I have a PCI modem (US Robotics model 3CP5610A).  The bus walk places 
the modem at port 0xd400 and irq 9 (from /proc/pci).  I use setserial 
to set the port and irq values, but setserial changes my irq 9 request 
to irq 2.  The man page for setserial has the statement, "On AT class 
machines, IRQ2 is seen as IRQ9 and Linux will interpret it in this 
way."  This may be fine for AT class machines, but this is disaster for 
PCI - irq 2 is assigned to the process controller for cascade 
interrupts.

This technique for configuring PCI modems is documented at 
http://www.o2.net/~gromitkc/3cp5610.txt with a pointer to this page 
from Red Hat.