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Date:	Wed, 26 Dec 2007 17:34:04 -0800
From:	"Siva Prasad" <sprasad@...io.net>
To:	<linux-kernel@...r.kernel.org>
Subject: Device node - How does kernel know about it

Hi,

How do the device nodes work as an interface between user and kernel
programs, and how to go debugging it? 
This is as part of my debugging effort on an embedded board.

* It all started with the problem of "not printing" any thing that comes
from ramdisk (echo and printf statements), while kernel printk's work
perfectly fine.
* Ramdisk is also executing fine, just that prints are not coming out of
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() or
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 that
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.

Now going back to the original question...
How does a kernel know about device nodes and how to link with it.
Basically I believe interface between user programs and kernel is lost
at device nodes.

Appreciate any help in continuing my debugging efforts.

Thanks
Siva


Download attachment "memtest.c" of type "application/octet-stream" (4715 bytes)

Download attachment "testmemtest.c" of type "application/octet-stream" (574 bytes)

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