Device node - How does kernel know about it

Device node - How does kernel know about it

[Siva Prasad]

[-- Attachment #1: Type: text/plain, Size: 1967 bytes --]

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


[-- Attachment #2: memtest.c --]
[-- Type: application/octet-stream, Size: 4715 bytes --]


/* Appropriate Kernel header files */
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/poll.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/mm.h>
#include <linux/delay.h>
#include <linux/random.h>
#include <asm/io.h>

/* Defaults */
#define DEFAULT_MAJOR	168
#define DEFAULT_NAME	"memtest"

/* Parameters that can be set by insmod */
static int major = 	DEFAULT_MAJOR;
static char* name = 	DEFAULT_NAME;	

#define uint unsigned int

/* Function Prototypes */
ssize_t memtest_read (struct file *, char *, size_t, loff_t *);
ssize_t memtest_write (struct file *, const char *, size_t, loff_t *);
int memtest_ioctl(struct inode *, struct file *, unsigned int, unsigned long);
int memtest_open (struct inode *, struct file *);
int memtest_release (struct inode *, struct file *);


MODULE_AUTHOR("Siva Prasad.");
MODULE_DESCRIPTION("This a a very simple memory test module");
MODULE_LICENSE("GPL");


/*
 * This must be made static since it is use after the current
 * function returns. It holds the methods used by the driver.
 */
#define NUMMINORS 5
static struct file_operations memtest_fops = {
	.read = memtest_read,
	.write = memtest_write,
	.ioctl = memtest_ioctl,
	.open = memtest_open,
	.release = memtest_release,
	.owner = THIS_MODULE,
};


static dev_t memtestdev;
static struct cdev *memtest_cdev;


#define K(x) ((x) << (PAGE_SHIFT - 10))
#define PAGEFACTOR 32


/*
 * my_init_module()
 *
 * First function called when loaded with insmod. This registers the
 * char driver handler.
 *
 * Returns 0 if successfull, non-zero on failure
 */
int my_init_module(void)
{
	unsigned long total_memory, free_memory, page_size;
	unsigned long testable_memory, testable_pages;
	struct sysinfo meminformation;
	int retval;

	/* Load character driver */
    	memtest_cdev = cdev_alloc();
    	memtestdev = MKDEV(major,0);
	retval = register_chrdev_region(memtestdev,NUMMINORS,name);

	/* Fail to load if register_chardriver fails */
	if (retval < 0)
		return retval; 

	/* If major = 0, it if auto-allocated */
	if (!major) 
		major = retval;

	printk(KERN_EMERG "Memtest module registered with major = %d\n",major);

	memtest_cdev->ops = &memtest_fops;
	memtest_cdev->owner = THIS_MODULE;
	retval = cdev_add(memtest_cdev,memtestdev,NUMMINORS);

	si_meminfo(&meminformation);

	total_memory = K(meminformation.totalram);  // in Kbytes
	total_memory *= 1024;  			    // Regular bytes
	free_memory = K(meminformation.freeram);
	free_memory *= 1024;

	page_size = PAGE_SIZE * PAGEFACTOR;

	printk(KERN_EMERG "System Information...\n");
	printk(KERN_EMERG "Total Memory     = %uK.\n", (uint)(total_memory/1024));
	printk(KERN_EMERG "Free Memory      = %uK.\n", (uint)(free_memory/1024));
	printk(KERN_EMERG "Test Page Size   = %08X.\n", (uint)page_size);

	testable_memory = (free_memory/100)*50;
	testable_pages = testable_memory / (page_size);

	return retval;
}



/*
 * my_cleanup_module()
 *
 * Called when module is removed by insmod. 
 */
void my_cleanup_module(void)
{
	printk(KERN_EMERG "memtest Cleaning up the module.\n");

	cdev_del(memtest_cdev);
	/* Unregister character driver. */
	unregister_chrdev_region(memtestdev,NUMMINORS);
}


