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To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Don Dutile <ddutile@redhat.com>
References: <20210610220541.GA2779926@bjorn-Precision-5520>
From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <ad142e90-86f1-4749-898f-702ae2b7cf51@deltatee.com>
Date:   Thu, 10 Jun 2021 16:25:53 -0600
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Subject: Re: [PATCH v1 1/6] PCI/P2PDMA: Rename upstream_bridge_distance() and
 rework documentation
Precedence: bulk


On 2021-06-10 4:05 p.m., Bjorn Helgaas wrote:
> The new text is:
> 
>   If there are two virtual functions of the same device behind the same
>   bridge port then PCI_P2PDMA_MAP_BUS_ADDR and a distance of 2 will be
>   returned (one step down to the PCIe switch, then one step back to the
>   same device).
> 
> I *think* this includes two functions of the same multi-function
> device, or two virtual functions of the same device, right?  In both
> cases, the two devices are obviously behind the same bridge port.

Yes, that's correct, if it's the same device it must be behind the same
bridge port; so dropping the "behind the same bridge port" is a good idea.

> 
> Is this usage of "down to the PCIe switch" the common usage in P2PDMA?
> I normally think of going from an endpoint to a switch as being "up"
> toward the CPU.  But PCIe made it all confusing by putting downstream
> ports at the upstream end of links and vice versa.

Good point. I've been casually saying "down to", but you are right "up"
makes a lot more sense given the downstream/upstream terminology.

> We also have a bit of a mix in terminology between "bridge," "switch,"
> "bridge port."  I'd probably write something like:
> 
>   If they are two functions of the same device behind the same bridge,
>   return PCI_P2PDMA_MAP_BUS_ADDR and a distance of 2 (one hop up to
>   the bridge, then one hop back down to another function of the same
>   device).
> 
> No need to repost for this; just let me know what you think and I can
> tweak accordingly.

What you've written sounds good to me, but I might have just dropped the
"behind the same bridge" entirely given your feedback above.

Thanks!

Logan