On Fri, Jun 22, 2012 at 10:53 AM, Arnd Bergmann [off-list ref] wrote:

On Friday 22 June 2012, Bjorn Helgaas wrote:

quoted

The requirement (if there is one) isn't anything related to PC-ness.
I just don't understand how things can actually work if two host
bridges both claim the same bus number. ?If we do a config read to
that bus, both bridges should claim it and turn it into config cycles
on their respective root buses, and we should get two responses. ?I
would expect the second response to cause an "unexpected response"
machine check or similar.

But each PCI domain has its own config space, so if you do a config
read for one bus, it's always relative to the domain.

Oh, I see!  I totally missed the fact that each host bridge was in its
own domain.  If each has its own domain, then the bus number aperture
can certainly be [bus 00-ff] and there's no problem.