Major and minor numers - mknod

[BrickTop]

How can I find out the available major numbers and also whats the rule of thumb for using minor numbers??

Thanks in advance..

[basant]

Major numbers

For ODDI and pre-DDI 8 drivers, the major number identifies the device class or group, such as a controller for several terminals. It is used, for example, to tell the kernel which device driver's open routine to call. The major number is assigned, sequentially, to each device driver by the Installable Driver Tools (idtools) during driver installation.

Character major numbers and block major numbers are assigned separately for devices that are exclusively block or character. This means that two separate special files for two different device drivers may appear to have the same number assigned to them. A device that supports both block and character access (for example, the floppy driver), may have different major numbers for the character and block device files.

Minor numbers

For ODDI and pre-DDI 8 drivers, the minor number identifies a specific device, such as a single terminal. Minor numbers are assigned to special files by the driver writer in another system configuration file called the Node file

Minor numbers are typically used to distinguish subdevices, but they can also be used to convey other information. For example, consider a floppy disk controller that can read and write data from floppies in several formats, and can also manage two floppy drives. When a special file associated with the floppy driver is opened, the minor number associated with the file, and available to the open( ) routine as one of its arguments, is used to identify to the floppy driver both which drive to access, and what format to assume for the I/O operation. In this particular case, the least significant bit of the minor number could be used to identify the drive, and the remaining bits used to indicate the format.

A device special file is uniquely identified by its major and minor number:

#ll

brw-r----- 1 bin sys  31    0x09f000 Jul 7 15:55  c9t15d0

block device                                     major #    minor #                                               file name

the major number of a certain driver can be determined using thelsdev(1M) command, e.g.:

# lsdev  -b  31

Character             Block               Driver                 Class

188                        31                     sdisk                 disk

# lsdev | grep lvm

             64               64                   lv                          lvm

Make Node (mknod)

The mknod command should only be used to create non-standard device files like e.g. for the SCSI pass through driver (spt) or the volume group control file:

# mknod name b|c major minor

where above terms are

name    name of the special file

b|c          block or character special

major     the major number

minor     the minor number

select a unique minor number for the VG:

# ll /dev/*/group
crw-r--r-- 1 root sys 64       0x000000 Apr 4  2001  /dev/vg00/group                     
crw-r--r-- 1 root sys 64       0x010000 Oct 26 15:52 /dev/vg01/group
crw-r--r-- 1 root sys 64       0x020000 Aug 2  15:49 /dev/vgsap/group
                      Major    Minor
                      Number   Number

create the VG control file (group file):
# mkdir /dev/vgnew
# mknod /dev/vgnew/group c 64 0x030000
when we create device file for volume group, major number (64) remain unchanged ,
we take next available minor number after 0x020000.

[Patrick Wallek]

If you run the command 'lsdev' the first column lists major numbers and their associated drivers.

As far as minor numbers go -- It depends on what you are doing.  If it is LVM work (with the group file for example) then just use the next available minor number.

If you are working with other device files, like tape drives, then the minor number can control tape drive options.  You can look at other device files for examples.

[Matti_Kurkela]

The "lsdev" command will tell you the major numbers that are currently in use, and the device driver each one  is assigned to.

The usage of the minor device numbers is independently determined by each driver: the rule of thumb would be "read the documentation of the driver", typically in the section 7 of the man pages. (see "ls /usr/share/man/man7.Z")

Sometimes the driver names listed by lsdev don't match exactly to man page names, so some knowledge (or trial&error) is required. Example (HP-UX 11.23):

# lsdev
    Character     Block       Driver          Class
        0          -1         cn              pseudo
        1          -1         asio0           tty
[... a lot of device entries...]
      203          -1         sctl            ctl
[... still more...]

 Character device 203 is associated with driver "sctl", the SCSI pass-through driver.

"man 7 sctl" produces no results, because the man page name is "scsi_ctl".

"man 7 scsi_ctl" produces a man page that includes instructions for constructing appropriate minor numbers for SCSI pass-through device nodes.

But if you need major numbers for LVM VG creation/import operations, the rules are simple:

• for LVM 1.0 (the only version of LVM on HP-UX 11.23 and older) the major number is always 64.
• for LVM 2.x (available on HP-UX 11.31+ only) the major number is 128.
• for /dev/vg*/group devices, the minor number must be 0xNN0000, where NN is a two-digit hexadecimal number that must be a) unique for this VG on the system, and b) between 0 and the value of kernel parameter "maxvgs". For example, if maxvgs is 10 (decimal), then the NN component for the VG group device can be between 00..0A (inclusive). Note that value 00 is normally reserved for vg00.

If you're developing a new driver for HP-UX, the Driver Development Kit probably has advice for choosing the major number for your driver. Once you have a major number, you can decide how to allocate minor numbers as you see fit.

[BrickTop]

Thank you for your responses. How do we assign points now?!?

[Matti_Kurkela]

Just click on the "kudos" star at the left column, next to the post you feel deserves recognition.

With the former ITRC points system, you could assign points to your own threads only; but this forum allows assigning kudos to any posts except your own.

You can also mark your thread as "solved", and mark the message(s?) that contained the best/accepted solution(s?).