Originally from Gerd...

I've just noticed that a hole left in the recent changes which should
allow the usb v4l drivers to unregister with open file handles.  The
drivers itself handle it just fine, but video_generic_ioctl() will barf
when called on unregistered devices.  Oops.

One way to fix this is to expect drivers call the helper function and
pass a pointer for the function doing the actual work, i.e. handle it
this way:

driver_ioctl(inode,file,cmd,userptr)
	-> video_usercopy(inode,file,cmd,userptr,func)
		copy_from_user(...)
		-> func(inode,file,cmd,kernelptr);
		copy_to_user(...)

Patch against 2.5.7-pre2 below. It updates videodev.[ch] and adapts
usbvideo.c to show how the driver changes will look like.

Note that this change makes the usercopy helper function a very generic
one, it probably could be used for other drivers to (as long as the API
has sane magic numbers based on _IO*(...) defines) as there is no
video4linux-related stuff in there any more.  So we might think of
renaming it an moving it to some more central place (fs/ioctl.c maybe).