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    SubjectRe: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory
    On Wed, Mar 14, 2018 at 12:03 PM, Logan Gunthorpe <logang@deltatee.com> wrote:
    >
    >
    > On 14/03/18 12:51 PM, Bjorn Helgaas wrote:
    >> You are focused on PCIe systems, and in those systems, most topologies
    >> do have an upstream switch, which means two upstream bridges. I'm
    >> trying to remove that assumption because I don't think there's a
    >> requirement for it in the spec. Enforcing this assumption complicates
    >> the code and makes it harder to understand because the reader says
    >> "huh, I know peer-to-peer DMA should work inside any PCI hierarchy*,
    >> so why do we need these two bridges?"
    >
    > Yes, as I've said, we focused on being behind a single PCIe Switch
    > because it's easier and vaguely safer (we *know* switches will work but
    > other types of topology we have to assume will work based on the spec).
    > Also, I have my doubts that anyone will ever have a use for this with
    > non-PCIe devices.

    P2P over PCI/PCI-X is quite common in devices like raid controllers.
    It would be useful if those configurations were not left behind so
    that Linux could feasibly deploy offload code to a controller in the
    PCI domain.

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