Originally Posted by zander
It looks like the driver rejects the various 1600x1200 modes due to display device and TMDS encoder limitations; if one of the 1600x1200 modes worked with an older driver, you could run X with -logverbose 6 and that driver, identify which mode was set, determine the differences in the driver's mode validation and (at your own risk), override the problematic parts of the 100.14.xx driver's mode validation (see the README for details).
It's only now I've got time to get back on this issue. I installed the latest driver, .19, and added the following to xorg.conf:
Option "ModeValidation" "NoMaxPClkCheck"
Now, the X server does start in 1600x1200 as before. However, there's a new issue: it does not restore the text console correctly anymore when the X server exits. The DFP complains that the sync values are out of range, 120 kHz H, 149 Hz V (obviously wrong).
So there's still a regression as compared to the previous driver. AFAICT, the values received from the DFP are the same in both versions, but they are interpreted and used in different ways by each drivers.
Any other idea why?