I have a PCI device with a DEC 21150 PCI/PCI bridge that uses the GPIOx
pins for a specific purpose. Is there some way to find the right pcitag_t for
the bridge that the driver for my chip is actually behind?
  I know this sort of violates layering, but I already had that conversation
with the hardware designers.

  So, given a "busnum" is there a way to generate the right pcitag_t for the 
bridge onto that bus? Obviously, it fails for busnum 0. 

  Is this a reasonable thing to add to pci_machdep.c?
  hmm. not even sure where the root pcitag_t get's created...

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