599  Otherwise you give it a pointer to a struct mutex_lock and before the
 600  unlocked_ioctl file operation is called this lock will be taken by the
 601  core and released afterwards. See the next section for more details.
 602
 603- queue: a pointer to the struct vb2_queue associated with this device node.
 604  If queue is non-NULL, and queue->lock is non-NULL, then queue->lock is
 605  used for the queuing ioctls (VIDIOC_REQBUFS, CREATE_BUFS, QBUF, DQBUF,
 606  QUERYBUF, PREPARE_BUF, STREAMON and STREAMOFF) instead of the lock above.
 607  That way the vb2 queuing framework does not have to wait for other ioctls.
 608  This queue pointer is also used by the vb2 helper functions to check for
 609  queuing ownership (i.e. is the filehandle calling it allowed to do the
 610  operation).
 611
 612- prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
 613  If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
 614  If you want to have a separate priority state per (group of) device node(s),
 615  then you can point it to your own struct v4l2_prio_state.
 616
 617- parent: you only set this if v4l2_device was registered with NULL as
 618  the parent device struct. This only happens in cases where one hardware
 619  device has multiple PCI devices that all share the same v4l2_device core.
 620
 621  The cx88 driver is an example of this: one core v4l2_device struct, but
 622  it is used by both an raw video PCI device (cx8800) and a MPEG PCI device
 623  (cx8802). Since the v4l2_device cannot be associated with a particular
 624  PCI device it is setup without a parent device. But when the struct
 625  video_device is setup you do know which parent PCI device to use.
 626
 627- flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework
 628  handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct
 629  v4l2_fh. Eventually this flag will disappear once all drivers use the core
 630  priority handling. But for now it has to be set explicitly.
 631
 632If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
 633in your v4l2_file_operations struct.
 634
 635Do not use .ioctl! This is deprecated and will go away in the future.
 636
 637In some cases you want to tell the core that a function you had specified in
 638your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this
 639function before video_device_register is called:
 640
 641void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd);
 642
 643This tends to be needed if based on external factors (e.g. which card is
 644being used) you want to turns off certain features in v4l2_ioctl_ops without
 645having to make a new struct.
 646
 647The v4l2_file_operations struct is a subset of file_operations. The main
 648difference is that the inode argument is omitted since it is never used.
 649
 650If integration with the media framework is needed, you must initialize the
 651media_entity struct embedded in the video_device struct (entity field) by
 652calling media_entity_init():
 653
 654        struct media_pad *pad = &my_vdev->pad;
 655        int err;
 656
 657        err = media_entity_init(&vdev->entity, 1, pad, 0);
 658
 659The pads array must have been previously initialized. There is no need to
 660manually set the struct media_entity type and name fields.
 661
 662A reference to the entity will be automatically acquired/released when the
 663video device is opened/closed.
 664
 665ioctls and locking
 666------------------
 667
 668The V4L core provides optional locking services. The main service is the
 669lock field in struct video_device, which is a pointer to a mutex. If you set
 670this pointer, then that will be used by unlocked_ioctl to serialize all ioctls.
 671
 672If you are using the videobuf2 framework, then there is a second lock that you
 673can set: video_device->queue->lock. If set, then this lock will be used instead
 674of video_device->lock to serialize all queuing ioctls (see the previous section
 675for the full list of those ioctls).
 676
 677The advantage of using a different lock for the queuing ioctls is that for some
 678drivers (particularly USB drivers) certain commands such as setting controls
 679can take a long time, so you want to use a separate lock for the buffer queuing
 680ioctls. That way your VIDIOC_DQBUF doesn't stall because the driver is busy
 681changing the e.g. exposure of the webcam.
 682
 683Of course, you can always do all the locking yourself by leaving both lock
 684pointers at NULL.
 685
 686If you use the old videobuf then you must pass the video_device lock to the
 687videobuf queue initialize function: if videobuf has to wait for a frame to
 688arrive, then it will temporarily unlock the lock and relock it afterwards. If
 689your driver also waits in the code, then you should do the same to allow other
 690processes to access the device node while the first process is waiting for
 691something.
 692
 693In the case of videobuf2 you will need to implement the wait_prepare and
 694wait_finish callbacks to unlock/lock if applicable. If you use the queue->lock
 695pointer, then you can use the helper functions vb2_ops_wait_prepare/finish.
