我對linux理解之v4l2

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我們先看具體sensor slave怎麼註冊到v4l2的：
static struct v4l2_int_ioctl_desc ov5642_ioctl_desc[] = {//ioctl與對應的序號聯絡在一起，在v4l2層將被轉換成固定的名字
{vidioc_int_dev_init_num, (v4l2_int_ioctl_func *)ioctl_dev_init},
{vidioc_int_dev_exit_num, ioctl_dev_exit},

    {vidioc_int_s_power_num, (v4l2_int_ioctl_func *)ioctl_s_power},
    {vidioc_int_g_ifparm_num, (v4l2_int_ioctl_func *)ioctl_g_ifparm},
/*    {vidioc_int_g_needs_reset_num,
                (v4l2_int_ioctl_func *)ioctl_g_needs_reset}, */
/*    {vidioc_int_reset_num, (v4l2_int_ioctl_func *)ioctl_reset}, */

    {vidioc_int_init_num, (v4l2_int_ioctl_func *)ioctl_init},
/*    {vidioc_int_enum_fmt_cap_num,
                (v4l2_int_ioctl_func *)ioctl_enum_fmt_cap}, */
/*    {vidioc_int_try_fmt_cap_num,
                (v4l2_int_ioctl_func *)ioctl_try_fmt_cap}, */
    {vidioc_int_g_fmt_cap_num, (v4l2_int_ioctl_func *)ioctl_g_fmt_cap},

/*    {vidioc_int_s_fmt_cap_num, (v4l2_int_ioctl_func *)ioctl_s_fmt_cap}, */
    {vidioc_int_g_parm_num, (v4l2_int_ioctl_func *)ioctl_g_parm},
    {vidioc_int_s_parm_num, (v4l2_int_ioctl_func *)ioctl_s_parm},
/*    {vidioc_int_queryctrl_num, (v4l2_int_ioctl_func *)ioctl_queryctrl}, */
    {vidioc_int_g_ctrl_num, (v4l2_int_ioctl_func *)ioctl_g_ctrl},
    {vidioc_int_s_ctrl_num, (v4l2_int_ioctl_func *)ioctl_s_ctrl},
};
static struct v4l2_int_slave ov5642_slave = {//slave
    .ioctls = ov5642_ioctl_desc,
    .num_ioctls = ARRAY_SIZE(ov5642_ioctl_desc),
};

static struct v4l2_int_device ov5642_int_device = {
    .module = THIS_MODULE,
    .name = "ov5642",
    .type = v4l2_int_type_slave,
    .u = {
        .slave = &ov5642_slave,
    },
};
v4l2_int_device_register(&ov5642_int_device);//註冊v4l2_int_device：
int v4l2_int_device_register(struct v4l2_int_device *d)
{
    if (d->type == v4l2_int_type_slave)
        sort(d->u.slave->ioctls, d->u.slave->num_ioctls,//按照序號儲存，加快訪問速度
             sizeof(struct v4l2_int_ioctl_desc),
             &ioctl_sort_cmp, NULL);
    mutex_lock(&mutex);
    list_add(&d->head, &int_list);//無論是slave還是master都會新增到int_list中
    v4l2_int_device_try_attach_all();//都會做匹配動作
    mutex_unlock(&mutex);

    return 0;
}
我們看下v4l2匹配函式v4l2_int_device_try_attach_all():
void v4l2_int_device_try_attach_all(void)
{
    struct v4l2_int_device *m, *s;

    list_for_each_entry(m, &int_list, head) {//對int_list中每個master
        if (m->type != v4l2_int_type_master)
            continue;

        list_for_each_entry(s, &int_list, head) {//對int_list中的每個salve
            if (s->type != v4l2_int_type_slave)
                continue;

            /* Slave is connected? */
            if (s->u.slave->master)//slave中master已經被賦值說明已經連線起來
                continue;

