[question] v4l read() operation

Hi,

If I register a video device with this fops:

static const struct v4l2_file_operations v4l2_fops = {
        .owner      = THIS_MODULE,
        .open        = xxx_open,
        .unlocked_ioctl = xxx_unlocked_ioctl,
        .poll           = xxx_poll,
        .mmap      = xxx_mmap,
};

then if I "cat" the device

$ cat /dev/video0

Who is supporting read() ?

I thought it could be v4l2_read(),
however this function seems to return EINVAL:

static ssize_t v4l2_read(struct file *filp, char __user *buf,
                size_t sz, loff_t *off)
{
        struct video_device *vdev = video_devdata(filp);
        int ret = -ENODEV;

        if (!vdev->fops->read)
                return -EINVAL;
        if (vdev->lock && mutex_lock_interruptible(vdev->lock))
                return -ERESTARTSYS;
        if (video_is_registered(vdev))
                ret = vdev->fops->read(filp, buf, sz, off);
        if (vdev->lock)
                mutex_unlock(vdev->lock);
        return ret;
}

Thanks,
Ezequiel.

Hi Ezequiel

2012/2/25 Ezequiel Garc?a [off-list ref]:

Hi,

If I register a video device with this fops:

static const struct v4l2_file_operations v4l2_fops = {
? ? ? ?.owner ? ? ?= THIS_MODULE,
? ? ? ?.open ? ? ? ?= xxx_open,
? ? ? ?.unlocked_ioctl = xxx_unlocked_ioctl,
? ? ? ?.poll ? ? ? ? ? = xxx_poll,
? ? ? ?.mmap ? ? ?= xxx_mmap,
};

then if I "cat" the device

$ cat /dev/video0

Who is supporting read() ?

I thought it could be v4l2_read(),
however this function seems to return EINVAL:

static ssize_t v4l2_read(struct file *filp, char __user *buf,
? ? ? ? ? ? ? ?size_t sz, loff_t *off)
{
? ? ? ?struct video_device *vdev = video_devdata(filp);
? ? ? ?int ret = -ENODEV;

? ? ? ?if (!vdev->fops->read)
? ? ? ? ? ? ? ?return -EINVAL;
? ? ? ?if (vdev->lock && mutex_lock_interruptible(vdev->lock))
? ? ? ? ? ? ? ?return -ERESTARTSYS;
? ? ? ?if (video_is_registered(vdev))
? ? ? ? ? ? ? ?ret = vdev->fops->read(filp, buf, sz, off);
? ? ? ?if (vdev->lock)
? ? ? ? ? ? ? ?mutex_unlock(vdev->lock);
? ? ? ?return ret;
}

I'm not all that familiar with v4l, but based on what you've posted,
you need to populate the read routine in your v4l2_fops structure to
support read.

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com

Hi Dave,

2012/2/25 Dave Hylands [off-list ref]:

I'm not all that familiar with v4l, but based on what you've posted,
you need to populate the read routine in your v4l2_fops structure to
support read.

My bad! You are totally right: I forgot about my webcam.
When I did:

$ cat /dev/video0

I was actually getting data from my webcam (which supports read),
and not from the easycap device, which is /dev/video1.

So, everything makes sense now:

$ cat /dev/video1

...
open("/dev/video1", O_RDONLY|O_LARGEFILE) = 3
...
read(3, 0x8835000, 32768)               = -1 EINVAL (Invalid argument)

This is clearly shown by the piece of code mentioned before.

Thanks and sorry for the noise,
Ezequiel.