//-------------------------------------------------------------------
//	dram.c
//
//	This module uses the 'kmap()' function to provide access to
//	all the installed physical memory (includes "high memory").
//	Access here is 'read-only' because we deem it too hazardous
//	to proper system functioning let users arbitrary modify the
//	various parts of physical memory (which includes "critical"
//	kernel data structures).  We implement an 'llseek()' method 
//	so users can find out how much physical ram is installed.    
//	
//	NOTE: Developed and tested with Linux kernel version 2.4.18
//
//	programmer: ALLAN CRUSE
//	written on: 26 FEB 2003
//	revised on: 23 APR 2003
//-------------------------------------------------------------------

#define	__KERNEL__
#define	  MODULE  

#include <linux/module.h>	// for init_module() 
#include <asm/uaccess.h>	// for copy_to_user() 
#include <linux/mm.h>		// for num_physpages
#include <linux/highmem.h>	// for kmap(), kunmap()
#include <asm/page.h>		

#define	SUCCESS	0

static char modname[] = "dram";
static int my_major = 253;	// note static major assignment 
static unsigned long ramtop;

static ssize_t
my_read( struct file *file, char *buf, size_t count, loff_t *pos )
{
	struct page	*pp;
	void		*from;
	int		page_number, page_indent, more;
	
	// we cannot read beyond the end-of-ram
	if ( *pos >= ramtop ) return 0;

	// determine which physical page to temporarily map
	// and how far into that page to begin reading from 
	page_number = *pos / PAGE_SIZE;
	page_indent = *pos % PAGE_SIZE;
	
	// map the designated physical page into kernel space
	pp = &mem_map[ page_number ];
	from = kmap( pp ) + page_indent;
	
	// cannot reliably read beyond the end of this mapped page
	if ( page_indent + count > PAGE_SIZE ) count = PAGE_SIZE - page_indent;

	// now transfer count bytes from mapped page to user-supplied buffer 	
	more = copy_to_user( buf, from, count );
	
	// ok now to discard the temporary page mapping
	kunmap( pp );
	
	// an error occurred if less than count bytes got copied
	if ( more ) return -EFAULT;
	
	// otherwise advance file-pointer and report number of bytes read
	*pos += count;
	return	count;
}

static loff_t my_llseek( struct file *file, loff_t offset, int whence )
{
	loff_t	newpos = -1;

	switch( whence )
		{
		case 0: newpos = offset; break;			// SEEK_SET
		case 1: newpos = file->f_pos + offset; break; 	// SEEK_CUR
		case 2: newpos = ramtop + offset; break; 	// SEEK_END
		}
	if (( newpos < 0 )||( newpos > ramtop )) return -EINVAL;
	file->f_pos = newpos;
	return	newpos;
}

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


int init_module( void )
{
	printk( "<1>\nInstalling \'%s\' module ", modname );
	printk( "(major=%d)\n", my_major );
	
	ramtop = num_physpages * PAGE_SIZE;
	printk( "<1>  ramtop=%08X (%d MB)\n", ramtop, ramtop >> 20 );
	return 	register_chrdev( my_major, modname, &my_fops );
}

void cleanup_module( void )
{
	unregister_chrdev( my_major, modname );
	printk( "<1>Removing \'%s\' module\n", modname );
}

MODULE_LICENSE("GPL");