We know that due to the hardware, in many cases no amount of probing during installation will retrieve the right values for xorg.conf, sound, etc and it leaves one to do it by hand. For example, ALL G3 iMacs need a horizontal frequency value of 58-62, and a vertical value of 75-117. Yet xorg is virtually guaranteed to get this wrong, mandating a manual edit. G4 eMacs need similar attention with different values. Again, it isn't a probing issue, but it lays with the Apple hardware not being accurate during probing.
ok, this one really helps.. from man radeon
Option "MacModel" "string"
Used to specify Mac models for connector tables and quirks. If
you have a powerbook or mini with DVI that does not work prop-
erly, try the alternate options as Apple does not seem to pro-
vide a good way of knowing whether they use internal or external
TMDS for DVI. Only valid on PowerPC.
ibook -- ibooks
powerbook-external -- Powerbooks with external DVI
powerbook-internal -- Powerbooks with integrated DVI
powerbook-vga -- Powerbooks with VGA rather than DVI
powerbook-duallink -- powerbook-external alias
powerbook -- powerbook-internal alias
mini-external -- Mac Mini with external DVI
mini-internal -- Mac Mini with integrated DVI
mini -- mini-external alias
imac-g5-isight -- iMac G5 iSight
emac -- eMac G4
The default value is undefined.
there are some scripts in gentoo portage: