https://docs.kernel.org/userspace-api/media/index.html
Video for Linux API
7.1 V4L2 close()
7.2 V4L2 ioctl()
支持的 ioctls 定义在 videodev2.h 中。
7.3 VIDIOC_CREATE_BUFS
Create buffers for Memory Mapped or User Pointer or DMA Buffer I/O.
可以用作 VIDIOC_REQBUFS 的替代。
struct v4l2_create_buffers {
__u32 index;
__u32 count;
__u32 memory;
struct v4l2_format format;
__u32 capabilities;
__u32 flags;
__u32 max_num_buffers;
__u32 reserved[5];
};
app:
count: 要创建的 buffer 的数量。memory: 要创建的 buffer 的内存类型。enum v4l2_memory.format: 要创建的 buffer 的格式。通常需要先通过 VIDIOC_TRY_FMT 或者 VIDIOC_G_FMT 获取。flags: 要创建的 buffer 的 flags.
kernel:
index: 返回创建的 buffers 中第一个 buffer 的 index。
capabilities: 返回创建的 buffers 的 capabilities.
max_num_buffers: 返回创建的 buffers 的最大数量。
7.4 VIDIOC_CROPCAP
Information about the video cropping and scaling abilities.
获取 driver 支持的可裁剪的区域大小。
struct v4l2_cropcap {
__u32 type;
struct v4l2_rect bounds;
struct v4l2_rect defrect;
struct v4l2_fract pixelaspect;
};
app:
type: enum v4l2_buf_type.
kernel:
bounds: 返回 driver 支持的可裁剪的区域大小。defrect: 返回 driver 支持的默认的裁剪区域大小。pixelaspect: 返回 driver 支持的像素的宽高比，默认是 1:1，Other common values are 54/59 for PAL and SECAM, 11/10 for NTSC.
7.8 VIDIOC_DQEVENT
Dequeue event from video device
struct v4l2_event {
__u32 type;
union {
struct v4l2_event_vsync vsync;
struct v4l2_event_ctrl ctrl;
struct v4l2_event_frame_sync frame_sync;
struct v4l2_event_src_change src_change;
struct v4l2_event_motion_det motion_det;
__u8 data[64];
} u;
__u32 pending;
__u32 sequence;
#ifdef __KERNEL__
struct __kernel_timespec timestamp;
#else
struct timespec timestamp;
#endif
__u32 id;
__u32 reserved[8];
};
app:
none, 不需要传入参数。
kernel:
type: event type, 定义在 videodev2.h 中。
V4L2_EVENT_ALL: 只用于 VIDIOC_UNSUBSCRIBE_EVENT 中取消所有 event 订阅。V4L2_EVENT_VSYNC: trigger on vertical sync.V4L2_EVENT_EOS: trigger on end of stream. 一般在 MPEG decoders 中报告最后的 MPEG stream decode 完成。V4L2_EVENT_CTRL: trigger on control change. 调用 VIDIOC_S_CTRL or VIDIOC_S_EXT_CTRL 的 file handler 不会收到该 event。V4L2_EVENT_FRAME_SYNC: trigger on frame begin.V4L2_EVENT_SOURCE_CHANGE: trigger on source parameter change.V4L2_EVENT_MOTION_DET: trigger on motion detection change.V4L2_EVENT_PRIVATE_START: base number of driver private events.
pending: 除了当前 event，待处理的 events.
sequence: event sequence number. 每个订阅的 event 发生后递增。
timestamp: event timestamp.
id: event source 的 ID.
struct v4l2_event_vsync {
/* Can be V4L2_FIELD_ANY, _NONE, _TOP or _BOTTOM */
__u8 field; // upcoming field
} __attribute__ ((packed));
struct v4l2_event_ctrl {
__u32 changes; // bitmask tells what changed, see V4L2_EVENT_CTRL_CH_XXX
__u32 type; // control type, see enum v4l2_ctrl_type
union {
__s32 value; // control value
__s64 value64;
};
__u32 flags; // control flags, see V4L2_CTRL_FLAG_XXX
__s32 minimum; // control minimum value
__s32 maximum; // control maximum value
__s32 step; // control step
__s32 default_value; // control default value
};
struct v4l2_event_frame_sync {
__u32 frame_sequence; // frame sequence number
};
struct v4l2_event_src_change {
__u32 changes; // bitmask tells source change, see V4L2_EVENT_SRC_CH_RESOLUTION
};
struct v4l2_event_motion_det {
__u32 flags; // V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ
__u32 frame_sequence; // frame sequence number, only valid when flags = V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ
__u32 region_mask; // report motion 的 region bitmask
};
7.14 VIDIOC_ENUM_FMT
Enumerate image formats.
