乙個硬體裝置可能包含多個子裝置，比如乙個電視卡除了有capture裝置，可能還有vbi裝置或者fm tunner。而v4l2_device就是所有這些裝置的根節點，負責管理所有的子裝置，可將該抽象為soc的資料採集器，如csi、mipi、isp等soc的控制器。
/**
* struct v4l2_device - main struct to for v4l2 device drivers
* * @dev: pointer to struct device.
* @mdev: pointer to struct media_device, may be null.
* @subdevs: used to keep track of the registered subdevs
* @lock: lock this struct; can be used by the driver as well
* if this struct is embedded into a larger struct.
* @name: unique device name, by default the driver name + bus id
* @notify: notify operation called by some sub-devices.
* @ctrl_handler: the control handler. may be %null.
* @prio: device's priority state
* @ref: keep track of the references to this struct.
* @release: release function that is called when the ref count
* goes to 0.
* * each instance of a v4l2 device should create the v4l2_device struct,
* either stand-alone or embedded in a larger struct.
* * it allows easy access to sub-devices (see v4l2-subdev.h) and provides
* basic v4l2 device-level support.
* * .. note::
* * #) @dev->driver_data points to this struct.
* #) @dev might be %null if there is no parent device
*/struct v4l2_device ;
int v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev)
/* set name to driver name + device name if it is empty. */
if (!v4l2_dev->name[0])
snprintf(v4l2_dev->name, sizeof(v4l2_dev->name), "%s %s",
dev->driver->name, dev_name(dev));
if (!dev_get_drvdata(dev))
dev_set_drvdata(dev, v4l2_dev);
return 0;
}export_symbol_gpl(v4l2_device_register);
static void v4l2_device_release(struct kref *ref)
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