We've gone through similar rounds of debugging our XHCI bits on illumos. PCI and USB sniffing hardware is expensive and we have access to neither. You can get

We've gone through similar rounds of debugging our XHCI bits on illumos. PCI and USB sniffing hardware is expensive and we have access to neither. You can get a long way by becoming comfortable with the specification, reading and getting review on your source, and by using the debugging tools on your OS (we have MDB and DTrace, etc).