获取支持的 pixel format.
struct v4l2_fmtdesc {
__u32 index;
__u32 type;
__u32 flags;
__u8 description[32];
__u32 pixelformat;
__u32 mbus_code;
};
app:
index: app 要查询的 format index.type: capture 设备传 V4L2_BUF_TYPE_VIDEO_CAPTURE/V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE.mbus_code: app 传入的 mbus_code, MEDIA_BUS_FMT_XXX, 可以用来限制枚举的 formats, 只有 driver 置起 V4L2_CAP_IO_MC flag 才有效，否则为 0。
kernel:
description: drvier 返回的 format 的 description. vendor driver 可以不用设置，v4l2 framework 会自动设置。
pixelformat: driver 返回的 format(fourcc code).
7.15 VIDIOC_ENUM_FRAMESIZES
Enumerate frame sizes
struct v4l2_frmsizeenum {
__u32 index;
__u32 pixel_format;
__u32 type;
union {
struct v4l2_frmsize_discrete discrete;
struct v4l2_frmsize_stepwise stepwise;
};
};
app:
index: app 要查询的 frame size index.pixel_format: app 要查询的 pixel format.一般是 7.14 返回的 pixelformat.
kernel:
type: frame size type, enum v4l2_frmsizetypes, 有 discrete, continuous, stepwise.discrete: 返回 discrete frame size, 是固定的长和宽。stepwise: 返回 stepwise frame size, 是可变的，有最小值，最大值，步长，continuous 类型步长为 1。
7.16 VIDIOC_ENUM_FRAMEINTERVALS
Enumerate frame intervals
struct v4l2_frmivalenum {
__u32 index;
__u32 pixel_format;
__u32 width;
__u32 height;
__u32 type;
union {
struct v4l2_fract discrete;
struct v4l2_frmival_stepwise stepwise;
};
}
app:
index: app 要查询的 frame interval index.pixel_format: app 要查询的 pixel format，一般是 7.14 返回的 pixelformat.width: app 要查询的 width, 一般是 7.15 返回的 width.height: app 要查询的 height, 一般是 7.15 返回的 height.
kernel:
type: frame interval type, enum v4l2_frmivaltypes, 有 discrete, continuous, stepwise.discrete: 返回 discrete frame interval.stepwise: 返回 stepwise frame interval.
7.21 VIDIOC_EXPBUF
Export a buffer as a DMABUF file descriptor
dma buf export
// TODO:
7.24 VIDIOC_G_CROP, VIDIOC_S_CROP
Get or set the current cropping rectangle
struct v4l2_crop {
__u32 type;
struct v4l2_rect c;
};
VIDIOC_G_CROP: 获取当前的 cropping rectangle.
app:
type: enum v4l2_buf_type.
kernel:
c: 返回当前的 cropping rectangle.
VIDIOC_S_CROP: 设置当前的 cropping rectangle.
app:
type: enum v4l2_buf_type.
c: 设置当前的 cropping rectangle.
7.25 VIDIOC_G_CTRL, VIDIOC_S_CTRL
get or set the value of control.
v4l2_ctrl id 参考 v4l2-controls.h.
struct v4l2_control {
__u32 id;
__s32 value;
};
VIDIOC_G_CTRL: 获取 control 的 value.
app:
id: v4l2_ctrl id.
kernel:
value: driver 返回 value.
VIDIOC_S_CTRL: 设置 control 的 value.
app:
id: v4l2_ctrl id.
value: 要设置的 value.
kernel:
7.29 VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS
Get or set the value of several controls, try control values.
app 传入 count, which, controls, reserved, 并且初始化好所有的 v4l2_ext_control.
struct v4l2_ext_controls {
__u32 which;
__u32 count;
__u32 error_idx;
__s32 request_fd;
struct v4l2_ext_control *controls;
};
which: V4L2_CTRL_WHICH_CUR_VAL/V4L2_CTRL_WHICH_DEF_VAL/V4L2_CTRL_WHICH_REQUEST_VAL.
count: controls 数组的数量。
error_idx: driver 返回发生错误的 control index.
request_fd:
controls: control 数组。
struct v4l2_ext_control {
__u32 id;
__u32 size;
union {
__s32 value;
__s64 value64;
char __user *string;
__u8 __user *p_u8;
__u16 __user *p_u16;
__u32 __user *p_u32;
__s32 __user *p_s32;
__s64 __user *p_s64;
// ...
void __user *ptr;
};
}
id: control id.
size: 通常为 0, 对于 pointer control 要设置为发送或接收的 payload 大小。
value: 要设置的 control value.
