V4L2驱动简单分析一

下面以内核源码中的radio-tea5764.c来简单说明一下V4L2 radio驱动的注册流程
1、首先按照通常的设备定义文件系统接口

/* File system interface */ 
 /*文件系统通用接口即供应用层使用的接口*/
    static const struct v4l2_file_operations tea5764_fops = {
        .owner      = THIS_MODULE,
        .open           = tea5764_open,
        .release        = tea5764_close,
        .ioctl      = video_ioctl2,/*通过ioctl接口来调用v4l2的接口*/
    };

//v4l2接口

static const struct v4l2_ioctl_ops tea5764_ioctl_ops = {
        .vidioc_querycap    = vidioc_querycap,
        .vidioc_g_tuner     = vidioc_g_tuner,
        .vidioc_s_tuner     = vidioc_s_tuner,
        .vidioc_g_audio     = vidioc_g_audio,
        .vidioc_s_audio     = vidioc_s_audio,
        .vidioc_g_input     = vidioc_g_input,
        .vidioc_s_input     = vidioc_s_input,
        .vidioc_g_frequency = vidioc_g_frequency,
        .vidioc_s_frequency = vidioc_s_frequency,
        .vidioc_queryctrl   = vidioc_queryctrl,
        .vidioc_g_ctrl      = vidioc_g_ctrl,
        .vidioc_s_ctrl      = vidioc_s_ctrl,
    };

tea5764_fops结构体其实跟普通的字符设备类似，填充基本的操作即可。其中，.ioctl指向的接口video_ioctl2是v4l2接口的入口，这个就是v4l2驱动的关键。

接口video_ioctl2具体如下：

long video_ioctl2(struct file *file,unsigned int cmd, unsigned long arg);

它是v4l2核心提供的标准控制接口，与我们平时自己去实现设备的ioctl有所区别，video_ioctl2的代码不需要我们去实现，但是它通过file指针来区分每一个设备，再使用cmd来区分每一个专用接口，即每一个命令对应于v4l2_ioctl_ops里面的接口，arg就是应用传入的参数。所以，我们在实现V4L2驱动时，就是去实现v4l2_ioctl_ops里面的专用接口，使用这些接口去读写设备。

2、具体V4L2 radio设备驱动注册流程

//自定义一个tea5764设备结构体
struct tea5764_device {
    struct i2c_client       *i2c_client;
    struct video_device     *videodev;
    struct tea5764_regs     regs;
    struct mutex            mutex;
    int             users;
};

//定义一个video_device设备
/* V4L2 interface */
static struct video_device tea5764_radio_template = {
    .name       = "TEA5764 FM-Radio",
    .fops           = &tea5764_fops,/*一套通用文件系统接口供应用层使用*/
    .ioctl_ops  = &tea5764_ioctl_ops,/*一套v4l2接口供tea5764_fops的.ioctl通过命令调用*/
    .release    = video_device_release,
};

//radio-tea5764是一个ic2设备，所以注册一般在probe里面进行
/* I2C probe: check if the device exists and register with v4l if it is */
static int __devinit tea5764_i2c_probe(struct i2c_client *client,
                    const struct i2c_device_id *id)
{
    struct tea5764_device *radio;
    struct tea5764_regs *r;
    int ret;

    PDEBUG("probe");
    radio = kmalloc(sizeof(struct tea5764_device), GFP_KERNEL);
    if (!radio)
        return -ENOMEM;

    mutex_init(&radio->mutex);
    radio->i2c_client = client;
    ret = tea5764_i2c_read(radio);
    if (ret)
        goto errfr;
    r = &radio->regs;
    PDEBUG("chipid = %04X, manid = %04X", r->chipid, r->manid);
    if (r->chipid != TEA5764_CHIPID ||
        (r->manid & 0x0fff) != TEA5764_MANID) {
        PWARN("This chip is not a TEA5764!");
        ret = -EINVAL;
        goto errfr;
    }
    //分配一个video_device空间
    radio->videodev = video_device_alloc();
    if (!(radio->videodev)) {
        ret = -ENOMEM;
        goto errfr;
    }
    //把tea5764_radio_template填充到radio->videodev
    memcpy(radio->videodev, &tea5764_radio_template,
        sizeof(tea5764_radio_template));

    i2c_set_clientdata(client, radio);
    video_set_drvdata(radio->videodev, radio);

    //注册V4L2设备         
    //注册完成后 应用层就可以使用ioctl的接口传入命令
    //来调用v4l2的接口了即tea5764_ioctl_ops的接口
    //生成/dev/radiox设备名
    ret = video_register_device(radio->videodev, VFL_TYPE_RADIO, radio_nr);
    if (ret < 0) {
        PWARN("Could not register video device!");
        goto errrel;
    }

    /* initialize and power off the chip */
    tea5764_i2c_read(radio);
    tea5764_set_audout_mode(radio, V4L2_TUNER_MODE_STEREO);
    tea5764_mute(radio, 1);
    tea5764_power_down(radio);

    PINFO("registered.");
    return 0;
errrel:
    video_device_release(radio->videodev);
errfr:
    kfree(radio);
    return ret;
}