videobuf2-core.h中的vb2_buffer，记录了v4l2_buffer ，驱动可以对vb2_buffer的v4l2_buffer进行操控，
vb2_buffer是v4l2框架层的代码，而v4l2_buffer也是用户空间的应用程序，可以读取的一个数据结构。
videobuf2-core.h：

 
 /**
 * struct vb2_buffer - represents a video buffer
  * @v4l2_buf: struct v4l2_buffer associated with this buffer; can
 * be read by the driver and relevant entries can be
 * changed by the driver in case of CAPTURE types
 * (such as timestamp)
 * @v4l2_planes: struct v4l2_planes associated with this buffer; can


 * (such as bytesused); NOTE that even for single-planar
 * types,the v4l2_planes[0] struct should be used
 * instead of v4l2_buf for filling bytesused - drivers
 * should use the vb2_set_plane_payload() function for that
  * @vb2_queue: the queue to which this driver belongs
 * @num_planes: number of planes in the buffer
 * on an internal driver queue
 * @state: current buffer state; do not change  * @queued_entry: entry on the queued buffers list,which holds all
  * buffers queued from userspace
  * @done_entry: entry on the list that stores all buffers ready to
  * be dequeued to userspace
 * @planes: private per-plane information; do not change
 */
 struct vb2_buffer {
         struct v4l2_buffer      v4l2_buf;
         struct v4l2_plane       v4l2_planes[VIDEO_MAX_PLANES];
 
         struct vb2_queue        *vb2_queue;
 
         unsigned int            num_planes;
 
/* Private: internal use only */
         enum vb2_buffer_state   state;
         struct list_head        queued_entry;
         struct list_head        done_entry;
 
         struct vb2_plane        planes[];
 };

videodev2.h，用户应用程序与驱动交互：

 * struct v4l2_buffer - video buffer info
 * @index: id number of the buffer
 * @type: enum v4l2_buf_type; buffer type (type == *_MPLANE for
 * multiplanar buffers);  * @bytesused: number of bytes occupied by data in the buffer (payload);
  * unused (set to 0) for multiplanar buffers
 * @flags: buffer informational flags
 * @field: enum v4l2_field; field order of the image in the buffer
 * @timestamp: frame timestamp
 * @timecode: frame timecode
 * @sequence: sequence count of this frame
 * @memory: enum v4l2_memory; the method,in which the actual video data is
 * passed
@L_403_66@ * @offset: for non-multiplanar buffers with memory == V4L2_MEMORY_MMAP;
 * offset from the start of the device memory for this plane,
 * (or a "cookie" that should be passed to mmap() as offset)
 * @userptr: for non-multiplanar buffers with memory == V4L2_MEMORY_USERPTR;
 * a userspace pointer pointing to this buffer
 * @fd: for non-multiplanar buffers with memory == V4L2_MEMORY_DMABUF;
 * a userspace file descriptor associated with this buffer
 * @planes: for multiplanar buffers; userspace pointer to the array of plane
 * info structs for this buffer
 * @length: size in bytes of the buffer (NOT its payload) for single-plane
 * buffers (when type != *_MPLANE); number of elements in the
 * planes array for multi-plane buffers
 * @input: input number from which the video data has has been captured
 *
 * Contains data exchanged by application and driver using one of the Streaming
 * I/O methods.

 struct v4l2_buffer {
@L_502_85@         __u32                   index;
                   type;
                   bytesused;
                   flags;
                   field;
         struct timeval          timestamp;
         struct v4l2_timecode    timecode;
                   sequence;
 
         /* memory location */
                   memory;
         union {
                            offset;
                 unsigned long   userptr;
                 struct  *planes;
                 __s32           fd;
         } m;
                   length;
                   reserved2;
                   reserved;
 };

vb2_buf维护了两个线性链表，可以参考 http://blog.sina.com.cn/s/blog_602f87700101bmvu.html