7.31 VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT
Get or set the data format, try a format.
struct v4l2_format {
__u32 type;
union {
struct v4l2_pix_format pix;
struct v4l2_pix_format_mplane pix_mp;
} fmt;
};
app:
type: enum v4l2_buf_type.
kernel:
fmt: driver 返回的 format.
struct v4l2_pix_format {
__u32 width;
__u32 height;
__u32 pixelformat;
__u32 field;
__u32 bytesperline;
__u32 sizeimage;
__u32 colorspace;
__u32 priv;
__u32 flags;
union {
__u32 ycbcr_enc;
__u32 hsv_enc;
};
__u32 quantization;
__u32 xfer_func;
};
width: picture width in pixels.
height: picture height in pixels. 如果 field 是 V4L2_FIELD_TOP/BOTTOM/ALTERNATE,
则 height 代表 field 中的 height pixels, 否则是 frame 中的 height pixels.
pixelformat: picture format in fourcc code.
field: enum v4l2_field 场的类型。
bytesperline: 每行的字节数。
sizeimage: 图像总字节数。
colorspace: enum v4l2_colorspace.
priv: app 设置为 V4L2_PIX_FMT_PRIV_MAGIC, 那么后面的 extended fields 才会有效。
flags: 有 V4L2_PIX_FMT_FLAG_PREMUL_ALPHA 和 V4L2_PIX_FMT_FLAG_SET_CSC.
ycbcr_enc: enum v4l2_ycbcr_encoding.
hsv_enc: enum v4l2_hsv_encoding.
quantization: enum v4l2_quantization.
xfer_func: enum v4l2_xfer_func.
struct v4l2_pix_format_mplane {
__u32 width;
__u32 height;
__u32 pixelformat;
__u32 field;
__u32 colorspace;
struct v4l2_plane_pix_format plane_fmt[VIDEO_MAX_PLANES];
__u8 num_planes;
__u8 flags;
union {
__u8 ycbcr_enc;
__u8 hsv_enc;
};
__u8 quantization;
__u8 xfer_func;
}
plane_fmt: 每个平面具体的信息。
num_planes: 平面数量。
struct v4l2_plane_pix_format {
__u32 sizeimage;
__u32 bytesperline;
}
sizeimage: 平面最大包含的字节数。
bytesperline: 每个平面一行的字节数。
VIDIOC_G_FMT: 获取当前的 format.
VIDIOC_S_FMT: app 设置好所有 field.
VIDIOC_TRY_FMT: 检查 hardware 是否支持 fmt，不会真正设置 format。
7.37 VIDIOC_G_PARM, VIDIOC_S_PARM
Get or set streaming parameters. 可以设置 fps.
struct v4l2_streamparm {
__u32 type;
union {
struct v4l2_captureparm capture;
struct v4l2_outputparm output;
__u8 raw_data[200];
} parm;
}
app:
type: enum v4l2_buf_type.
kernel:
parm: driver 返回的 streamparm.
7.38 VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY
7.39 VIDIOC_G_SELECTION, VIDIOC_S_SELECTION
struct v4l2_selection {
__u32 type;
__u32 target;
__u32 flags;
struct v4l2_rect r;
__u32 reserved[9];
};
app:
type: buffer type
target: V4L2_SET_TGT_CROP/V4L2_SET_TGT_COMPOSE
flags: V4L2_SEL_FLAG_GE/V4L2_SEL_FLAG_LE
r: selection rectangle
kernel:
r: adjusted rectangle
7.45 VIDIOC_PREPARE_BUF
可以在 QBUF 之前做一些 cache invalidation or cleaning 的工作。
7.46 VIDIOC_QBUF, VIDIOC_DQBUF
Exchange a buffer with the driver.
