如果你有多个设备节点，则很难知道何时可以安全地为可热插拔设备注销v4l2_device。为此，v4l2_device具有引用计数支持。每次调用video_register_device时增加计数，释放该设备节点时减小计数。当引用计数达到零时，则会调用v4l2_device_release回调。你可以在那里进行最终清理。如果创建了其他设备节点，则也可以通过调用以下方式手动增加和减少引用计数：v4l2_device_get。或：v4l2_device_put。由于初始引用计数为因此还需要在disconnect回调或remove回调中调用v4l2_device_put，否则引用计数永远不会达到0。1v4l2_devicefunctionsanddatastructuresstructv4l2_device是V4L2设备驱动程序的主要结构体。定义：
struct v4l2_device {
struct device *dev;
struct media_device *mdev;
struct list_head subdevs;
spinlock_t lock;
char name[V4L2_DEVICE_NAME_SIZE];
void (*notify)(struct v4l2_subdev *sd, unsigned int notification, void *arg);
struct v4l2_ctrl_handler *ctrl_handler;
struct v4l2_prio_state prio;
struct kref ref;
void (*release)(struct v4l2_device *v4l2_dev);
};static atomic_t drv_instance = ATOMIC_INIT(0);
…
instance = v4l2_device_set_name(&v4l2_dev, “foo”, &drv_instance);
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