 696
 697The implementation of a hotplug disconnect should also take the lock from
 698video_device before calling v4l2_device_disconnect. If you are also using
 699video_device->queue->lock, then you have to first lock video_device->queue->lock
 700followed by video_device->lock. That way you can be sure no ioctl is running
 701when you call v4l2_device_disconnect.
 702
 703video_device registration
 704-------------------------
 705
 706Next you register the video device: this will create the character device
 707for you.
 708
 709        err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
 710        if (err) {
 711                video_device_release(vdev); /* or kfree(my_vdev); */
 712                return err;
 713        }
 714
 715If the v4l2_device parent device has a non-NULL mdev field, the video device
 716entity will be automatically registered with the media device.
 717
 718Which device is registered depends on the type argument. The following
 719types exist:
 720
 721VFL_TYPE_GRABBER: videoX for video input/output devices
 722VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
 723VFL_TYPE_RADIO: radioX for radio tuners
 724
 725The last argument gives you a certain amount of control over the device
 726device node number used (i.e. the X in videoX). Normally you will pass -1
 727to let the v4l2 framework pick the first free number. But sometimes users
 728want to select a specific node number. It is common that drivers allow
 729the user to select a specific device node number through a driver module
 730option. That number is then passed to this function and video_register_device
 731will attempt to select that device node number. If that number was already
 732in use, then the next free device node number will be selected and it
 733will send a warning to the kernel log.
 734
 735Another use-case is if a driver creates many devices. In that case it can
 736be useful to place different video devices in separate ranges. For example,
 737video capture devices start at 0, video output devices start at 16.
 738So you can use the last argument to specify a minimum device node number
 739and the v4l2 framework will try to pick the first free number that is equal
 740or higher to what you passed. If that fails, then it will just pick the
 741first free number.
 742
 743Since in this case you do not care about a warning about not being able
 744to select the specified device node number, you can call the function
 745video_register_device_no_warn() instead.
 746
 747Whenever a device node is created some attributes are also created for you.
 748If you look in /sys/class/video4linux you see the devices. Go into e.g.
 749video0 and you will see 'name' and 'index' attributes. The 'name' attribute
 750is the 'name' field of the video_device struct.
 751
 752The 'index' attribute is the index of the device node: for each call to
 753video_register_device() the index is just increased by 1. The first video
 754device node you register always starts with index 0.
 755
 756Users can setup udev rules that utilize the index attribute to make fancy
 757device names (e.g. 'mpegX' for MPEG video capture device nodes).
 758
 759After the device was successfully registered, then you can use these fields:
 760
 761- vfl_type: the device type passed to video_register_device.
 762- minor: the assigned device minor number.
 763- num: the device node number (i.e. the X in videoX).
 764- index: the device index number.
 765
 766If the registration failed, then you need to call video_device_release()
 767to free the allocated video_device struct, or free your own struct if the
 768video_device was embedded in it. The vdev->release() callback will never
 769be called if the registration failed, nor should you ever attempt to
 770unregister the device if the registration failed.
 771
 772
 773video_device cleanup
 774--------------------
 775
 776When the video device nodes have to be removed, either during the unload
 777of the driver or because the USB device was disconnected, then you should
 778unregister them:
 779
 780        video_unregister_device(vdev);
 781
 782This will remove the device nodes from sysfs (causing udev to remove them
 783from /dev).
 784
 785After video_unregister_device() returns no new opens can be done. However,
 786in the case of USB devices some application might still have one of these
 787device nodes open. So after the unregister all file operations (except
 788release, of course) will return an error as well.
 789
 790When the last user of the video device node exits, then the vdev->release()
 791callback is called and you can do the final cleanup there.
 792
 793Don't forget to cleanup the media entity associated with the video device if
 794it has been initialized:
 795
 796        media_entity_cleanup(&vdev->entity);
 797
 798This can be done from the release callback.
 799
 800
 801video_device helper functions
 802-----------------------------
 803
 804There are a few useful helper functions:
 805
 806- file/video_device private data
 807
 808You can set/get driver private data in the video_device struct using:
 809
 810void *video_get_drvdata(struct video_device *vdev);
 811void video_set_drvdata(struct video_device *vdev, void *data);
 812
 813Note that you can safely call video_set_drvdata() before calling
 814video_register_device().
