On Mon, Mar 26, 2018 at 12:11:38PM +0100, Jonathan Cameron wrote:

On Tue, 13 Mar 2018 10:43:55 -0600
Logan Gunthorpe [off-list ref] wrote:

quoted

It turns out that root ports that support P2P are far less common than 
anyone thought. So it will likely have to be a white list.

This came as a bit of a surprise to our PCIe architect.

His follow up was whether it was worth raising an ECR for the PCIe spec
to add a capability bit to allow this to be discovered.  This might
long term avoid the need to maintain the white list for new devices.

So is it worth having a long term solution for making this discoverable?

It was surprising to me that there's no architected way to discover
this.  It seems like such an obvious thing that I guess I assumed the
omission was intentional, i.e., maybe there's something that makes it
impractical, but it would be worth at least asking somebody in the
SIG.  It seems like for root ports in the same root complex, at least,
there could be a bit somewhere in the root port or the RCRB (which
Linux doesn't support yet).