I don't think this is correct. A Root Port defines a hierarchy domain (I'm looking at PCIe r4.0, sec 1.3.1). The capability to route peer-to-peer transactions *between* hierarchy domains is optional. I think this means a Root Complex is not required to route transactions from one Root Port to another Root Port. This doesn't say anything about routing between two different devices below a Root Port. Those would be in the same hierarchy domain and should follow the conventional PCI routing rules. Of course, since a Root Port has one link that leads to one device, they would probably be different functions of a single multi-function device, so I don't know how practical it would be to test this.
Yes, given that there's only one device below a root port it will either be a switch or a multi-function device. In the multi-function device case, I'm pretty sure the spec disallows routing-to-self so doing a P2P transaction in that sense isn't going to work unless the device specifically supports it and intercepts the traffic before it gets to the port.
But, if we're talking about multi-function devices it should be able to do everything within it's own driver so it's not exactly Peer-to-Peer. Still, if someone has such hardware I think it's up to them to add support for this odd situation.
Logan
