register_chrdev vs. cdev_add

register_chrdev vs. cdev_add

[Hinko Kočevar]

Hi,

Testing 2.6.26 (GIT) on cris architecture shown that old style of registering 
char devices with register_chrdev() vs. new style with cdev_init() and 
cdev_add() does not produce the same results.

static unsigned int htrc110_major = 123;
static unsigned int htrc110_minor = 2;

CODE:
dev = MKDEV(htrc110_major, htrc110_minor);
ret = register_chrdev_region(dev, 1, mod_name);
ret = register_chrdev(dev, mod_name, &htrc110_fops);

Using char driver with 123 major and 2 minor and registering it with 
register_chrdev() shows this in /proc/devices:
# cat /proc/devices
Character devices:
   1 mem
   2 pty
   3 ttyp
   4 ttyS
   4 ttyS
   5 /dev/tty
   5 /dev/console
   5 /dev/ptmx
  10 misc
  29 fb
  90 mtd
123 carneol-htrc110
128 ptm
136 pts
180 usb
128974850 carneol-htrc110
189 usb_device
253 usb_endpoint
254 usbmon

CODE:
dev = MKDEV(htrc110_major, htrc110_minor);
ret = register_chrdev_region(dev, 1, mod_name);
cdev_init(&htrc110_dev->cdev, &htrc110_fops);
htrc110_dev->cdev.owner = THIS_MODULE;
htrc110_dev->cdev.ops = &htrc110_fops;
ret = cdev_add(&htrc110_dev->cdev, dev, 1);

While using cdev_init() and cdev_add():
# cat /proc/devices
Character devices:
   1 mem
   2 pty
   3 ttyp
   4 ttyS
   4 ttyS
   5 /dev/tty
   5 /dev/console
   5 /dev/ptmx
  10 misc
  29 fb
  90 mtd
123 carneol-htrc110
128 ptm
136 pts
180 usb
189 usb_device
253 usb_endpoint
254 usbmon

This is just heads up, I know that the old interface to registering char devices 
will be dropped, but when...

regards,
Hinko

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Re: register_chrdev vs. cdev_add

[Jonathan Corbet]

> dev = MKDEV(htrc110_major, htrc110_minor);
> ret = register_chrdev_region(dev, 1, mod_name);
> ret = register_chrdev(dev, mod_name, &htrc110_fops);

If you want to use register_chrdev(), do not make a separate call to
register_chrdev_region().  Those two are not meant to work together.
register_chrdev() implicitly allocates a region of 256 minors for you.

jon