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

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Message-ID: <c717eb06-1e83-f0f3-4d6c-15531aa808e0@deltatee.com>
Date:   Mon, 26 Mar 2018 09:46:24 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     Sinan Kaya <okaya@...eaurora.org>, 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, Stephen Bates <sbates@...thlin.com>,
        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>,
        Eric Wehage <Eric.Wehage@...wei.com>
Subject: Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory



On 26/03/18 08:01 AM, Bjorn Helgaas wrote:
> On Mon, Mar 26, 2018 at 12:11:38PM +0100, Jonathan Cameron wrote:
>> On Tue, 13 Mar 2018 10:43:55 -0600
>> Logan Gunthorpe <logang@...tatee.com> wrote:
>>> It turns out that root ports that support P2P are far less common than 
>>> anyone thought. So it will likely have to be a white list.
>>
>> This came as a bit of a surprise to our PCIe architect.
>>
>> His follow up was whether it was worth raising an ECR for the PCIe spec
>> to add a capability bit to allow this to be discovered.  This might
>> long term avoid the need to maintain the white list for new devices.
>>
>> So is it worth having a long term solution for making this discoverable?
> 
> It was surprising to me that there's no architected way to discover
> this.  It seems like such an obvious thing that I guess I assumed the
> omission was intentional, i.e., maybe there's something that makes it
> impractical, but it would be worth at least asking somebody in the
> SIG.  It seems like for root ports in the same root complex, at least,
> there could be a bit somewhere in the root port or the RCRB (which
> Linux doesn't support yet).

Yes, I agree. It would be a good long term solution to have this bit in
the spec. That would avoid us needing to create a white list for new
hardware. However, I expect it would be years before we can rely on it
so someone may yet implement that white list.

Logan