Device node - How does kernel know about it

Siva Prasad sprasad at
Fri Dec 28 14:27:17 EST 2007

Thank you Jon and Nicholas.

I already have "console=ttyS0" in the kernel command line. That is not
helping me.

I looked at the major/minor numbers with a good working system and it
looks correct for the nodes created in ramdisk.

What is the kernel routine that is first called when there is, for
example a read() function call from user program? 
I would like to start debugging from there and see if any thing at all
happens when there is a call. Appreciate your help with this question.


-----Original Message-----
From: Nicholas Mc Guire [mailto:der.herr at] 
Sent: Friday, December 28, 2007 12:39 AM
To: Siva Prasad
Cc: linuxppc-dev at; linuxppc-embedded at
Subject: Re: Device node - How does kernel know about it

Hash: SHA1

> * Ramdisk is also executing fine, just that prints are not coming out
> serial. I can see the execution of various user programs with a printk
> in sys_execve() routine. Ramdisk has all the required files like
> /dev/console, /dev/ttyS0, etc.
> * Looking further into tty driver, I noticed that call to tty_write()
> do_tty_write() is not happening at all. So, somewhere the interface
> between kernel and user program is lost.
> * Just to check it out, I tried to write a small kernel module and a
> test program.
>  - Attached memtest.c module (not really testing memory there. :-))
>  - Attached testmemtest.c user program, that just open's it and reads
> the information
>  - Created a device node using "mknod /dev/memtest c 168 0"
>  - When I do "insmod memtest.ko" inside the ramdisk bootup scripts, I
> could see all the printk's on the console
>  - When I execute "testmemtest" next in the same script, it does not
> display the printk inside of memtest.c module. This only indicates
> read call did not really go to the kernel side.
>  - Just to check my program's validity, I checked on a similar machine
> and all the code works fine.
>  - "uname -r" also matches with what I built. So, chances of exiting
> from open call because of mismatch is remote. Since userland cannot
> print, I have no idea what exactly is happening there.
The kernel will simply look at the major:minor numbers - so maybe you
simply have a wrong major/minor for /dev/ttyS0 ? in that case you will
see nothing but other than that most things will go on working.

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