V4L2 device instance
--------------------

Each device instance is represented by a struct :c:type:`v4l2_device`.
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 by calling:

	:c:func:`v4l2_device_register <v4l2_device_register>`
	(dev, :c:type:`v4l2_dev <v4l2_device>`).

Registration will initialize the :c:type:`v4l2_device` struct. If the
dev->driver_data field is ``NULL``, it will be linked to
:c:type:`v4l2_dev <v4l2_device>` argument.

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 :c:type:`v4l2_device` instance. This is achieved by a
``dev_set_drvdata()`` call before registering the V4L2 device instance.
They must also set the struct :c:type:`v4l2_device` mdev field to point to a
properly initialized and registered :c:type:`media_device` instance.

If :c:type:`v4l2_dev <v4l2_device>`\ ->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 :c:func:`v4l2_device_register` then it will
be untouched. If dev is ``NULL``, then you **must** setup
:c:type:`v4l2_dev <v4l2_device>`\ ->name before calling
:c:func:`v4l2_device_register`.

You can use :c:func:`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
:c:type:`v4l2_dev <v4l2_device>` 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``.

V4L2 devices are unregistered by calling:

	:c:func:`v4l2_device_unregister`
	(:c:type:`v4l2_dev <v4l2_device>`).

If the dev->driver_data field points to :c:type:`v4l2_dev <v4l2_device>`,
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 :c:type:`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:

	:c:func:`v4l2_device_disconnect`
	(:c:type:`v4l2_dev <v4l2_device>`).

This does *not* unregister the subdevs, so you still need to call the
:c:func:`v4l2_device_unregister` function for that. If your driver is not
hotpluggable, then there is no need to call :c:func:`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:

.. code-block:: c

	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:

.. code-block:: c

	static atomic_t drv_instance = ATOMIC_INIT(0);

	static int 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 :c:type:`v4l2_device` for hotpluggable devices. For this
purpose :c:type:`v4l2_device` has refcounting support. The refcount is
increased whenever :c:func:`video_register_device` is called and it is
decreased whenever that device node is released. When the refcount reaches
zero, then the :c:type:`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:

	:c:func:`v4l2_device_get`
	(:c:type:`v4l2_dev <v4l2_device>`).

or:

	:c:func:`v4l2_device_put`
	(:c:type:`v4l2_dev <v4l2_device>`).

Since the initial refcount is 1 you also need to call
:c:func:`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.

v4l2_device functions and data structures
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. kernel-doc:: include/media/v4l2-device.h