            /* Slave wants to attach to master? */
            if (s->u.slave->attach_to[0] != 0
                && strncmp(m->name, s->u.slave->attach_to,
                       V4L2NAMESIZE))
                continue;

            if (!try_module_get(m->module))
                continue;

            s->u.slave->master = m;//說明slave找到了master
            if (m->u.master->attach(s)) {//執行master的匹配函式
                s->u.slave->master = NULL;
                module_put(m->module);
                continue;
            }
        }
    }
}
上面即是slave註冊到v4l2的過程，也許你會比較的奇怪，開始那些ioctl哪去呢？系統怎麼訪問呢？下面我們就分析這個過程。
我們在v4l2-int-device.h中有這樣的定義：
/* IOCTL command numbers. */
enum v4l2_int_ioctl_num {
    /*
    *
    * "Proper" V4L ioctls, as in struct video_device.
    *
    */
    vidioc_int_enum_fmt_cap_num = 1,
    vidioc_int_g_fmt_cap_num,
    vidioc_int_s_fmt_cap_num,
    vidioc_int_try_fmt_cap_num,
    vidioc_int_queryctrl_num,
    vidioc_int_g_ctrl_num,
    vidioc_int_s_ctrl_num,
    vidioc_int_cropcap_num,
    vidioc_int_g_crop_num,
    vidioc_int_s_crop_num,
    vidioc_int_g_parm_num,
    vidioc_int_s_parm_num,
    vidioc_int_querystd_num,
    vidioc_int_s_std_num,
    vidioc_int_s_video_routing_num,
......
};
V4L2_INT_WRAPPER_1(enum_fmt_cap, struct v4l2_fmtdesc, *);
V4L2_INT_WRAPPER_1(g_fmt_cap, struct v4l2_format, *);
V4L2_INT_WRAPPER_1(s_fmt_cap, struct v4l2_format, *);
V4L2_INT_WRAPPER_1(try_fmt_cap, struct v4l2_format, *);
V4L2_INT_WRAPPER_1(queryctrl, struct v4l2_queryctrl, *);
V4L2_INT_WRAPPER_1(g_ctrl, struct v4l2_control, *);
V4L2_INT_WRAPPER_1(s_ctrl, struct v4l2_control, *);
V4L2_INT_WRAPPER_1(cropcap, struct v4l2_cropcap, *);
V4L2_INT_WRAPPER_1(g_crop, struct v4l2_crop, *);
V4L2_INT_WRAPPER_1(s_crop, struct v4l2_crop, *);
V4L2_INT_WRAPPER_1(g_parm, struct v4l2_streamparm, *);
V4L2_INT_WRAPPER_1(s_parm, struct v4l2_streamparm, *);
V4L2_INT_WRAPPER_1(querystd, v4l2_std_id, *);
V4L2_INT_WRAPPER_1(s_std, v4l2_std_id, *);
V4L2_INT_WRAPPER_1(s_video_routing, struct v4l2_routing, *);
我們看下V4L2_INT_WRAPPER_1這個巨集定義：
#define V4L2_INT_WRAPPER_1(name, arg_type, asterisk)            \
    static inline int vidioc_int_##name(struct v4l2_int_device *d,    \
                        arg_type asterisk arg)    \
    {                                \
        return v4l2_int_ioctl_1(d, vidioc_int_##name##_num,    \
                    (void *)(unsigned long)arg);    \
    }                                \
                                    \
    static inline struct v4l2_int_ioctl_desc            \
    vidioc_int_##name##_cb(int (*func)                \
                   (struct v4l2_int_device *,        \
                arg_type asterisk))            \
    {                                \
        struct v4l2_int_ioctl_desc desc;            \
                                    \
        desc.num = vidioc_int_##name##_num;            \
        desc.func = (v4l2_int_ioctl_func *)func;        \
                                    \
        return desc;                        \
    }
我們舉例來說，V4L2_INT_WRAPPER_1(s_ctrl, struct v4l2_control, *)，那也就是有了這樣的定義：
    static inline int vidioc_int_s_ctrl(struct v4l2_int_device *d,
                        arg_type asterisk arg)
    {
        return v4l2_int_ioctl_1(d, vidioc_int_s_ctrl_num,
                    (void *)(unsigned long)arg);
    }

    static inline struct v4l2_int_ioctl_desc
    vidioc_int_s_ctrl_cb(int (*func)
                   (struct v4l2_int_device *,
                arg_type asterisk))
    {
        struct v4l2_int_ioctl_desc desc;

        desc.num = vidioc_int_s_ctrl_num;
        desc.func = (v4l2_int_ioctl_func *)func;