// * Returns number of bytes read
ssize_t memtest_read (struct file *filp, char *buf, size_t nbytes, loff_t* ppos)
{
        int tr;
	char stringpass[] = "Dude, here is the info from kernel to user";

	printk(KERN_EMERG "memtest READ.\n");

	tr = 80;
        tr = tr - copy_to_user(buf, stringpass, tr);

        return tr;
}


// * Returns the number of bytes written
ssize_t memtest_write (struct file *filp, const char *buf, size_t nbytes, loff_t* ppos)
{
	printk(KERN_EMERG "memtest WRITE: %d bytes\n",(int)nbytes);

	if (!nbytes)
		return 0;

	return nbytes;
}


// * Returns 0 for success
int memtest_ioctl(struct inode *inode , struct file * filp, unsigned int cmd, unsigned long arg)
{
	printk(KERN_EMERG "memtest IOCTL: cmd = %d\n",cmd);

	// Just return, as there is nothing to do in open.
	return -ENOTTY;
}


// * Returns 0 on success, non-zero on error
int memtest_open (struct inode * inode, struct file * filp)
{
	unsigned int minor = MINOR(inode->i_rdev);

	printk(KERN_EMERG "memtest OPEN: Major=%d Minor=%d Mode=%08x\n",
			MAJOR(inode->i_rdev), MINOR(inode->i_rdev),filp->f_mode);

	if (minor >3 )
		return -ENXIO;

	// Just return, as there is nothing to do in open.
	return 0;
}


// * Returns 0 on success, non-zero on error
int memtest_release (struct inode *inode, struct file * filp)
{
	printk(KERN_EMERG "memtest RELEASED:\n");

	// Just return, as there is nothing to do in release.
	return 0;
}


module_init(my_init_module);
module_exit(my_cleanup_module);


[-- Attachment #3: testmemtest.c --]
[-- Type: application/octet-stream, Size: 574 bytes --]


#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

#define DEVICE "/dev/memtest"
#define BUFFER_SIZE     50

int main(void)
{
	int fd, retval, i;
	char buffer[BUFFER_SIZE*80];
	unsigned long buflength = 0;


	fd = open(DEVICE, O_RDONLY);
	printf("testmemtest: open returned fd=%d.\n", fd);
	if (fd < 0)
	{
		printf("Could not open the device `%s'.\n", DEVICE);
		return -1;
	}

	retval = read(fd, buffer, BUFFER_SIZE*80);
	if (retval < 1)
		printf("Error reading information from %s.\n", DEVICE);

	for (i=0; i<retval; i++) printf("%c", buffer[i]);

	close(fd);

}

Re: Device node - How does kernel know about it

[Phillip Susi]

Siva Prasad wrote:
> 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.

The filesystem sets specific bits in the mode mask and elsewhere in the 
inode to mark the file as a dev node, and which major/minor device 
number it should be linked to.  The kernel device drivers register to 
handle a given device number.

> * 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.

So you did you pass the console=ttyS0 parameter to the kernel?  Did you 
configure your inittab to spawn the getty on the serial port instead of 
/dev/ttyN?  You might want to take a look at the Linux Serial Console 
HOWTO.

RE: Device node - How does kernel know about it

[Siva Prasad]

Thanks for the response Phillip. See below for response.

I already am using "console=ttyS0" as well as set the inittab correctly.

I would like to debug this node behavior. Can you please let me know
which routine gets called first when there is, for example a read() call
from user program. Eventually it will result in a call to what ever read
routine registered, but that must be a call back function or some thing.
Right?

- Siva


-----Original Message-----
From: Phillip Susi [mailto:psusi@cfl.rr.com] 
Sent: Thursday, December 27, 2007 12:10 PM
To: Siva Prasad
Cc: linux-kernel@vger.kernel.org
Subject: Re: Device node - How does kernel know about it

Siva Prasad wrote:
> 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.

The filesystem sets specific bits in the mode mask and elsewhere in the 
inode to mark the file as a dev node, and which major/minor device 
number it should be linked to.  The kernel device drivers register to 
handle a given device number.

> * 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.

So you did you pass the console=ttyS0 parameter to the kernel?  Did you 
configure your inittab to spawn the getty on the serial port instead of 
/dev/ttyN?  You might want to take a look at the Linux Serial Console 
HOWTO.