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To:     Bjorn Helgaas <helgaas@kernel.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>
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From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <c717eb06-1e83-f0f3-4d6c-15531aa808e0@deltatee.com>
Date:   Mon, 26 Mar 2018 09:46:24 -0600
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Subject: Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory
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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@deltatee.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