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

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.

A switch shows up as two or more virtual bridges (per the PCIe v4 Spec
1.3.3) which explains the existing get_upstream_bridge_port() function.

In any case, we'll look at generalizing this by looking for a common
upstream port in the next revision of the patch set.


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