Since the complete re-write in 2.6.10, some PowerMacs (At least PowerMac 5500
and PowerMac G3 Beige rev A) with ATI Mach64 chip have suffered from unstable
columns in their framebuffer image. This seems to depend on a value (4) read
from PLL_EXT_CNTL register, which leads to incorrect DSP config parameters to
be written to the chip. This patch uses a value calculated by aty_init_pll_ct
instead, as a starting point.

Signed-off-by: Risto Suominen <Risto.Suominen@gmail.com>
---
The testing is done on kernel version 2.6.18.

--- a/drivers/video/aty/mach64_ct.c.org	2006-09-20 06:42:06.000000000 +0300
+++ b/drivers/video/aty/mach64_ct.c	2009-01-22 14:28:13.000000000 +0200
@@ -7,6 +7,7 @@
 #include <linux/delay.h>
 #include <asm/io.h>
 #include <video/mach64.h>
+#include <asm/machdep.h>
 #include "atyfb.h"
 
 #undef DEBUG
@@ -536,6 +537,14 @@ static int __init aty_init_pll_ct(const 
 	pll->ct.xclk_post_div_real = postdividers[xpost_div];
 	pll->ct.mclk_fb_div = q * pll->ct.xclk_post_div_real / 8;
 
+#ifdef CONFIG_PPC
+	if (machine_is(powermac)) {
+		/* Override PLL_EXT_CNTL & 0x07. */
+		pll->ct.xclk_post_div = xpost_div;
+		pll->ct.xclk_ref_div = 1;
+	}
+#endif
+
 #ifdef DEBUG
 	pllmclk = (1000000 * pll->ct.mclk_fb_mult * pll->ct.mclk_fb_div) /
 			(par->ref_clk_per * pll->ct.pll_ref_div);