/*
  Note: Previously this program did not unlock the extended registers,
        so it only worked if the registers happened to be unlocked.

  This program sets the 'memory clock' of Cirrus 5424/26/28 cards.
  The first three values could be set by utility programs that
  came with my card (AVGA3), but somewhat higher values seem to work (on my
  card at least). It may be that better and more recent Cirrus cards use a
  higher value as boot-up default. It should depend on DRAM speed, but it
  seems to be more dependant on the card logic.
  
  I have the impression that many Cirrus 542x cards suffer from horrible
  BIOS version/DRAM timing misconfurations. Perhaps even some versions of
  MS-Windows drivers change the MCLK register. In any case, the boot-up BIOS
  default (0x1c) may be inappropriately low for the type of DRAM timing most
  cards use.

  Using a higher memory clock gives a very significant performance improvement;
  with high dot clock modes (like 640x480x16M or 1150x900x256) performance can
  be more than twice that of the standard 50 MHz clock. This goes for both
  (VLB) framebuffer access and accelerated features (bitblt). This also helps
  XFree86 server performance, but only if the XFree86 Cirrus driver doesn't
  set the memory clock register (it should work for XFree86 1.3 and 2.0).
  Use at your own risk!

  Note that the 'dot clock' is something entirely different. There does not
  seem to be much correlation between the two (i.e. if a high dot clock gives
  screen problems, using a high memory clock is not likely to fix it, other
  than improving speed).

*/


#define NEW_MCLK 0x26


#include <stdlib.h>
#include <unistd.h>
#include <vga.h>
#include "../libvga.h"	/* For port I/O macros. */

void main() {
	vga_init();
	if (vga_getcurrentchipset() != CIRRUS) {
		printf("Not a Cirrus.\n");
		exit(-1);
	}

	/* Unlock extended registers. */
       	outb(0x3c4, 0x06);
       	outb(0x3c5, 0x12);

	outb(0x3c4, 0x1f);

	/*
		Tested on VLB AVGA3 CL-GD5426 on 40MHz VLB
		0x1c		50 MHz (boot-up default)
		0x21		59
		0x22		62
		0x23 OK
		0x24 OK
		0x25 OK
		0x26 OK		[This corresponds to the highest value
				mentioned in the databook (which does
				not mean that it is supposed to work
				on all cards); the book also says the
				parts are not specified for more than
				50 MHz].
		0x27 occasional loose pixels
		0x28 occasional problems
		0x29 some problems
		0x2A 16M/textmode problems
		0x2c 256/32k color problems
	*/
	printf("Old MCLK value: %02x\n", inb(0x3c5));
	outb(0x3c5, NEW_MCLK);
	printf("New MCLK value: %02x\n", NEW_MCLK);

	exit(0);
}
