> I suppose a "pcitag_t *pa_bridgetag" member could be added to pci_attach_args.
> If non-NULL, it points to the tag for the bridge that is the PCI's parent,
> and it would obviously be NULL for the root bus.

My dim memory claims Manuel Bouyer wanted something similar some time ago to 
distinguish different IDE controllers in VIA southbridges, that unfortunately
used the same PCI identifiers (so he needed to examine some properties of
the bridge to tell whether UDMA 100 was supported).

IIRC he solved his problem - maybe worth looking, but maybe I'm completely
misremembering this or it does not apply to this context.

Martin