Re: [PATCH v2 01/10] PCI/P2PDMA: Support peer to peer memory

Logan Gunthorpe
2018-03-01T23:06:47Z

> 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