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writing devicedrivers

Hello,

I'm reading Linux device drivers 2nd edition by Alessandro Rubini & Jonathan Corbet.
there is a piece of code in chapter 3 (http://www.xml.com/ldd/chapter/book/ch03.html) I don't understand:

int scull_open(struct inode *inode, struct file *filp)
{
 Scull_Dev *dev; /* device information */
 int num = NUM(inode->i_rdev);
 int type = TYPE(inode->i_rdev);

 /*
  * If private data is not valid, we are not using devfs
  * so use the type (from minor nr.) to select a new f_op
  */
 if (!filp->private_data && type) {
  if (type > SCULL_MAX_TYPE) return -ENODEV;
  filp->f_op = scull_fop_array[type];
  return filp->f_op->open(inode, filp); /* dispatch to specific open */
 }

If I read correctly the open syscall will set filp->private_data to NULL. Since
open in only used once to get a filedescriptor (I presume) the code in the if-block will never be 
excuted and there will be no dispatching for different minor devicefiles.
Anyone who can tell me where I'm going wrong?

Richard

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