[PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory

[Bjorn Helgaas]

On Tue, Mar 13, 2018 at 10:31:55PM +0000, Stephen  Bates wrote:
> >> It sounds like you have very tight hardware expectations for this to work
> >> at this moment. You also don't want to generalize this code for others and
> >> address the shortcomings.
> >  No, that's the way the community has pushed this work
> 
> Hi Sinan
> 
> Thanks for all the input. As Logan has pointed out the switch
> requirement is something that has evolved over time based on input
> from the community. You are more than welcome to have an opinion on
> this (and you have made that opinion clear ;-)). Over time the
> patchset may evolve from its current requirements but right now we
> are aligned with the feedback from the community.

This part of the community hasn't been convinced of the need to have
two bridges, e.g., both an Upstream Port and a Downstream Port, or two
conventional PCI bridges, above the peers.

Every PCI-to-PCI bridge is required to support routing transactions
between devices on its secondary side.  Therefore, I think it is
sufficient to verify that the potential peers share a single common
upstream bridge.  This could be a conventional PCI bridge, a Switch
Downstream Port, or a Root Port.

I've seen the response that peers directly below a Root Port could not
DMA to each other through the Root Port because of the "route to self"
issue, and I'm not disputing that.  But enforcing a requirement for
two upstream bridges introduces a weird restriction on conventional
PCI topologies, makes the code hard to read, and I don't think it's
necessary.

If it *is* necessary because Root Ports and devices below them behave
differently than in conventional PCI, I think you should include a
reference to the relevant section of the spec and check directly for a
Root Port.  I would prefer that over trying to exclude Root Ports by
looking for two upstream bridges.

Bjorn