function of major/minor device number

[Alexandru Juncu]

2012/2/12 hz hanks <hankshz at gmail.com>:
> 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 <alex.juncu at rosedu.org>:
>> 2012/2/12 hz hanks <hankshz at gmail.com>:
>>> 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.