According to the PReP specification section 6.1.6 "System Interrupt
Assignments", all PCI interrupts are routed via IRQ 15.
In the case of the 40p machine this isn't quite true in that it has a routing
quirk: the LSI SCSI device is always routed to IRQ 13. At least Linux and
NetBSD compare the model name presented by the firmware to "IBM PPS Model
6015", and if it matches will active this quirk.
There is also a slight issue in that whilst the legacy PReP machine is still
present in the codebase, the old IRQ routing must still be preserved. This is
done by introducing a new "is-legacy-prep" qdev property to the raven PCI host
bridge which preserves the old routing for -M prep until that code is finally
removed.