Add a helper method to find a PCI-e root port of a device.

[jhb]

Summary

Add a new helper function for PCI devices to locate the upstream
PCI-express root port of a given PCI device.

This function is of limited usefulness. I use it in a pending change to
cxgbe(4), but I put it in pci.c since it is a generic PCI function. If
the function is dubbed to be too dubious I can just leave it internal
to cxgbe(4) instead of exposing it publically.

Test Plan

• I use this in a pending change to cxgbe(4).

[kib]

You added me because jah and I managed to produce sys/x86/iommu/busdma_dmar.c:dmar_get_requester(), right ? pci_find_pcie_root_port() is mostly a simplified version of the dmar_get_requester(). The later e.g. handles buggy pcie-pci bridges which do not report pcie capability (I have one of them in my test machine).

Ideally, the functions would be merged someday.

[jhb]

Yes, I did use the dmar bits as a reference though I think we discussed those when you implemented them. It's not quite the same as the dmar routine though (I wanted to pull out a shared routine initially). We could possibly have something like 'is_a_pci_device(device_t dev)' that checked if the parent had the pci devclass, but that wouldn't be efficient to use in a walk like this.