special resolution with xorg and neomagic

[Nic Duerr]

Hi!

I recently got this sony picturebook subnotebook and after some
struggling I managed to get gentoo working on it.
But there is a problem with the ultrawidescreen-resolution 1024x480 and
xorg (or maybe solely the neomagic driver?!). Although I added a
ModeLine for 1024x480 I always end up 1024x768 when starting X with the
lower half of the screen 'missing'.
After examining the logfiles I found this:

[---snip---]
(II) NEOMAGIC(0): Removing mode (1024x480) that won't display properly
on LCD
(II) NEOMAGIC(0): Not using mode "1024x480" (unknown reason)
(II) NEOMAGIC(0): Not using mode "1024x480" (no mode of this name)
[---snip---]

I'm absolutely sure that 1024x480 is the proper mode for this LCD but I
haven't found a solution yet how to tell Xorg/neomagic to use this one
and not removing it on startup.

So is there any way to fix this? Where does the information come from
about which modes can be "displayed properly" and which ones can't? Is
there any option to override these mode removements?
If not, where exactly does this take place? Which sourcefiles could I
try to hack to make it work?

Thank you a lot!
bye
Nic