- every pci device which implements _PRT acpi method is considered to
be a pci-to-pci bridge
- acpi driver constructs a hierarchy of pci-to-pci bridges
- when pci driver identifies a pci-to-pci bridge it tells acpi driver
what is the primary and the secondary bus for this device
- when pci requests IRQ routing information from acpi, it passes the
bus number too to be able to identify the device accurately
