function of major/minor device number

2012/2/12 hz hanks [off-list ref]:

Hey Alexandru Juncu

Thank you so much! You are so helpful for me.

As far as I'm concerned, the point that confuses me is the definition
of ONE device driver. For example, I would consider a file of some
source codes, that manipulate the GPIO and use ?misc_register to
register as ?miscdevice, as one device driver. However, I know that it
will share the major device number 10 with other device drivers such
as AD/DA. So how kernel works like that? In other words, how kernel
works with major/minor device number? For example, will the kernel
allocate some specific resources to drivers with major number 4
because it knows those drivers are for Serial ports. How about the
minor number, will the kernel care about that?

Hello!

First of all, you should always reply to the list, because when there
are 100 people rather than one, you have a bigger chance of getting an
answer.

Is hard to define ONE device driver. Linux is a monolithic kernel.
It's one big code. But linux is also modular and you can have
dynamically loaded modules. But it's very flexible... you can have a
module (a .ko file) that is a driver for two devices (not going to
find this, but it could happend).

You usually have one .ko for one device driver for one device. This
would mean one major. For example, a device driver for all the SATA
devices.

[0] http://en.wikipedia.org/wiki/Monolithic_kernel

Thank you again!

2012/2/12 Alexandru Juncu [off-list ref]:

quoted

2012/2/12 hz hanks [off-list ref]:

quoted

Hi, all

I have a question about major/minor device number. I know that one can
use int register_chrdev_region(dev_t from, unsigned count, const char
*name); to register a driver with a unique pair of major/minor device
number. But what's the difference between two drivers: in one case
they have same major device number but different minor device number;
on another case they have different major device number but same minor
device number. When I searched the Internet, it said that major device
number is to identify the driver while the minor device number is to
identify specific hardware. But I'm still confused because it seems
legal to share the major device number with different drivers.

Please help me?Thank you!

Hello!

I would redirect you to the /dev fs to do a long listing of its
contents and notice the pattern.
Devices with the same major, are usually handled by the same device
driver. If that device driver needs to handle several devices, it uses
an array of minors.

Example:

crw-rw---- ? 1 root ?dialout ? 4, ?64 2011-12-12 16:25 ttyS0
crw-rw---- ? 1 root ?dialout ? 4, ?65 2011-12-12 16:25 ttyS1
crw-rw---- ? 1 root ?dialout ? 4, ?66 2011-12-12 16:25 ttyS2
crw-rw---- ? 1 root ?dialout ? 4, ?67 2011-12-12 16:25 ttyS3

Notice that all the Serial ports have the same major (4) but different
minors (64-67). If you do a "cat /proc/devices" you will notice that
the major 4 is registered by ttyS device driver. ?"ttyS" is the name
variable in the register_chrdev_region call.

A device driver can register zero, one ore more majors. And for each
of that major, it can a number of minors (the count parameter in
register_chrdev_region).

Two devices having the same minor but different majors is just a
coincidence, they have nothing in common. It's just an obvious thing
because a major has more minors so there will be more minors ?that
majors.

Hope this helps.