首先附上，Linux内核网站：
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v4l2非常棒的一篇文章
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内核代码查看：
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一个博客文档：
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https://lwn.net/Articles/204545/

别人整理

关于linux内核头函数：
/usr/src/xxx

系列文

在其内部有media文件夹，里面含有v4l2有关的内核函数。

我们的设别驱动都是要依赖这些内核函数，但是平时这些内核函数是不存在的，只有你把设备挂载后才会出现：

如果你要实现一个v4l2驱动，只要简单进行对下属函数重写就可以了：

struct v4l2_ioctl_ops {/* ioctl callbacks *//* VIDIOC_QUERYCAP handler */int (*vidioc_querycap)(struct file *file, void *fh,struct v4l2_capability *cap);/* VIDIOC_ENUM_FMT handlers */int (*vidioc_enum_fmt_vid_cap)(struct file *file, void *fh,struct v4l2_fmtdesc *f);int (*vidioc_enum_fmt_vid_overlay)(struct file *file, void *fh,struct v4l2_fmtdesc *f);int (*vidioc_enum_fmt_vid_out)(struct file *file, void *fh,struct v4l2_fmtdesc *f);int (*vidioc_enum_fmt_vid_cap_mplane)(struct file *file, void *fh,struct v4l2_fmtdesc *f);};static const struct v4l2_file_operations mvx_v4l2_fops = {.owner          = THIS_MODULE,.open           = mvx_v4l2_open,.release        = mvx_v4l2_release,.poll           = mvx_v4l2_poll,.unlocked_ioctl = video_ioctl2,.mmap           = mvx_v4l2_mmap
};static const struct v4l2_ioctl_ops mvx_v4l2_ioctl_ops = {.vidioc_querycap                = mvx_v4l2_vidioc_querycap,.vidioc_enum_fmt_vid_cap        = mvx_v4l2_vidioc_enum_fmt_vid_cap,.vidioc_enum_fmt_vid_out        = mvx_v4l2_vidioc_enum_fmt_vid_out,.vidioc_enum_fmt_vid_cap_mplane = mvx_v4l2_vidioc_enum_fmt_vid_cap,.vidioc_enum_fmt_vid_out_mplane = mvx_v4l2_vidioc_enum_fmt_vid_out,.vidioc_enum_framesizes         = mvx_v4l2_vidioc_enum_framesizes,.vidioc_g_fmt_vid_cap           = mvx_v4l2_vidioc_g_fmt_vid_cap,.vidioc_g_fmt_vid_cap_mplane    = mvx_v4l2_vidioc_g_fmt_vid_cap,.vidioc_g_fmt_vid_out           = mvx_v4l2_vidioc_g_fmt_vid_out,.vidioc_g_fmt_vid_out_mplane    = mvx_v4l2_vidioc_g_fmt_vid_out,.vidioc_s_fmt_vid_cap           = mvx_v4l2_vidioc_s_fmt_vid_cap,.vidioc_s_fmt_vid_cap_mplane    = mvx_v4l2_vidioc_s_fmt_vid_cap,.vidioc_s_fmt_vid_out           = mvx_v4l2_vidioc_s_fmt_vid_out,.vidioc_s_fmt_vid_out_mplane    = mvx_v4l2_vidioc_s_fmt_vid_out,.vidioc_try_fmt_vid_cap         = mvx_v4l2_vidioc_try_fmt_vid_cap,.vidioc_try_fmt_vid_cap_mplane  = mvx_v4l2_vidioc_try_fmt_vid_cap,.vidioc_try_fmt_vid_out         = mvx_v4l2_vidioc_try_fmt_vid_out,.vidioc_try_fmt_vid_out_mplane  = mvx_v4l2_vidioc_try_fmt_vid_out,.vidioc_g_crop                  = mvx_v4l2_vidioc_g_crop,.vidioc_streamon                = mvx_v4l2_vidioc_streamon,.vidioc_streamoff               = mvx_v4l2_vidioc_streamoff,.vidioc_encoder_cmd             = mvx_v4l2_vidioc_encoder_cmd,.vidioc_try_encoder_cmd         = mvx_v4l2_vidioc_try_encoder_cmd,.vidioc_decoder_cmd             = mvx_v4l2_vidioc_decoder_cmd,.vidioc_try_decoder_cmd         = mvx_v4l2_vidioc_try_decoder_cmd,.vidioc_reqbufs                 = mvx_v4l2_vidioc_reqbufs,.vidioc_create_bufs             = mvx_v4l2_vidioc_create_bufs,.vidioc_querybuf                = mvx_v4l2_vidioc_querybuf,.vidioc_qbuf                    = mvx_v4l2_vidioc_qbuf,.vidioc_dqbuf                   = mvx_v4l2_vidioc_dqbuf,.vidioc_subscribe_event         = mvx_v4l2_vidioc_subscribe_event,.vidioc_unsubscribe_event       = v4l2_event_unsubscribe,.vidioc_default                 = mvx_v4l2_vidioc_default
};