7.47 VIDIOC_QUERYBUF
Query the status of a buffer.
struct v4l2_buffer {
__u32 index;
__u32 type;
__u32 bytesused;
__u32 flags;
__u32 field;
struct __kernel_v4l2_timeval timestamp;
struct v4l2_timecode timecode;
__u32 sequence;
__u32 memory;
union {
__u32 offset;
unsigned long userptr;
struct v4l2_plane *planes;
__s32 fd;
} m;
__u32 length;
union {
__s32 request_fd;
__u32 reserved;
};
};
app:
type: enum v4l2_buf_type.
index: buffer index.
planes: 指向用户层 struct v4l2_plane 数组。length: 对于 multi planes, 需要指定为 v4l2_plane 数组大小。
kernel:
memory: enum v4l2_memory.byteused: multi plane 为 0.bytesused: 已经使用的字节数。
7.48 VIDIOC_QUERYCAP
查询 v4l2 设备支持的功能，返回 struct v4l2_capability 结构体。所有 app 程序在 open 后都要执行。
struct v4l2_capability caps;
ioctl(fd, VIDIOC_QUERYCAP, &caps);
struct v4l2_capability {
__u8 driver[16];
__u8 card[32];
__u8 bus_info[32];
__u32 version;
__u32 capabilities;
__u32 device_caps;
__u32 reserved[3];
};
driver: 驱动名称。
card: 设备名称。
bus_info: bus 名称，比如 platform:xxx
version: kernel 版本。
capabilities: 物理设备支持的功能。看着和 device_caps 差不多，多一个 V4L2_CAP_DEVICE_CAPS?
device_caps: 设备支持的功能。
7.49 ioctls VIDIOC_QUERYCTRL, VIDIOC_QUERY_EXT_CTRL and VIDIOC_QUERYMENU
Enumerate controls and menu control items.
int ioctl(int fd, VIDIOC_QUERYCTRL, struct v4l2_queryctrl *argp);
int ioctl(int fd, VIDIOC_QUERY_EXT_CTRL, struct v4l2_query_ext_ctrl *argp);
int ioctl(int fd, VIDIOC_QUERYMENU, struct v4l2_querymenu *argp);
struct v4l2_queryctrl {
__u32 id;
__u32 type; /* enum v4l2_ctrl_type */
__u8 name[32]; /* Whatever */
__s32 minimum; /* Note signedness */
__s32 maximum;
__s32 step;
__s32 default_value;
__u32 flags;
__u32 reserved[2];
};
id: control id. app 设置。 type: enum v4l2_ctrl_type. driver 返回。 name: minimum: maximum: step: default_value: flags: control flags. driver 返回。
struct v4l2_querymenu {
__u32 id;
__u32 index;
union {
__u8 name[32]; /* Whatever */
__s64 value;
};
__u32 reserved;
}
id: control id, app 设定。 index: 该 menu 选项中的第 index 个，app 设定。 name: driver 返回的 menu control string, V4L2_CTRL_TYPE_MENU 类型 control 适用。 value: driver 返回的 menu control value, V4L2_CTRL_TYPE_INTEGER_MENU 类型 control 适用。
7.52 VIDIOC_REQBUFS
Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O.
struct v4l2_requestbuffers {
__u32 count;
__u32 type; /* enum v4l2_buf_type */
__u32 memory; /* enum v4l2_memory */
__u32 capabilities;
__u8 flags;
__u8 reserved[3];
};
count: app 传入要 request 的 buf 数量，driver 返回实际的 buf 数量。 type: app 传入的 v4l2_buf_type. memory: usersapce 设置为 V4L2_MEMORY_MMAP/V4L2_MEMORY_DMABUF/V4L2_MEMORY_USERPTR. capabilities: driver 返回的 capabilities. 可以在 app 开头通过设置 count=0, memory=V4L2_MEMORY_MMAP 和 type 来获取 driver 的 capabilities。 flags: 可以传入 V4L2_MEMORY_FLAG_NON_COHERENT 来分配 non-coherent 内存。只有在 mmap io 和 driver capabilities 支持 V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS 才有效
7.53 VIDIOC_REMOVE_BUFS
7.55 VIDIOC_STREAMON, VIDIOC_STREAMOFF
7.56 VIDIOC_SUBDEV_ENUM_FRAME_INTERVAL
Enumerate frame intervals
struct v4l2_subdev_frame_interval_enum {
__u32 index;
__u32 pad;
__u32 code;
__u32 width;
__u32 height;
struct v4l2_fract interval;
__u32 which;
__u32 stream;
__u32 reserved[7];
}
app:
index: 要 get 的 frame interval index.pad: pad id.code: bus format code，一般是 7.58 中 get 的 code.which: 见 7.60 的 which.
width: frame width, 一般是 7.57 中 get 的 width.height: frame height, 一般是 7.57 中 get 的 height.
kernel:
interval: 返回 frame interval.
