Linux 下UVC&V4L2技术简介(三)

先抛开V4L2的架构以及为用户提供的操作接口不提。我们在了解一个设备怎么塞入一个框架的时候，我们其实最关键的就是看到代码以及结构体的描述。下面我们就来看一下这个摘自内核的描述：
文件取自linux-xlnx-xilinx-v2017.4\include\media文件夹下，大部分我们用的结构体都在v4l2-device.h和v4l2-dev.h中有描述。

/**
 * struct video_device - Structure used to create and manage the V4L2 device
 *	nodes.
 *
 * @entity: &struct media_entity
 * @intf_devnode: pointer to &struct media_intf_devnode
 * @pipe: &struct media_pipeline
 * @fops: pointer to &struct v4l2_file_operations for the video device
 * @device_caps: device capabilities as used in v4l2_capabilities
 * @dev: &struct device for the video device
 * @cdev: character device
 * @v4l2_dev: pointer to &struct v4l2_device parent
 * @dev_parent: pointer to &struct device parent
 * @ctrl_handler: Control handler associated with this device node.
 *	 May be NULL.
 * @queue: &struct vb2_queue associated with this device node. May be NULL.
 * @prio: pointer to &struct v4l2_prio_state with device's Priority state.
 *	 If NULL, then v4l2_dev->prio will be used.
 * @name: video device name
 * @vfl_type: V4L device type
 * @vfl_dir: V4L receiver, transmitter or m2m
 * @minor: device node 'minor'. It is set to -1 if the registration failed
 * @num: number of the video device node
 * @flags: video device flags. Use bitops to set/clear/test flags
 * @index: attribute to differentiate multiple indices on one physical device
 * @fh_lock: Lock for all v4l2_fhs
 * @fh_list: List of &struct v4l2_fh
 * @dev_debug: Internal device debug flags, not for use by drivers
 * @tvnorms: Supported tv norms
 *
 * @release: video device release() callback
 * @ioctl_ops: pointer to &struct v4l2_ioctl_ops with ioctl callbacks
 *
 * @valid_ioctls: bitmap with the valid ioctls for this device
 * @disable_locking: bitmap with the ioctls that don't require locking
 * @lock: pointer to &struct mutex serialization lock
 *
 * .. note::
 *	Only set @dev_parent if that can't be deduced from @v4l2_dev.
 */

struct video_device
{
#if defined(CONFIG_MEDIA_CONTROLLER)
	struct media_entity entity;
	struct media_intf_devnode *intf_devnode;
	struct media_pipeline pipe;
#endif
	const struct v4l2_file_operations *fops;

	u32 device_caps;

	/* sysfs */
	struct device dev;
	struct cdev *cdev;

	struct v4l2_device *v4l2_dev;
	struct device *dev_parent;

	struct v4l2_ctrl_handler *ctrl_handler;

	struct vb2_queue *queue;

	struct v4l2_prio_state *prio;

	/* device info */
	char name[32];
	int vfl_type;
	int vfl_dir;
	int minor;
	u16 num;
	unsigned long flags;
	int index;

	/* V4L2 file handles */
	spinlock_t		fh_lock;
	struct list_head	fh_list;

	int dev_debug;

	v4l2_std_id tvnorms;

	/* callbacks */
	void (*release)(struct video_device *vdev);
	const struct v4l2_ioctl_ops *ioctl_ops;
	DECLARE_BITMAP(valid_ioctls, BASE_VIDIOC_PRIVATE);

	DECLARE_BITMAP(disable_locking, BASE_VIDIOC_PRIVATE);
	struct mutex *lock;
};

#define media_entity_to_video_device(__e) \
	container_of(__e, struct video_device, entity)
/* dev to video-device */
#define to_video_device(cd) container_of(cd, struct video_device, dev)

/**
 * __video_register_device - register video4linux devices
 *
 * @vdev: struct video_device to register
 * @type: type of device to register
 * @nr:   which device node number is desired:
 * 	(0 == /dev/video0, 1 == /dev/video1, ..., -1 == first free)
 * @warn_if_nr_in_use: warn if the desired device node number
 *        was already in use and another number was chosen instead.
 * @owner: module that owns the video device node
 *
 * The registration code assigns minor numbers and device node numbers
 * based on the requested type and registers the new device node with
 * the kernel.
 *
 * This function assumes that struct video_device was zeroed when it
 * was allocated and does not contain any stale date.
 *
 * An error is returned if no free minor or device node number could be
 * found, or if the registration of the device node failed.
 *
 * Returns 0 on success.
 *
 * Valid values for @type are:
 *
 *	- %VFL_TYPE_GRABBER - A frame grabber
 *	- %VFL_TYPE_VBI - Vertical blank data (undecoded)
 *	- %VFL_TYPE_RADIO - A radio card
 *	- %VFL_TYPE_SUBDEV - A subdevice
 *	- %VFL_TYPE_SDR - Software Defined Radio
 *	- %VFL_TYPE_TOUCH - A touch sensor
 *
 * .. note::
 *
 *	This function is meant to be used only inside the V4L2 core.
 *	Drivers should use video_register_device() or
 *	video_register_device_no_warn().
 */
int __must_check __video_register_device(struct video_device *vdev, int type,
		int nr, int warn_if_nr_in_use, struct module *owner);

/**
 *  video_register_device - register video4linux devices
 *
 * @vdev: struct video_device to register
 * @type: type of device to register
 * @nr:   which device node number is desired:
 * 	(0 == /dev/video0, 1 == /dev/video1, ..., -1 == first free)
 *
 * Internally, it calls __video_register_device(). Please see its
 * documentation for more details.
 *
 * .. note::
 * 	if video_register_device fails, the release() callback of
 *	&struct video_device structure is *not* called, so the caller
 *	is responsible for freeing any data. Usually that means that
 *	you video_device_release() should be called on failure.
 */
static inline int __must_check video_register_device(struct video_device *vdev,
		int type, int nr)
{
	return __video_register_device(vdev, type, nr, 1, vdev->fops->owner);
}

/**
 *  video_register_device_no_warn - register video4linux devices
 *
 * @vdev: struct video_device to register
 * @type: type of device to register
 * @nr:   which device node number is desired:
 * 	(0 == /dev/video0, 1 == /dev/video1, ..., -1 == first free)
 *
 * This function is identical to video_register_device() except that no
 * warning is issued if the desired device node number was already in use.
 *
 * Internally, it calls __video_register_device(). Please see its
 * documentation for more details.
 *
 * .. note::
 * 	if video_register_device fails, the release() callback of
 *	&struct video_device structure is *not* called, so the caller
 *	is responsible for freeing any data. Usually that means that
 *	you video_device_release() should be called on failure.
 */
static inline int __must_check video_register_device_no_warn(
		struct video_device *vdev, int type, int nr)
{
	return __video_register_device(vdev, type, nr, 0, vdev->fops->owner);
}