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


    On 26/03/18 08:01 AM, Bjorn Helgaas wrote:
    > On Mon, Mar 26, 2018 at 12:11:38PM +0100, Jonathan Cameron wrote:
    >> On Tue, 13 Mar 2018 10:43:55 -0600
    >> Logan Gunthorpe <logang@deltatee.com> wrote:
    >>> 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).

    Yes, I agree. It would be a good long term solution to have this bit in
    the spec. That would avoid us needing to create a white list for new
    hardware. However, I expect it would be years before we can rely on it
    so someone may yet implement that white list.

    Logan

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