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To:     Bjorn Helgaas <helgaas@kernel.org>
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From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <b75e83ad-bb83-1f8d-1751-0989cc2493f1@deltatee.com>
Date:   Wed, 14 Mar 2018 13:03:43 -0600
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Subject: Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory
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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.

Logan