1、在Linux/videodev2.h中定义了枚举类型和关键结构体，这些都是对app用户而言的：

{V4L2_FIELD_ANY           = 0, /* driver can choose from none,top, bottom, interlaced depending on whatever it thinksis approximate ... */V4L2_FIELD_NONE          = 1, /* this device has no fields ... */V4L2_FIELD_TOP           = 2, /* top field only */V4L2_FIELD_BOTTOM        = 3, /* bottom field only */V4L2_FIELD_INTERLACED    = 4, /* both fields interlaced */V4L2_FIELD_SEQ_TB        = 5, /* both fields sequential into one buffer, top-bottom order */V4L2_FIELD_SEQ_BT        = 6, /* same as above + bottom-top order */V4L2_FIELD_ALTERNATE     = 7, /* both fields alternating into separate buffers */V4L2_FIELD_INTERLACED_TB = 8, /* both fields interlaced, top field first and the top field is transmitted first */V4L2_FIELD_INTERLACED_BT = 9, /* both fields interlaced, top field  first and the bottom field is transmitted first */
};enum v4l2_buf_type {V4L2_BUF_TYPE_VIDEO_CAPTURE        = 1,V4L2_BUF_TYPE_VIDEO_OUTPUT         = 2,V4L2_BUF_TYPE_VIDEO_OVERLAY        = 3,V4L2_BUF_TYPE_VBI_CAPTURE          = 4,V4L2_BUF_TYPE_VBI_OUTPUT           = 5,V4L2_BUF_TYPE_SLICED_VBI_CAPTURE   = 6,V4L2_BUF_TYPE_SLICED_VBI_OUTPUT    = 7,V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY = 8,V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE = 9,V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE  = 10,V4L2_BUF_TYPE_SDR_CAPTURE          = 11,V4L2_BUF_TYPE_SDR_OUTPUT           = 12,V4L2_BUF_TYPE_META_CAPTURE         = 13,V4L2_BUF_TYPE_META_OUTPUT	   = 14,/* Deprecated, do not use */V4L2_BUF_TYPE_PRIVATE              = 0x80,
};enum v4l2_memory {V4L2_MEMORY_MMAP             = 1,V4L2_MEMORY_USERPTR          = 2,V4L2_MEMORY_OVERLAY          = 3,V4L2_MEMORY_DMABUF           = 4,
};struct v4l2_capability {__u8	driver[16];__u8	card[32];__u8	bus_info[32];__u32   version;__u32	capabilities;__u32	device_caps;__u32	reserved[3];
};
struct v4l2_pix_format {__u32			width;__u32			height;__u32			pixelformat;__u32			field;		/* enum v4l2_field */__u32			bytesperline;	/* for padding, zero if unused */__u32			sizeimage;__u32			colorspace;	/* enum v4l2_colorspace */__u32			priv;		/* private data, depends on pixelformat */__u32			flags;		/* format flags (V4L2_PIX_FMT_FLAG_*) */union {/* enum v4l2_ycbcr_encoding */__u32			ycbcr_enc;/* enum v4l2_hsv_encoding */__u32			hsv_enc;};__u32			quantization;	/* enum v4l2_quantization */__u32			xfer_func;	/* enum v4l2_xfer_func */
};
/* RGB formats */
#define V4L2_PIX_FMT_RGB332  v4l2_fourcc('R', 'G', 'B', '1') /*  8  RGB-3-3-2     */
#define V4L2_PIX_FMT_RGB444  v4l2_fourcc('R', '4', '4', '4') /* 16  xxxxrrrr ggggbbbb */
#define V4L2_PIX_FMT_ARGB444 v4l2_fourcc('A', 'R', '1', '2') /* 16  aaaarrrr ggggbbbb */
#define V4L2_PIX_FMT_XRGB444 v4l2_fourcc('X', 'R', '1', '2') /* 16  xxxxrrrr ggggbbbb */
#define V4L2_PIX_FMT_RGBA444 v4l2_fourcc('R', 'A', '1', '2') /* 16  rrrrgggg bbbbaaaa */
#define V4L2_PIX_FMT_RGBX444 v4l2_fourcc('R', 'X', '1', '2') /* 16  rrrrgggg bbbbxxxx */
#define V4L2_PIX_FMT_ABGR444 v4l2_fourcc('A', 'B', '1', '2') /* 16  aaaabbbb ggggrrrr */
#define V4L2_PIX_FMT_XBGR444 v4l2_fourcc('X', 'B', '1', '2') /* 16  xxxxbbbb ggggrrrr */struct v4l2_fmtdesc {__u32		    index;             /* Format number      */__u32		    type;              /* enum v4l2_buf_type */__u32               flags;__u8		    description[32];   /* Description string */__u32		    pixelformat;       /* Format fourcc      */__u32		    reserved[4];
};
struct v4l2_plane {__u32			bytesused;__u32			length;union {__u32		mem_offset;unsigned long	userptr;__s32		fd;} m;__u32			data_offset;__u32			reserved[11];
};
struct v4l2_buffer {__u32			index;__u32			type;__u32			bytesused;__u32			flags;__u32			field;struct timeval		timestamp;struct v4l2_timecode	timecode;__u32			sequence;/* memory location */__u32			memory;union {__u32           offset;unsigned long   userptr;struct v4l2_plane *planes;__s32		fd;} m;__u32			length;__u32			reserved2;union {__s32		request_fd;__u32		reserved;};
};
struct v4l2_output {__u32	     index;		/*  Which output */__u8	     name[32];		/*  Label */__u32	     type;		/*  Type of output */__u32	     audioset;		/*  Associated audios (bitfield) */__u32	     modulator;         /*  Associated modulator */v4l2_std_id  std;__u32	     capabilities;__u32	     reserved[3];
};#define VIDIOC_QUERYCAP		 _IOR('V',  0, struct v4l2_capability)
#define VIDIOC_ENUM_FMT         _IOWR('V',  2, struct v4l2_fmtdesc)
#define VIDIOC_G_FMT		_IOWR('V',  4, struct v4l2_format)
#define VIDIOC_S_FMT		_IOWR('V',  5, struct v4l2_format)
#define VIDIOC_REQBUFS		_IOWR('V',  8, struct v4l2_requestbuffers)
#define VIDIOC_QUERYBUF		_IOWR('V',  9, struct v4l2_buffer)
#define VIDIOC_G_FBUF		 _IOR('V', 10, struct v4l2_framebuffer)