[SOLVED] my device driver is dropping characters

jamesbon · 09-23-2010, 11:59 AM

I wrote a small hello world type of character device driver.
When I type echo -n "abcdef" > /dev/bond
and do a cat /dev/bond
then only last "f" of above input abcdef is displayed rest nothing is displayed.
I asked this question earlier and some people suggested me some modifications
I have done and experimented all that but I am unable to catch the error.

Can some one point out the error?
Here is the code

Code:

/* Necessary includes for device drivers */
#include <linux/init.h>
//#include <linux/config.h>
#include <linux/module.h>
#include <linux/kernel.h> /* printk() */
#include <linux/slab.h> /* kmalloc() */
#include <linux/fs.h> /* everything... */
#include <linux/errno.h> /* error codes */
#include <linux/types.h> /* size_t */
#include <linux/proc_fs.h>
#include <linux/fcntl.h> /* O_ACCMODE */
#include <asm/system.h> /* cli(), *_flags */
#include <asm/uaccess.h> /* copy_from/to_user */

MODULE_LICENSE("Dual BSD/GPL");

/* Declaration of memory.c functions */
int memory_open(struct inode *inode, struct file *filp);
int memory_release(struct inode *inode, struct file *filp);
ssize_t memory_read(struct file *filp, char *buf, size_t count, loff_t *f_pos);
ssize_t memory_write(struct file *filp, char *buf, size_t count, loff_t *f_pos);
void memory_exit(void);
int memory_init(void);

/* Structure that declares the usual file */
/* access functions */
struct file_operations memory_fops = {
  read: memory_read,
  write: memory_write,
  open: memory_open,
  release: memory_release
};
/* Declaration of the init and exit functions */
module_init(memory_init);
module_exit(memory_exit);

/* Global variables of the driver */
/* Major number */
int memory_major = 60;
/* Buffer to store data */
char *memory_buffer;

int memory_init(void) {
  int result;

  /* Registering device */
  result = register_chrdev(memory_major, "bond", &memory_fops);
  if (result < 0) {
    printk(KERN_ALERT  "memory: cannot obtain major number %d\n", memory_major);
    return result;
  }

  /* Allocating memory for the buffer */
  memory_buffer = kmalloc(1, GFP_KERNEL); 
  if (!memory_buffer) { 
    result = -ENOMEM;
    goto fail; 
  } 
  memset(memory_buffer, 0, 10);

  printk(KERN_ALERT "Inserting bond module\n"); 
  return 0;

  fail: 
    memory_exit(); 
    return result;
}


void memory_exit(void) {
  /* Freeing the major number */
  unregister_chrdev(memory_major, "bond");

  /* Freeing buffer memory */
  if (memory_buffer) {
    kfree(memory_buffer);
  }

  printk( KERN_ALERT "Removing bond module\n");

}


int memory_open(struct inode *inode, struct file *filp) {

  /* Success */
  return 0;
}


int memory_release(struct inode *inode, struct file *filp) {
 
  /* Success */
  return 0;
}


ssize_t memory_read(struct file *filp, char *buf, 
                    size_t count, loff_t *f_pos) { 
 
  /* Transfering data to user space */ 
  copy_to_user(buf,memory_buffer,10);

  /* Changing reading position as best suits */ 
  if (*f_pos == 0) { 
    *f_pos+=1; 
    return 1; 
  } else { 
    return 0; 
  }
}

ssize_t memory_write( struct file *filp, char *buf,
                      size_t count, loff_t *f_pos) {

  char *tmp;

  tmp=buf+count-1;
  copy_from_user(memory_buffer,tmp,10);
  return 1;
}

linux_hy · 09-26-2010, 03:14 AM

it maybe as following the red font

ssize_t memory_write( struct file *filp, char *buf,
size_t count, loff_t *f_pos) {

char *tmp;

//tmp=buf+count-1;
tmp=buf; copy_from_user(memory_buffer,tmp,10);
return 1;
}

archieval · 09-26-2010, 03:18 AM

What error are you expecting when your driver does not return any error code? Put the necessary negative error return values, and ensure your variables point to valid memory addresses.

Regards,
archieval

jamesbon · 09-26-2010, 03:53 AM

Ok here are the mistakes I did
Mistake 1)

Code:

 memory_buffer = kmalloc(1, GFP_KERNEL);

allocated just 1 byte
Mistake 2)

Code:

  /* Changing reading position as best suits */
  if (*f_pos == 0) {
    *f_pos+=1;
    return 1;
  } else {
    return 0;
  }

read one character when *f_pos is at the start of file and otherwise reads nothing:
Mistake 3
in fucntion memory_write only saving the last character written:

Code:

...
  tmp=buf+count-1; // see this is the last one in "buf"
  copy_from_user(memory_buffer,tmp,1); // and that's all you copy

So these were the mistakes.
The functions that worked was

Code:

ssize_t memory_read(struct file *filp, char __user *buf, 
		size_t count, loff_t *f_pos) 
{
	if (*f_pos > 0)
		return 0;
	if (count > strlen(memory_buffer))
		count = strlen(memory_buffer);
	copy_to_user(buf,memory_buffer,count);
	*f_pos = *f_pos + count;

	return count; 
}

Similarly

Code:

ssize_t memory_write( struct file *filp, const char __user *buf,
		size_t count, loff_t *f_pos) 
{
	copy_from_user(memory_buffer, buf, count);
	return count;
}