I can't chose preferred mode from edid without configuration in xorg.conf

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hi All

I have a machine, which is connected by LCD. From log file I saw the preferred mode -1680x1050"x59.9 from edid was found in probed mode list. however it is strange that even if in driver I set the mode(1680x1050"x59.9) type as  (M_T_DRIVER | M_T_PREFERRED), xorg still chose 1024x768 from default mode list. Yes I can set up preferred option in xorg.conf but I don't want to pollute xorg.conf, what else i need to do?

Thanks a lot & Best Regards
Yan