Interesting scanpci question

Running XFree86 4.3 on a powepc-based IBM p615 RISC.   I recompiled from
source, and am now trying to start X.   I've configured the XF86Config to
look at my card, which is a Matrox G400 AGP.      /proc/pci shows the card
at  'Bus 290, device 0, function 0.       I changed XF86Config to use the
BusID  "PCI:122:0:0"  but I still get the message   "No devices detected".

Is there a limitation in scanpci that won't allow detection of cards above
Bus id 255?    How do I fake out scanpci to think it sees the card?