 815
 816And this function:
 817
 818struct video_device *video_devdata(struct file *file);
 819
 820returns the video_device belonging to the file struct.
 821
 822The video_drvdata function combines video_get_drvdata with video_devdata:
 823
 824void *video_drvdata(struct file *file);
 825
 826You can go from a video_device struct to the v4l2_device struct using:
 827
 828struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
 829
 830- Device node name
 831
 832The video_device node kernel name can be retrieved using
 833
 834const char *video_device_node_name(struct video_device *vdev);
 835
 836The name is used as a hint by userspace tools such as udev. The function
 837should be used where possible instead of accessing the video_device::num and
 838video_device::minor fields.
 839
 840
 841video buffer helper functions
 842-----------------------------
 843
 844The v4l2 core API provides a set of standard methods (called "videobuf")
 845for dealing with video buffers. Those methods allow a driver to implement
 846read(), mmap() and overlay() in a consistent way.  There are currently
 847methods for using video buffers on devices that supports DMA with
 848scatter/gather method (videobuf-dma-sg), DMA with linear access
 849(videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
 850(videobuf-vmalloc).
 851
 852Please see Documentation/video4linux/videobuf for more information on how
 853to use the videobuf layer.
 854
 855struct v4l2_fh
 856--------------
 857
 858struct v4l2_fh provides a way to easily keep file handle specific data
 859that is used by the V4L2 framework. New drivers must use struct v4l2_fh
 860since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY)
 861if the video_device flag V4L2_FL_USE_FH_PRIO is also set.
 862
 863The users of v4l2_fh (in the V4L2 framework, not the driver) know
 864whether a driver uses v4l2_fh as its file->private_data pointer by
 865testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is
 866set whenever v4l2_fh_init() is called.
 867
 868struct v4l2_fh is allocated as a part of the driver's own file handle
 869structure and file->private_data is set to it in the driver's open
 870function by the driver.
 871
 872In many cases the struct v4l2_fh will be embedded in a larger structure.
 873In that case you should call v4l2_fh_init+v4l2_fh_add in open() and
 874v4l2_fh_del+v4l2_fh_exit in release().
 875
 876Drivers can extract their own file handle structure by using the container_of
 877macro. Example:
 878
 879struct my_fh {
 880        int blah;
 881        struct v4l2_fh fh;
 882};
 883
 884...
 885
 886int my_open(struct file *file)
 887{
 888        struct my_fh *my_fh;
 889        struct video_device *vfd;
 890        int ret;
 891
 892        ...
 893
 894        my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
 895
 896        ...
 897
 898        v4l2_fh_init(&my_fh->fh, vfd);
 899
 900        ...
 901
 902        file->private_data = &my_fh->fh;
 903        v4l2_fh_add(&my_fh->fh);
 904        return 0;
 905}
 906
 907int my_release(struct file *file)
 908{
 909        struct v4l2_fh *fh = file->private_data;
 910        struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
 911
 912        ...
 913        v4l2_fh_del(&my_fh->fh);
 914        v4l2_fh_exit(&my_fh->fh);
 915        kfree(my_fh);
 916        return 0;
 917}
 918
 919Below is a short description of the v4l2_fh functions used:
 920
 921void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
 922
 923  Initialise the file handle. This *MUST* be performed in the driver's
 924  v4l2_file_operations->open() handler.
 925
 926void v4l2_fh_add(struct v4l2_fh *fh)
 927
 928  Add a v4l2_fh to video_device file handle list. Must be called once the
 929  file handle is completely initialized.
 930
 931void v4l2_fh_del(struct v4l2_fh *fh)
 932
 933  Unassociate the file handle from video_device(). The file handle
 934  exit function may now be called.
 935
 936void v4l2_fh_exit(struct v4l2_fh *fh)
 937
 938  Uninitialise the file handle. After uninitialisation the v4l2_fh
 939  memory can be freed.
 940
 941
 942If struct v4l2_fh is not embedded, then you can use these helper functions:
 943
 944int v4l2_fh_open(struct file *filp)
 945
 946  This allocates a struct v4l2_fh, initializes it and adds it to the struct
 947  video_device associated with the file struct.
 948
 949int v4l2_fh_release(struct file *filp)
 950
 951  This deletes it from the struct video_device associated with the file
 952  struct, uninitialised the v4l2_fh and frees it.
