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SubjectV4L2 & request_module("char-major-...")
Date
Hi all,

I'm in the process of converting v4l2-dev.c (2.6.27, in earlier kernels
it was called videodev.c) from using register_chrdev() to using
register_chrdev_region() and cdev_add().

The problem I have is that register_chrdev provides a file_operations
struct. The video_open() function in there performs this piece of code
when a video device is opened:

vfl = video_device[minor];
if (vfl == NULL) {
mutex_unlock(&videodev_lock);
request_module("char-major-%d-%d", VIDEO_MAJOR, minor);
mutex_lock(&videodev_lock);
vfl = video_device[minor];
if (vfl == NULL) {
mutex_unlock(&videodev_lock);
unlock_kernel();
return -ENODEV;
}
}

It checks if a V4L2 driver registered this particular minor, if not then
it tries to load the appropriate module with a char-major-x-x alias. If
still no luck, then we bail out.

Now, as I understand it this can only happen if someone used mknod to
create device nodes and is not using udev. It's not clear to me however
how this can ever work: the char-major alias relies on the fact that
someone has to link the minor number with an actual V4L driver, but you
do not generally know what minor number will be used by a specific
driver (depends on load order, etc). Or am I missing something?

My questions are:

1) is this code still relevant?

2) if so, how can I replace this code when I switch to cdev since in
that case there is no longer a video_open() that can be used for this.

3) I also saw after some googling this proposed patch:
http://linux.derkeiler.com/Mailing-Lists/Kernel/2003-09/2925.html
It adds a MODULE_ALIAS_CHARDEV_MAJOR line for videodev.c. Either this
patch was never applied or quickly removed in videodev.c since it's not
there. I'm not sure how this relates to the request_module in the
current code and whether it should be added after all or not.

I had a hard time finding any useful information about this, so I hope
someone can shed some light on this. It's the last piece of the puzzle
that I need before I can drag the old and venerable v4l2-dev.c formerly
known as videodev.c into the 21st century :-)

Regards,

Hans


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