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