Valid VCO range on 855GME platform

["David Müller (ELSOFT AG)"]

Hello

I have a problem using a 1024x768 at 60Hz LVDS panel attached to a 855GME
based system running under Linux-2.6.39.

As long as the system is under VGA BIOS control, everything is fine
(panel refresh rate is 60.1Hz).

As soon as the i915 Linux driver is taking over control, the refresh
rate raises to 61.4Hz.

As far as i can see the i915 driver is (wrongly) reprogramming the Pipe
B PLL to a higher dot clock as shown below:

PLL values set up by VGA BIOS:
N=4, M1=18, M2=12, P1=1, P2=14
 => dot clock: 65.142 MHz, refresh rate: 60.1Hz

PLL values set up by i915 driver:
N=5, M1=22, M2=14, P1=1, P2=14
 => dot clock: 66.612MHz, refresh rate: 61.4Hz


According to "intel_PLL_is_valid()" the PLL values used by the VGA BIOS
are invalid because of a "vco out of range" condition.

Is the "I8XX_VCO_MIN" in "drivers/gpu/drm/i915/intel_display.c"
correctly set or should it be set to a lower value?


Dave