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To:     Bjorn Helgaas <helgaas@kernel.org>,
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From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <8de5d3dd-a78f-02d5-0eea-4365364143b6@deltatee.com>
Date:   Tue, 13 Mar 2018 17:21:20 -0600
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Subject: Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory
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On 13/03/18 05:08 PM, Bjorn Helgaas wrote:
> On Tue, Mar 13, 2018 at 10:31:55PM +0000, Stephen  Bates wrote:
> If it *is* necessary because Root Ports and devices below them behave
> differently than in conventional PCI, I think you should include a
> reference to the relevant section of the spec and check directly for a
> Root Port.  I would prefer that over trying to exclude Root Ports by
> looking for two upstream bridges.

Well we've established that we don't want to allow root ports.

But we need to, at a minimum, do two pci_upstream_bridge() calls...

Remember, we need to check that a number of devices are behind the same
switch. So we need to find a _common_ upstream port for several devices.
Excluding the multifunction device case (which I don't think is
applicable for reasons we've discussed before), this will *never* be the
first upstream port for a given device. We need to find the upstream
port of the switch and ensure all devices share that same port. If a
device is behind a switch, it's pci_upstream_bridge() is the downstream
switch port which is unique to that device. So a peer device would have
a different pci_upstream_bridge() port but share the same
pci_upstream_bridge(pci_upstream_bridge()) port (assuming they are on
the same switch).

The alternative, is to walk the entire tree of upstream ports and check
that all devices have a common port somewhere in the tree that isn't the
root complex. Arguably this is the right way to do it and would support
a network of switches but is a fair bit more complex to code.

Logan