1600x1200 not working on T42(Mobile 9000) DVI

[Che-Wei Chang]

I installed Ubuntu 9.04 on IBM T42 with ATI Mobile 9000 chipset.
The T42 is connected to an external monitor, Dell 2007FP, through DVI cable.
The native resolution of 2007FP is 1600x1200.

I set up dual monitors with build-in System/Preferences/Display.
The display panel detected the 2007FP, and showed that it can work on
1600x1200 mode.
However, the external monitor turned to power saving
while I changed the display mode. In the mean time, T42 screen works.

The dual monitors mode works OK when I lower the resolution of 2007FP
to 1280x1024, 1152x864, or 1024x768. Then, I turned off T42 monitor in
the display panel, and turned 2007FP to 1600x1200, still not working.
Therefore, I think this should be due to the video driver, not the
configuration.

I googled dual monitor set-up, and found there is a bug in kernel
2.68.28, then I upgraded kernel to 2.68.30. However, not luck here.
Following is information given from xrandr with 1152x864 mode.

Screen 0: minimum 320 x 200, current 2176 x 864, maximum 2176 x 1632
VGA-0 disconnected (normal left inverted right x axis y axis)
DVI-0 connected 1152x864+0+0 (normal left inverted right x axis y
axis) 367mm x 275mm
1600x1200 60.0 +
1280x1024 75.0 60.0
1152x864 75.0*
1024x768 75.0 60.0
800x600 75.0 60.3
640x480 75.0 59.9
720x400 70.1
LVDS connected 1024x768+1152+0 (normal left inverted right x axis y
axis) 0mm x 0mm
1024x768 60.0*+ 60.0
800x600 60.3 59.9
640x480 59.9 59.4
S-video disconnected (normal left inverted right x axis y axis)

Any suggestion?