bus number into the bus argument. The bus number occupies the least
significant 8 bits. The PCI domain occupies the most significant 24
bits.
On the Altix 350, the PCI domain is a required parameter, but
changing the prototype of the pci_cfgreg*() functions to include a
separate domain argument has wide-spread consequences across the
supported architectures. We'd be changing a known interface.
Multiplexing is an acceptable kluge to give us what we need with
manageable impact. Note that the PCI bus number fits in 8 bits,
so the multiplexing of the domain is a backward compatible change.
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