先拋開V4L2的架構以及爲用戶提供的操作接口不提。我們在瞭解一個設備怎麼塞入一個框架的時候，我們其實最關鍵的就是看到代碼以及結構體的描述。下面我們就來看一下這個摘自內核的描述：
文件取自linux-xlnx-xilinx-v2017.4\include\media文件夾下，大部分我們用的結構體都在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);
}