linux-kernel - Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory

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Date:   Wed, 14 Mar 2018 13:03:43 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Stephen Bates <sbates@...thlin.com>,
        Sinan Kaya <okaya@...eaurora.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [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.

Logan