The USB debug capability is supposed to work even when almost nothing is work.
EHCI debug port requires PCI to be set up (at least partially) -- it requires a BAR.
Am I correct in understanding that this is about the same amount of work as is needed to reach the serial port super IO chips?
No, it's more work.
The BAR is just an address at which the controller will listen - right
Sort of.
- or does it have to be backed by system RAM?
Nope.
You typically need to set up quite a bit of the PCI subsystem for it to work -- including not only bridges and devices on the path to the PCI function you're interested in, but also any siblings of any such. And you need to do some work with the north bridge typically (open up some window there).
Segher