11月19日
==========



V4L2 Ioctl调用流程
++++++++++++++++++++++

上次培训时讲到，V4L2的功能大部分都是通过ioctl导出给用户使用的。那么今天就看看这些ioctl是如何对应到驱动中的代码的。

下面以VIDIOC_QUERYCAP为例讲解。

 .. code-block:: c

    static const struct file_operations v4l2_fops = {
    .owner = THIS_MODULE,
    .read = v4l2_read,
    .write = v4l2_write,
    .open = v4l2_open,
    .get_unmapped_area = v4l2_get_unmapped_area,
    .mmap = v4l2_mmap,
    .unlocked_ioctl = v4l2_ioctl, 
    #ifdef CONFIG_COMPAT
    .compat_ioctl = v4l2_compat_ioctl32,
    #endif
    .release = v4l2_release,
    .poll = v4l2_poll,
    .llseek = no_llseek,
    };

    vdev->cdev->ops = &v4l2_fops;
    vdev->cdev->owner = owner;
    ret = cdev_add(vdev->cdev, MKDEV(VIDEO_MAJOR, vdev->minor), 1);
    if (ret < 0) {
        printk(KERN_ERR "%s: cdev_add failed\n", __func__);
        kfree(vdev->cdev);
        vdev->cdev = NULL;
        goto cleanup;
    }
    
从上面的代码可以看出来，v4l2在注册字符设备时，将该字符设备的file_operations赋值为v4l2_fops。下面看看v4l2_ioctl的实现。

.. code-block:: c

    static long v4l2_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
    {
    struct video_device *vdev = video_devdata(filp);
    int ret = -ENODEV;

    if (vdev->fops->unlocked_ioctl) {
        if (vdev->lock && mutex_lock_interruptible(vdev->lock))
            return -ERESTARTSYS;
        if (video_is_registered(vdev))
            ret = vdev->fops->unlocked_ioctl(filp, cmd, arg);
        if (vdev->lock)
            mutex_unlock(vdev->lock);
    } else if (vdev->fops->ioctl) {
        static DEFINE_MUTEX(v4l2_ioctl_mutex);
        struct mutex *m = vdev->v4l2_dev ?
            &vdev->v4l2_dev->ioctl_lock : &v4l2_ioctl_mutex;

        if (cmd != VIDIOC_DQBUF && mutex_lock_interruptible(m))
            return -ERESTARTSYS;
        if (video_is_registered(vdev))
            ret = vdev->fops->ioctl(filp, cmd, arg);
        if (cmd != VIDIOC_DQBUF)
            mutex_unlock(m);
    } else
        ret = -ENOTTY;

    return ret;
    }

可以看到v4l2_ioctl的处理分为两种情况：unlocked_ioctl和正常的ioctl(具体区别后面在仔细讲)。不论哪种情况，都会调用vdev->fops->ioctl函数。看一下soc_camera驱动中此函数是如何实现的：

.. code-block:: c
    :emphasize-lines: 5
    
    static struct v4l2_file_operations soc_camera_fops = {
    .owner		= THIS_MODULE,
    .open		= soc_camera_open,
    .release	= soc_camera_close,
    .unlocked_ioctl	= video_ioctl2,
    .read		= soc_camera_read,
    .mmap		= soc_camera_mmap,
    .poll		= soc_camera_poll,
    };

一般情况下，unlocked_ioctl都是指向video_ioctl2函数的。那么我们接下来看看video_ioctl2的代码：

.. code-block:: c

    long video_ioctl2(struct file *file,
        unsigned int cmd, unsigned long arg)
    {
        return video_usercopy(file, cmd, arg, __video_do_ioctl);
    }

video_ioctl2函数接着调用video_usercopy函数。video_usercopy函数会对用户传递过来的参数检查，将用户空间的数据拷贝到内核空间，调用我们指定的__video_do_ioctl函数，并将内核空间的数据拷贝到用户空间。

接下来终于到分析关键函数了，__video_do_ioctl :

.. code-block:: c

    static long __video_do_ioctl(struct file *file,
        unsigned int cmd, void *arg)
    {
        struct video_device *vfd = video_devdata(file);
        const struct v4l2_ioctl_ops *ops = vfd->ioctl_ops;
        void *fh = file->private_data;
        struct v4l2_fh *vfh = NULL;
        int use_fh_prio = 0;
        long ret_prio = 0;
        long ret = -ENOTTY;
    
        if (ops == NULL) {
            printk(KERN_WARNING "videodev: \"%s\" has no ioctl_ops.\n",
                    vfd->name);
            return ret;
        }
    
        if ((vfd->debug & V4L2_DEBUG_IOCTL) &&
                    !(vfd->debug & V4L2_DEBUG_IOCTL_ARG)) {
            v4l_print_ioctl(vfd->name, cmd);
            printk(KERN_CONT "\n");
        }
    
        if (test_bit(V4L2_FL_USES_V4L2_FH, &vfd->flags)) {
            vfh = file->private_data;
            use_fh_prio = test_bit(V4L2_FL_USE_FH_PRIO, &vfd->flags);
        }
    
        if (use_fh_prio)
            ret_prio = v4l2_prio_check(vfd->prio, vfh->prio);
    
        switch (cmd) {
    
        /* --- capabilities ------------------------------------------ */
        case VIDIOC_QUERYCAP:
        {
            struct v4l2_capability *cap = (struct v4l2_capability *)arg;
    
            if (!ops->vidioc_querycap)
                break;
    
            cap->version = LINUX_VERSION_CODE;
            ret = ops->vidioc_querycap(file, fh, cap);
            if (!ret)
                dbgarg(cmd, "driver=%s, card=%s, bus=%s, "
                        "version=0x%08x, "
                        "capabilities=0x%08x, "
                        "device_caps=0x%08x\n",
                        cap->driver, cap->card, cap->bus_info,
                        cap->version,
                        cap->capabilities,
                        cap->device_caps);
            break;
            }
        ......
        return ret;
    }

由于代码太长，只节选了VIDIOC_QUERYCAP的代码。__video_do_ioctl做完参数检查后，就是判断cmd的值并作相应处理。基本上所有的case的操作都一样：判断ops结构体中所对应的函数是否有效，若有效则使用用户指定的参数调用，否则返回-ENOTTY.