//-------------------------------------------------------------------
//	mymmfb.c
//
//	This character driver gives an application direct access to
//	the display frame-buffer memory at a predetermined address,
//	but instead of doing this via the 'mmap()' system call, the 
//	required page-directory entries are directly installed when 
//	the device file is opened (and removed when the device file
//	is closed).  Doing this is a bold and risky scheme, because 
//	it completely bypasses the kernel memory-management system,
//	and there is no guarantee that the kernel will never try to 
//	'overwrite' our assigned frame-buffer region.  However, for
//	purposes of exploration it is reasonable to test this idea.   
//	One advantage it offers over the Linux 'mmap()' function is
//	that no additional ram is consumed (by creating page-tables
//	and a 'vm_area_struct' object).  But disadvantages exist as
//	well, which we can explore in our class lectures/exercises. 
//
//	NOTE: Developed and tested with Linux kernel version 2.4.20
//
//	programmer: ALLAN CRUSE
//	written on: 28 APR 2003
//-------------------------------------------------------------------

#define	__KERNEL__
#define	  MODULE  

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

#define	SUCCESS	0
#define	VGA_CLASS 0x030000	// for VGA Display Adapters
#define USERADDR 0x10000000	// fixed frame-buffer address

static char modname[] = "mysis";
static char devname[] = "/dev/vram";
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 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 = fb_size + offset; break;	// SEEK_END
		}
	if (( newpos < 0 )||( newpos > fb_base )) return -EINVAL;
	file->f_pos = newpos;
	return	newpos;
}



static int my_open( struct inode *inode, struct file *file )
{
	unsigned long	*pgd = (unsigned long*)current->mm->pgd;
	unsigned long	physaddr = fb_base;
	unsigned long	virtaddr = USERADDR;
	int		entry_index, entry_count, i;
	
	// map the frame-buffer to the predetermined user-space address  

	entry_index = virtaddr >> 22;		// index of starting entry 	
	entry_count = fb_size >> 22;		// divide by 4MB page-size

	for (i = 0; i < entry_count; i++) 
		{
		pgd[ entry_index ] = physaddr | 0x87;	// Present, 4MB-entry	
		physaddr += (4<<20);			// advance 4 megabytes
		++entry_index;
		}
	
	return	SUCCESS;	
}	



static int my_release( struct inode *inode, struct file *file )
{
	unsigned long	*pgd = (unsigned long*)current->mm->pgd;
	unsigned long	virtaddr = USERADDR;
	int		entry_index, entry_count, i;
	
	// unmap the frame-buffer at the agreed user-space address  

	entry_index = virtaddr >> 22;		// index of starting entry 	
	entry_count = fb_size >> 22;		// divide by 4MB page-size

	for (i = 0; i < entry_count; i++) 
		{
		pgd[ entry_index ] = 0;		// Page Not Present 	
		++entry_index;
		}
	
	return	SUCCESS;	
}	


static struct file_operations 
my_fops =	{
		owner:		THIS_MODULE,
		open:		my_open,
		llseek:		my_llseek,
		release:	my_release,	
		};



int init_module( void )
{
	struct pci_dev	*devp = NULL;
	u32		save, temp, i;

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

	// detect VGA device and store frame-buffer address
	devp = pci_find_class( VGA_CLASS, devp );
	if ( !devp ) return -ENODEV;
	fb_base = pci_resource_start( devp, 0 );

	// determine the frame-buffer's size
	temp = ~0;
	pci_read_config_dword( devp, 0x10, &save );
	pci_write_config_dword( devp, 0x10, temp );
	pci_read_config_dword( devp, 0x10, &temp );
	pci_write_config_dword( devp, 0x10, save );
	for (i = 4; i < 32; i++) if ( temp & (1<<i) ) break;
	fb_size = (1<<i);

	// show device identification and frame-buffer's location and size
	printk( "<1>  VGA Device: %s \n", devp->name );
	printk( "<1>  Frame-Buffer at %08X (%d MB)\n", fb_base, fb_size>>20 );
	
	// register this device-driver's operations with the kernel
	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");