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Subject: Re: [PATCH 2/8] PCI: Add pci_find_common_upstream_dev()
To:     Logan Gunthorpe <logang@deltatee.com>,
        Christoph Hellwig <hch@infradead.org>
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From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
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Date:   Wed, 28 Mar 2018 18:02:46 +0200
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Am 28.03.2018 um 17:47 schrieb Logan Gunthorpe:
>
> On 28/03/18 09:07 AM, Christian König wrote:
>> Am 28.03.2018 um 14:38 schrieb Christoph Hellwig:
>>> On Sun, Mar 25, 2018 at 12:59:54PM +0200, Christian König wrote:
>>>> From: "wdavis@nvidia.com" <wdavis@nvidia.com>
>>>>
>>>> Add an interface to find the first device which is upstream of both
>>>> devices.
>>> Please work with Logan and base this on top of the outstanding peer
>>> to peer patchset.
>> Can you point me to that? The last code I could find about that was from
>> 2015.
> The latest posted series is here:
>
> https://lkml.org/lkml/2018/3/12/830
>
> However, we've made some significant changes to the area that's similar
> to what you are doing. You can find lasted un-posted here:
>
> https://github.com/sbates130272/linux-p2pmem/tree/pci-p2p-v4-pre2
>
> Specifically this function would be of interest to you:
>
> https://github.com/sbates130272/linux-p2pmem/blob/0e9468ae2a5a5198513dd12990151e09105f0351/drivers/pci/p2pdma.c#L239
>
> However, the difference between what we are doing is that we are
> interested in the distance through the common upstream device and you
> appear to be finding the actual common device.

Yeah, that looks very similar to what I picked up from the older 
patches, going to read up on that after my vacation.

Just in general why are you interested in the "distance" of the devices?

And BTW: At least for writes that Peer 2 Peer transactions between 
different root complexes work is actually more common than the other way 
around.

So I'm a bit torn between using a blacklist or a whitelist. A whitelist 
is certainly more conservative approach, but that could get a bit long.

Thanks,
Christian.

>
> Thanks,
>
> Logan