2.6.13 char driver not working on 2.6.22

I looked at your code before I finished reading your post... my first suggestion was going to be to add an open() function, but now I see you already tried that. It shouldn't be necessary anyways, the char device code does not require an open() function.

What precise version of the kernel are you using, 2.6.22.what? There were about 15 different revisions of 2.6.22.

Another thought, and I have no real reason to think this might fix it, but it's consistent with other kernel drivers, is to add static const to the file_operations declaration:

  static const struct file_operations ik220_fops = {
  ...
  };

Another thing you might want to do, again for consistency with newer code, is to change the syntax of the structure initializers:

  static struct pci_driver ik220_driver = {
          .name          = DRV_NAME,
          .id_table      = ik220_tbl,
          .probe         = ik220_init_one,
          .remove        = ik220_remove_one,
  };

  static const struct file_operations ik220_fops = {
          .ioctl         = ik220_driver_ioctl,
  };

The "name: value" syntax was long ago deprecated by gcc.

Since the code dies before it gets to your open() function (when you have one), the next move is probably to add some debugging code to the kernel proper. A good starting point is the function chrdev_open() in the file fs/char_dev.c. That's where character devices are "opened", it's the function that calls your open() function.