//-------------------------------------------------------------------
//	vram.c
//
//	This character-mode Linux device-driver allows applications
//	to both read and write directly to graphics display memory.   
//	It invokes the kernel's PCI bus-interface routines, to find
//	the physical address of the display adapter's video memory;
//	it calls the kernel's 'ioremap()' and 'iounmap()' functions 
//	to setup and destroy temporary mappings of VRAM page-frames 
//	into the Pentium processor's virtual address-space; it uses 
//	the kernel's 'memcpy_toio()' and 'memcpy_fromio()' to write
//	and read these mapped pages of video memory. 
//
//	In this driver the device major number is hard-coded as 99,
//	and it is left to the user (or the system administrator) to
//	create the corresponding device-node, using the 'mknod' and 
//	'chmod' shell commands:
//
//		root# mknod /dev/vram c 99 0
//		root# chmod a+rw /dev/vram 
//	
//	NOTE: Written and tested using Linux kernel version 2.4.21.
//
//	programmer: ALLAN CRUSE
//	written on: 26 APR 2003
//	revised on: 18 NOV 2004 -- for Linux kernel version 2.4.26.
//-------------------------------------------------------------------

#include <linux/module.h>	// for init_module() 
#include <linux/pci.h>		// for pci_find_class() 

#define VGA_CLASS 0x030000	// for VGA Display Adapters
#define VRAM_SIZE (8 << 20)	// access only 8 megabytes

static char modname[] = "vram";
static struct pci_dev *devp = NULL;
static int my_major = 99;	// driver's assigned major-number
static unsigned long fb_base;	// physical address of frame-buffer
static unsigned long fb_size;	// size of the frame-buffer (bytes)

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

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

static ssize_t
my_write( struct file *file, const char *buf, size_t count, loff_t *pos )
{
	unsigned long	phys_address = fb_base + *pos;
	unsigned long	page_number = phys_address / PAGE_SIZE;
	unsigned long	page_indent = phys_address % PAGE_SIZE;
	unsigned long	where;
	void		*vaddr;

	// sanity check: we cannot write anything past end-of-vram
	if ( *pos >= fb_size ) return 0;

	// we can only write up to the end of the current page
	if ( page_indent + count > PAGE_SIZE ) count = PAGE_SIZE - page_indent;

	// ok, map the current page of physical vram to virtual address
	where = page_number * PAGE_SIZE;	// page-addrerss
	vaddr = ioremap( where, PAGE_SIZE );	// setup mapping
	
	// copy 'count' bytes from the caller's buffer to video memory
	memcpy_toio( vaddr + page_indent, buf, count );
	iounmap( vaddr );			// unmap memory
	
	// tell the kernel how many bytes were actually transferred
	*pos += count;
	return	count;
}

static ssize_t	
my_read( struct file *file, char *buf, size_t count, loff_t *pos )
{
	unsigned long	phys_address = fb_base + *pos;
	unsigned long	page_number = phys_address / PAGE_SIZE;
	unsigned long	page_indent = phys_address % PAGE_SIZE;
	unsigned long	where;
	void		*vaddr;

	// sanity check: we cannot read anything past end-of-vram
	if ( *pos >= fb_size ) return 0;

	// we can only read up to the end of the current page
	if ( page_indent + count > PAGE_SIZE ) count = PAGE_SIZE - page_indent;

	// ok, map the current page of physical vram to virtual address
	where = page_number * PAGE_SIZE;	// page-addrerss
	vaddr = ioremap( where, PAGE_SIZE );	// setup mapping
	
	// copy 'count' bytes from video memory to the caller's buffer 
	memcpy_fromio( buf, vaddr + page_indent, count );
	iounmap( vaddr );			// unmap memory
	
	// tell the kernel how many bytes were actually transferred
	*pos += count;
	return	count;
}

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


int init_module( void )
{
	printk( "<1>\nInstalling \'%s\' module ", modname );
	printk( "(major=%d) \n", my_major );

	// identify the video display device
	devp = pci_find_class( VGA_CLASS, devp );
	if ( !devp ) return -ENODEV;
	printk( "<1>  %s \n", devp->name );
	
	// determine location and length of the frame buffer
	fb_base = pci_resource_start( devp, 0 );
	fb_size = VRAM_SIZE;
	printk( "<1>  address-range of frame-buffer: " );
	printk( "%08lX-%08lX ", fb_base, fb_base+fb_size );
	printk( "(%lu MB) \n", fb_size >> 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");