function of major/minor device number

[hz hanks]

 Hey Alexandru Juncu

Thank you for your help again. Sorry about that mistake, I just didn't
notice the forward address when using quick reply. It won't happen
again.

Best.

2012/2/12 Alexandru Juncu <alex.juncu at rosedu.org>:
> 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.