linux-kernel - Re: [PATCH v5 06/13] PCI/P2PDMA: Add P2P DMA driver writer's documentation

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Date:   Fri, 31 Aug 2018 19:38:50 +0200
From:   Christian König <christian.koenig@....com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-block@...r.kernel.org
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        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>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v5 06/13] PCI/P2PDMA: Add P2P DMA driver writer's
 documentation

Am 31.08.2018 um 17:51 schrieb Logan Gunthorpe:
>
> 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.

Sounds like a plan to me.

If you can separate out adding the detection I can take a look adding 
this with my DMA-buf P2P efforts.

Christian.

>
> Logan