v4l2 学习
1。控制命令
VIDIOC_REQBUFS分配内存
VIDIOC_QUERYBUF //把VIDIOC_REQBUFS中分配的数据缓存转换成物理地址
VIDIOC_QUERYCAP //查询驱动功能
VIDIOC_ENUM_FMT //获取当前驱动支持的视频格式
VIDIOC_S_FMT //设置当前驱动的频捕获格式
VIDIOC_G_FMT //读取当前驱动的频捕获格式
VIDIOC_TRY_FMT //验证当前驱动的显示格式
VIDIOC_CROPCAP //查询驱动的修剪能力
VIDIOC_S_CROP //设置视频信号的矩形边框
VIDIOC_G_CROP /读取视频信号的矩形边框
VIDIOC_QBUF //把数据从缓存中读取出来
VIDIOC_DQBUF //把数据放回缓存队列
VIDIOC_STREAMON //开始视频显示函数
VIDIOC_STREAMOFF //结束视频显示函数
VIDIOC_QUERYSTD //检查当前视频设备支持的标准，例如PAL或NTSC
struct v4l2_subdev *v4l2_i2c_new_subdev_board(struct v4l2_device *v4l2_dev,
struct i2c_adapter *adapter, struct i2c_board_info *info,
const unsigned short *probe_addrs)
{
struct v4l2_subdev *sd = NULL;
struct i2c_client *client;
BUG_ON(!v4l2_dev);
//加载I2C驱动，通过modprobe加载
request_module(I2C_MODULE_PREFIX "%s", info->type);
创建i2c设备时从设备地址是明确的还是要通过探测。
在这两个调用中会向系统中增加sysfs，会把驱动和设备关联起来，并会调用i2c驱动中的probe接口
/* Create the i2c client */
if (info->addr == 0 && probe_addrs)
client = i2c_new_probed_device(adapter, info, probe_addrs,
NULL);
else
client = i2c_new_device(adapter, info);
/* Note: by loading the module first we are certain that c->driver
will be set if the driver was found. If the module was not loaded
first, then the i2c core tries to delay-load the module for us,
and then c->driver is still NULL until the module is finally
loaded. This delay-load mechanism doesn't work if other drivers
want to use the i2c device, so explicitly loading the module
is the best alternative. */
if (client == NULL || client->driver == NULL)
goto error;
/* Lock the module so we can safely get the v4l2_subdev pointer */
if (!try_module_get(client->driver->driver.owner))
goto error;
从客户端驱动私有数据获得SD
sd = i2c_get_clientdata(client);
注册子设备到系统
/* Register with the v4l2_device which increases the module's
use count as well. */
if (v4l2_device_register_subdev(v4l2_dev, sd))
sd = NULL;
/* Decrease the module use count to match the first try_module_get. */
module_put(client->driver->driver.owner);
error:
/* If we have a client but no subdev, then something went wrong and
we must unregister the client. */
if (client && sd == NULL)
i2c_unregister_device(client);
return sd;