 953
 954These two functions can be plugged into the v4l2_file_operation's open() and
 955release() ops.
 956
 957
 958Several drivers need to do something when the first file handle is opened and
 959when the last file handle closes. Two helper functions were added to check
 960whether the v4l2_fh struct is the only open filehandle of the associated
 961device node:
 962
 963int v4l2_fh_is_singular(struct v4l2_fh *fh)
 964
 965  Returns 1 if the file handle is the only open file handle, else 0.
 966
 967int v4l2_fh_is_singular_file(struct file *filp)
 968
 969  Same, but it calls v4l2_fh_is_singular with filp->private_data.
 970
 971
 972V4L2 events
 973-----------
 974
 975The V4L2 events provide a generic way to pass events to user space.
 976The driver must use v4l2_fh to be able to support V4L2 events.
 977
 978Events are defined by a type and an optional ID. The ID may refer to a V4L2
 979object such as a control ID. If unused, then the ID is 0.
 980
 981When the user subscribes to an event the driver will allocate a number of
 982kevent structs for that event. So every (type, ID) event tuple will have
 983its own set of kevent structs. This guarantees that if a driver is generating
 984lots of events of one type in a short time, then that will not overwrite
 985events of another type.
 986
 987But if you get more events of one type than the number of kevents that were
 988reserved, then the oldest event will be dropped and the new one added.
 989
 990Furthermore, the internal struct v4l2_subscribed_event has merge() and
 991replace() callbacks which drivers can set. These callbacks are called when
 992a new event is raised and there is no more room. The replace() callback
 993allows you to replace the payload of the old event with that of the new event,
 994merging any relevant data from the old payload into the new payload that
 995replaces it. It is called when this event type has only one kevent struct
 996allocated. The merge() callback allows you to merge the oldest event payload
 997into that of the second-oldest event payload. It is called when there are two
 998or more kevent structs allocated.
 999
1000This way no status information is lost, just the intermediate steps leading
1001up to that state.
1002
1003A good example of these replace/merge callbacks is in v4l2-event.c:
1004ctrls_replace() and ctrls_merge() callbacks for the control event.
1005
1006Note: these callbacks can be called from interrupt context, so they must be
1007fast.
1008
1009Useful functions:
1010
1011void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev)
1012
1013  Queue events to video device. The driver's only responsibility is to fill
1014  in the type and the data fields. The other fields will be filled in by
1015  V4L2.
1016
1017int v4l2_event_subscribe(struct v4l2_fh *fh,
1018                         struct v4l2_event_subscription *sub, unsigned elems,
1019                         const struct v4l2_subscribed_event_ops *ops)
1020
1021  The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
1022  is able to produce events with specified event id. Then it calls
1023  v4l2_event_subscribe() to subscribe the event.
1024
1025  The elems argument is the size of the event queue for this event. If it is 0,
1026  then the framework will fill in a default value (this depends on the event
1027  type).
1028
1029  The ops argument allows the driver to specify a number of callbacks:
1030  * add:     called when a new listener gets added (subscribing to the same
1031             event twice will only cause this callback to get called once)
1032  * del:     called when a listener stops listening
1033  * replace: replace event 'old' with event 'new'.
1034  * merge:   merge event 'old' into event 'new'.
1035  All 4 callbacks are optional, if you don't want to specify any callbacks
1036  the ops argument itself maybe NULL.
1037
1038int v4l2_event_unsubscribe(struct v4l2_fh *fh,
1039                           struct v4l2_event_subscription *sub)
1040
1041  vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
1042  v4l2_event_unsubscribe() directly unless it wants to be involved in
1043  unsubscription process.
1044
1045  The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The
1046  drivers may want to handle this in a special way.
1047
1048int v4l2_event_pending(struct v4l2_fh *fh)
1049
1050  Returns the number of pending events. Useful when implementing poll.
1051
1052Events are delivered to user space through the poll system call. The driver
1053can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait().
1054
1055There are standard and private events. New standard events must use the
1056smallest available event type. The drivers must allocate their events from
1057their own class starting from class base. Class base is
1058V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number.
1059The first event type in the class is reserved for future use, so the first
1060available event type is 'class base + 1'.
1061
1062An example on how the V4L2 events may be used can be found in the OMAP
10633 ISP driver (drivers/media/platform/omap3isp).
1064
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