7.57 VIDIOC_SUBDEV_ENUM_FRAME_SIZE
Enumerate media bus frame sizes
struct v4l2_subdev_frame_size_enum {
__u32 index;
__u32 pad;
__u32 code;
__u32 min_width;
__u32 max_width;
__u32 min_height;
__u32 max_height;
__u32 which;
__u32 stream;
__u32 reserved[7];
};
app:
index: 要 get 的 frame size index.pad: pad id.code: bus format code, 一般是 7.58 中 get 的 code.which: 见 7.60 的 which.
kernel:
min_width, max_width, min_height, max_height: 如果是 discrete frame size，那么 min 和 max 值相同。
7.58 VIDIOC_SUBDEV_ENUM_MBUS_CODE
Enumerate media bus formats.
struct v4l2_subdev_mbus_code_enum {
__u32 pad;
__u32 index;
__u32 code;
__u32 which;
__u32 flags;
__u32 stream;
__u32 reserved[6];
};
app:
pad: pad id.index: 要 get 的 mbus code index.which: 见 7.60 的 which.
kernel:
code: 返回 bus format, MEDIA_BUS_FMT_XXX.
flag: 告诉 app 一些 flag. V4L2_SUBDEV_MBUS_CODE_XXX
7.59 VIDIOC_SUBDEV_G_CROP, VIDIOC_SUBDEV_S_CROP
不推荐实现了，用 selection api 代替。
7.60 VIDIOC_SUBDEV_G_FMT, VIDIOC_SUBDEV_S_FMT
Get or set the data format on a subdev pad.
struct v4l2_subdev_format {
__u32 which;
__u32 pad;
struct v4l2_mbus_framefmt format;
};
struct v4l2_mbus_framefmt {
__u32 width;
__u32 height;
__u32 code;
__u32 field;
__u32 colorspace;
union {
__u16 ycbcr_enc; /* enum v4l2_ycbcr_encoding */
__u16 hsv_enc; /* enum v4l2_hsv_encoding */
};
__u16 quantization;
__u16 xfer_func;
__u16 flags;
};
VIDIOC_SUBDEV_S_FMT：
app：
which: Format to modified, 传入 enum v4l2_subdev_format_whence,
有 V4L2_SUBDEV_FORMAT_TRY 和 V4L2_SUBDEV_FORMAT_ACTIVE 两个值。前者用来 try format,
后者 apply to hardware.pad: pad id.format: 要设置的 struct v4l2_mbus_framefmt.
如果 app 传入的 format 不符合硬件要求，kernel 不应该返回错误，而是修改 format 返回给 userspace。
7.61 VIDIOC_SUBDEV_G_FRAME_INTERVAL, VIDIOC_SUBDEV_S_FRAME_INTERVAL
Get or set the frame interval on a subdev pad
struct v4l2_subdev_frame_interval {
__u32 pad;
struct v4l2_fract interval;
__u32 stream;
};
struct v4l2_fract {
__u32 numerator;
__u32 denominator;
};
app:
pad: pad number.interval: 要设置的 struct v4l2_fract.stream: stream api 相关。
7.63 VIDIOC_SUBDEV_G_SELECTION, VIDIOC_SUBDEV_S_SELECTION
7.65 VIDIOC_SUBDEV_QUERYCAP
7.66 VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT
Subscribe or unsubscribe event
struct v4l2_event_subscription {
__u32 type;
__u32 id;
__u32 flags;
__u32 reserved[5];
};
type: 要订阅的 event 类型，V4L2_EVENT_XXX.id: 根据 type 不同类型，有的需要 event source id.
flags: V4L2_EVENT_SUB_FL_XXX.
Media Controller API
4. Request API
5.4 MEDIA_IOC_DEVICE_INFO
获取 media device 的信息
struct media_device_info {
char driver[16];
char model[32];
char serial[40];
char bus_info[32];
__u32 media_version;
__u32 hw_revision;
__u32 driver_version;
__u32 reserved[31];
};
kernel:
填充所有的信息返回给 app.
5.5 MEDIA_IOC_G_TOPOLOGY
enumerate graph topology and graph element properties.
