Open Firmware Recommended Practice: Interrupt Mapping
http://www.openfirmware.org/1275/practice/imap/imap0_9d.pdf
+
+Additionally to the properties specified in the above standards a host bridge
+driver implementation may support the following properties:
+
+- linux,pci-domain:
+ If present this property assigns a fixed PCI domain number to a host bridge,
+ otherwise an unstable (across boots) unique number will be assigned.
+ It is required to either not set this property at all or set it for all
+ host bridges in the system, otherwise potentially conflicting domain numbers
+ may be assigned to root buses behind different host bridges. The domain
+ number for each host bridge in the system must be unique.