//-------------------------------------------------------------------
//	tsc.c
//
//	This character driver allows application programs to access
//	the current value of the cpu's Time Stamp Counter register,
//	as if it were a file of length 8 bytes (64 bits).
//
//	Note: Developed and tested with Linux kernel version 2.4.18.
//
//	programmer: ALLAN CRUSE
//	written on: 03 APR 2003
//-------------------------------------------------------------------

#define	__KERNEL__
#define	  MODULE  

#include <linux/module.h>	// for init_module() 
#include <asm/uaccess.h>	// for get_ds(), set_fs() 

#define	SUCCESS	0
#define	ACCESS	0666

static char modname[] = "tsc";
static char devname[] = "/dev/tsc";
static int my_major = 0;


static loff_t filesize = 8;
static unsigned char tsc_value[8];


static ssize_t
my_read( struct file *file, char *buf, size_t count, loff_t *pos )
{
	int	more;
	
	// sanity check: we cannot read beyond end-of-file
	if ( *pos >= filesize ) return -EINVAL;

	// if necessary, adjust the count of bytes to be read
	if ( *pos + count > filesize ) count = filesize - *pos;

	// assembly language is needed (to get cpu register-value)
	asm(" rdtsc ");
	asm(" movl %eax, tsc_value+0 ");
	asm(" movl %edx, tsc_value+4 ");

	// copy requested number of bytes (starting from *pos)
	more = copy_to_user( buf, tsc_value + *pos, count );
	if ( more ) return -EFAULT;

	// update *pos, and return the number of bytes transferred
	*pos += count;
	return	count;
}


static loff_t my_llseek( struct file *file, loff_t offset, int whence )
{
	loff_t	newpos;

	switch( whence )
		{
		case 0:	// SEEK_SET
			newpos = offset;
			break;
		case 1: // SEEK_CUR
			newpos = file->f_pos + offset; 
			break;
		case 2:	// SEEK_END
			newpos = filesize + offset;
			break;
		default: // should never occur
			return -EINVAL;
		}		

	if ( newpos < 0 ) return -EINVAL;
	file->f_pos = newpos;
	return	newpos;
}


static struct file_operations 
my_fops =	{
		owner:		THIS_MODULE,
		read:		my_read,
		llseek:		my_llseek,
		};


int init_module( void )
{
	dev_t	dev_id;
	mode_t	mode;

	printk( "<1>\nInstalling \'%s\' module\n", modname );

	my_major = register_chrdev( 0, modname, &my_fops );
	if ( my_major < 0 ) return  -EBUSY;

	set_fs( get_ds() );
	sys_unlink( devname ); 

	mode = S_IFCHR;
	dev_id = MKDEV( my_major, 0 );
	sys_mknod( devname, mode, dev_id );

	mode = ACCESS;
	sys_chmod( devname, mode );

	return	SUCCESS;
}


void cleanup_module( void )
{
	unregister_chrdev( my_major, modname );
	set_fs( get_ds() );
	sys_unlink( devname );

	printk( "<1>Removing \'%s\' module\n", modname );
}

MODULE_LICENSE("GPL");