典型的用法是调用该 ioctl 两次，第一次 media_v2_topology 结构体全部设置为 0，kernel 返回 topology_version 和 num_entities/interfaces/pads/links
在第二次调用 ioctl 之前，app 应该 allocate 好数组，把ptr_entities/interfaces/pads/links 指向这些分配好的数组。
struct media_v2_topology {
__u64 topology_version;
__u32 num_entities;
__u64 ptr_entities;
__u32 num_interfaces;
__u64 ptr_interfaces;
__u32 num_pads;
__u64 ptr_pads;
__u32 num_links;
__u64 ptr_links;
}
kernel:
topology_version: topology 版本。num_entities: entites 数量。ptr_entities：struct media_v2_entity 数组。num_interfaces: interfaces 数量。ptr_interfaces: struct media_v2_interface 数组。num_pads: pads 数量。ptr_pads: struct media_v2_pad 数组。num_links: links 数量。ptr_links: struct media_v2_link 数组。
struct media_v2_entity {
__u32 id;
char name[64];
__u32 function; /* Main function of the entity */
__u32 flags;
__u32 reserved[5];
} __attribute__ ((packed));
id: entity id
name: entity name
function: entity function. MEDIA_ENT_F_XXX
flags: entity flags. 只有在 MEDIA_MEDIA_V2_ENTITY_HAS_FLAGS(media_version in struct media_device_info) 被定义的时候才有效。
struct media_v2_interface {
__u32 id;
__u32 intf_type;
__u32 flags;
__u32 reserved[9];
union {
struct media_v2_intf_devnode devnode;
__u32 raw[16];
};
} __attribute__ ((packed));
struct media_v2_intf_devnode {
__u32 major;
__u32 minor;
} __attribute__ ((packed));
id: interface id
intf_type: interface type. MEDIA_INTF_T_XXX
flags: interface flags. 目前没有使用
devnode: 只有 device node interfaces 用到了，包含了 device node 的 major 和 minor 信息。
struct media_v2_pad {
__u32 id;
__u32 entity_id;
__u32 flags;
__u32 index;
__u32 reserved[4];
} __attribute__ ((packed));
id: pad id
entity_id: pad 附属的 entity id
flags: pad flags. MEDIA_PAD_FL_SINK/MEDIA_PAD_FL_SOURCE/MEDIA_PAD_FL_MUST_CONNECT
struct media_v2_link {
__u32 id;
__u32 source_id;
__u32 sink_id;
__u32 flags;
__u32 reserved[6];
} __attribute__ ((packed));
id: link id
source/sink_id: source/sink pad id
flags: link flags. MEDIA_LNK_FL_ENABLED/MEDIA_LNK_FL_IMMUTABLE/MEDIA_LNK_FL_DYNAMIC/MEDIA_LNK_FL_LINK_TYPE
MEDIA_LNK_FL_ENABLE: link enabled and can be used to transfer media data.
MEDIA_LNK_FL_IMMUTABLE: link enabled state 在 runtime 期间不能被修改，immutable link 一直是 enabled 状态。
5.6 MEDIA_IOC_ENUM_ENTITIES
Enumerate entities and their properties
用户层传入 entity 的 id，返回 entity 信息
struct media_entity_desc {
__u32 id;
char name[32];
__u32 type;
__u32 flags;
__u16 pads;
__u16 links;
__u32 reserved[4];
union {
struct {
__u32 major;
__u32 minor;
} dev;
__u8 raw[184];
};
}
app 传入：
id: 如果与上了 MEDIA_ENT_ID_FLAG_NEXT flag，则返回下一个 entity。
kernel 返回：
id: entity 真正的 id 号。nametypeflagspadslinkstype
5.7 MEDIA_IOC_ENUM_LINKS
Enumerate all pads and links for a given entity
struct media_links_enum {
__u32 entity;
struct media_pad_desc __user *pads;
struct media_link_desc __user *links;
__u32 reserved[4];
};
app 传入：
entity: entity id。
kernel 返回：
pads: struct media_pad_desc 数组
struct media_pad_desc {
__u32 entity; /* entity ID */
__u16 index; /* pad index */
__u32 flags; /* pad flags */
}
links: struct media_link_desc 数组
struct media_link_desc {
struct media_pad_desc source;
struct media_pad_desc sink;
__u32 flags;
}
5.8 MEDIA_IOC_SETUP_LINK
Modify the properties of a link
enable source pad 和 sink pad 的 link.
struct media_link_desc {
struct media_pad_desc source;
struct media_pad_desc sink;
__u32 flags;
}
app:
source: source pad desc.sink: sink pad desc.flags: MEDIA_LNK_FL_ENABLE.