Howto set a modeline for fbdev driver?

[Hans J. Koch]

I'm using X on top of fb using xserver-xorg-video-fbdev from latest Debian
sid. It's some embedded device (armel) and uses an 800x480 TFT with strange
timings. A framebuffer console works fine, I've got the fb device itself
setup correctly.
But if I start X, instead of simply copying the correct values, X insists on
calculating its own, which don't work (vsync much too short).

I tried to add a Modeline in my Monitor Section, but it looks like X simply
ignores it. No matter what I put there, the chosen modeline I see in the log
is always the same.

Any hints?

Thanks,
Hans