device number registration Post: 302673633

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Top Forums UNIX for Advanced & Expert Users device number registration Post 302673633 by dr_mabuse on Wednesday 18th of July 2012 07:35:01 AM

07-18-2012

Registered User

device number registration

I would THANK if you answer my doubts in DETAILS please.

Here's a sample code to create a device:

Code:

if(scull_major) {
  dev=MKDEV(scull_major, scull_minor);
  result=register_chrdev_region(dev, scull_nr_devs, "scull");
} else{
   result=alloc_chrdev_region(&dev, scull_minor, scull_nr_devs,  "scull");
 scull_major=MAJOR(dev);
}

if(result < 0)
{
   printk(KERN_WARNING "scull:can't get major %d\n", scull_major);
   return result;
}

1. I didn't understand the first line! In the second and third lines, a device is created using the given minor and major numbers (I am sure this is a static allocation) and register_chrdev creates multiple instances of it.
what about the first line? I can't understand what that if(major number) doing there, what's so relevant to it?

2. if the major number is not created it goes to while n creates a dynamic major number. Major number means the driver ID to which the device associates with, OK? but so far, how would the kernel know about which driver it shall connect it? it just allocates one dynamically!

3.

Quote:

register_chrdev_region works well if you know ahead of time exactly which device numbers you want. Often, However, you will not know which major numbers your device will use.

!!! what's so relevant between major number and device numbers in 3? major number is the driver number the device associates with and it's not the device number (that's minor number). So, why 3 notes that?

4. Why do we need a seperate script to read /proc/devices file?

dr_mabuse

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LEARN ABOUT SUNOS

getmajor

getmajor(9F)						   Kernel Functions for Drivers 					      getmajor(9F)

NAME

       getmajor - get major device number

SYNOPSIS

       #include <sys/types.h>
       #include <sys/mkdev.h>
       #include <sys/ddi.h>

       major_t getmajor(dev_t dev);

INTERFACE LEVEL

       Architecture independent level 1 (DDI/DKI).

PARAMETERS

       dev	Device number.

DESCRIPTION

       getmajor() extracts the major number from a device number.

RETURN VALUES

       The major number.

CONTEXT

       getmajor() can be called from user or interrupt context.

EXAMPLES

       Example 1: Using getmajor()

       The  following example shows both the  getmajor() and getminor(9F) functions used in a debug  cmn_err(9F) statement to return the major and
       minor numbers for the device supported by the driver.

	      dev_t dev;

	      #ifdef DEBUG
	      cmn_err(CE_NOTE,"Driver Started.	Major# = %d,
		   Minor# = %d", getmajor(dev), getminor(dev));
	      #endif

SEE ALSO

       cmn_err(9F), getminor(9F), makedevice(9F)

       Writing Device Drivers

WARNINGS

       No validity checking is performed.  If  dev is invalid, an invalid number is returned.

SunOS 5.10							    11 Apr 1991 						      getmajor(9F)