        return desc;
    }
也就是定義了這兩個行內函數。我們再看下v4l2_int_ioctl_1(d, vidioc_int_s_ctrl_num, (void *)(unsigned long)arg)：
int v4l2_int_ioctl_1(struct v4l2_int_device *d, int cmd, void *arg)
{
    return ((v4l2_int_ioctl_func_1 *)
        find_ioctl(d->u.slave, cmd,
               (v4l2_int_ioctl_func *)no_such_ioctl_1))(d, arg);
}
轉到find_ioctl：
static v4l2_int_ioctl_func *find_ioctl(struct v4l2_int_slave *slave, int cmd,//這裡的slave就上面定義的ov5642_slave
                       v4l2_int_ioctl_func *no_such_ioctl)
{
    const struct v4l2_int_ioctl_desc *first = slave->ioctls;//這裡其實就上面的ov5642_ioctl_desc
    const struct v4l2_int_ioctl_desc *last =
        first + slave->num_ioctls - 1;//slave->num_ioctls=ARRAY_SIZE(ov5642_ioctl_desc)

    while (first <= last) {
        const struct v4l2_int_ioctl_desc *mid;

        mid = (last - first) / 2 + first;//二分法

        if (mid->num < cmd)
            first = mid + 1;
        else if (mid->num > cmd)
            last = mid - 1;
        else
            return mid->func;//找到就返回具體的函式，具體的說這裡的函式就是ov5642_slave定義的ov5642_ioctl_desc中的具體函式！
    }

    return no_such_ioctl;
}
總結這裡的函式對應關係，以ov5642_ioctl_desc中的{vidioc_int_s_ctrl_num, (v4l2_int_ioctl_func *)ioctl_s_ctrl}為例，如果系統有這樣的函式vidioc_int_s_ctrl(...)使用，那也就是對應引用了
ov5642_slave的定義的ioctl_s_ctrl(...)。

下面看下master註冊過程：
static __init int camera_init(void)
{
    u8 err = 0;

    pr_debug("In MVC:camera_init\n");

    /* Register the device driver structure. */
    err = platform_driver_register(&mxc_v4l2_driver);//平臺註冊v4l2驅動
    if (err != 0) {
        pr_err("ERROR: v4l2 capture:camera_init: "
            "platform_driver_register failed.\n");
        return err;
    }

    return err;
}
mxc_v4l2_driver定義：
static struct platform_driver mxc_v4l2_driver = {
    .driver = {
           .name = "mxc_v4l2_capture",
           },
    .probe = mxc_v4l2_probe,
    .remove = mxc_v4l2_remove,
    .suspend = mxc_v4l2_suspend,
    .resume = mxc_v4l2_resume,
    .shutdown = NULL,
};
註冊成功將會執行mxc_v4l2_probe：
static int mxc_v4l2_probe(struct platform_device *pdev)
{
    /* Create g_cam and initialize it. */
    g_cam = kmalloc(sizeof(cam_data), GFP_KERNEL);
    if (g_cam == NULL) {
        pr_err("ERROR: v4l2 capture: failed to register camera\n");
        return -1;
    }
    init_camera_struct(g_cam, pdev);//初始化cam_data結構
    pdev->dev.release = camera_platform_release;

    /* Set up the v4l2 device and register it*/
    mxc_v4l2_int_device.priv = g_cam;//注意這裡的g_cam賦值，後面mattch函式中會用到
    /* This function contains a bug that won't let this be rmmod'd. */
    v4l2_int_device_register(&mxc_v4l2_int_device);//向v4l2註冊int_device

    /* register v4l video device */
    if (video_register_device(g_cam->video_dev, VFL_TYPE_GRABBER, video_nr)//註冊video裝置
        == -1) {
        kfree(g_cam);
        g_cam = NULL;
        pr_err("ERROR: v4l2 capture: video_register_device failed\n");
        return -1;
    }
    pr_debug("   Video device registered: %s #%d\n",
        g_cam->video_dev->name, g_cam->video_dev->minor);

    if (device_create_file(&g_cam->video_dev->dev,
                           &dev_attr_fsl_v4l2_capture_property))//建立fsl_v4l2_capture_property屬性檔案

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