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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Wed, 14 Mar 2018 12:28:04 -0700
Message-ID: <CAPcyv4g-A6fqdTahxX=aONSCgp1eEvBb=ZVMzbZUWUDmqBbj7Q@mail.gmail.com>
Subject: Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory
To:     Logan Gunthorpe <logang@deltatee.com>
Cc:     Bjorn Helgaas <helgaas@kernel.org>,
        Stephen Bates <sbates@raithlin.com>,
        Sinan Kaya <okaya@codeaurora.org>,
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        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
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        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Keith Busch <keith.busch@intel.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
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        Alex Williamson <alex.williamson@redhat.com>
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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.