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Date:   Fri, 31 Aug 2018 09:51:06 -0600
From:   Logan Gunthorpe <>
To:     Christian König <>,,,,,,
Cc:     Stephen Bates <>,
        Christoph Hellwig <>,
        Keith Busch <>,
        Sagi Grimberg <>,
        Bjorn Helgaas <>,
        Jason Gunthorpe <>,
        Max Gurtovoy <>,
        Dan Williams <>,
        Jérôme Glisse <>,
        Benjamin Herrenschmidt <>,
        Alex Williamson <>,
        Jonathan Corbet <>
Subject: Re: [PATCH v5 06/13] PCI/P2PDMA: Add P2P DMA driver writer's

On 31/08/18 02:08 AM, Christian König wrote:
>> +One of the biggest issues is that PCI doesn't require forwarding
>> +transactions between hierarchy domains, and in PCIe, each Root Port
>> +defines a separate hierarchy domain. To make things worse, there is no
>> +simple way to determine if a given Root Complex supports this or not.
>> +(See PCIe r4.0, sec 1.3.1). Therefore, as of this writing, the kernel
>> +only supports doing P2P when the endpoints involved are all behind the
>> +same PCI bridge, as such devices are all in the same PCI hierarchy
>> +domain, and the spec guarantees that all transacations within the
>> +hierarchy will be routable, but it does not require routing
>> +between hierarchies.
> Can we add a kernel command line switch and a whitelist to enable P2P 
> between separate hierarchies?

In future work, yes. But not for this patchset. This is definitely the
way I see things going, but we've chosen to start with what we've presented.


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