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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, /dev/radioX and
+   /dev/vtxX) and keeping track of device-node specific data.
+
+4) Filehandle-specific structs containing per-filehandle data.
+
+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).
+
+
+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 and link dev->driver_data
+to v4l2_dev. Registration will also set v4l2_dev->name to a value derived from
+dev (driver name followed by the bus_id, to be precise). You may change the
+name after registration if you want.
+
+You unregister with:
+
+        v4l2_device_unregister(struct v4l2_device *v4l2_dev);
+
+Unregistering will also automatically unregister all subdevs from the device.
+
+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 *dev,
+                                const struct pci_device_id *pci_id)
+{
+        ...
+        state->instance = atomic_inc_return(&drv_instance) - 1;
+}
+
+
+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.
+
+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_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(subdev, &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.
+
+A device (bridge) driver needs to register the v4l2_subdev with the
+v4l2_device:
+
+        int err = v4l2_device_register_subdev(device, subdev);
+
+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.
+
+You can unregister a sub-device using:
+
+        v4l2_device_unregister_subdev(subdev);
+
+Afterwards the subdev module can be unloaded and subdev->dev == NULL.
+
+You can call an ops function either directly:
+
+        err = subdev->ops->core->g_chip_ident(subdev, &chip);
+
+but it is better and easier to use this macro:
+
+        err = v4l2_subdev_call(subdev, 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(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(dev, 0, core, g_chip_ident, &chip);
+
+Any error except -ENOIOCTLCMD will exit the loop with that error. If no
+errors (except -ENOIOCTLCMD) occured, 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 subdev->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.
+
+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.
+
+
+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);
+
+Finally you need to make a command function to make driver->command()
+call the right subdev_ops functions:
+
+static int subdev_command(struct i2c_client *client, unsigned cmd, void *arg)
+{
+        return v4l2_subdev_command(i2c_get_clientdata(client), cmd, arg);
+}
+
+If driver->command is never used then you can leave this out. Eventually the
+driver->command usage should be removed from v4l.
+
+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.
+
+
+The bridge driver also has some helper functions it can use:
+
+struct v4l2_subdev *sd = v4l2_i2c_new_subdev(adapter, "module_foo", "chipid", 0x36);
+
+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. It gets
+the v4l2_device by calling i2c_get_adapdata(adapter), so you should make sure
+that adapdata is set to v4l2_device when you setup the i2c_adapter in your
+driver.
+
+You can also use v4l2_i2c_new_probed_subdev() which is very similar to
+v4l2_i2c_new_subdev(), except that it has an array of possible I2C addresses
+that it should probe. Internally it calls i2c_new_probed_device().
+
+Both functions return NULL if something went wrong.
+
+
+struct video_device
+-------------------
+
+Not yet documented.