[PATCH v5 3/3] pci, pci-thunder-ecam: Add driver for ThunderX-pass1 on-chip devices

On Tue, Feb 09, 2016 at 10:25:33AM +0100, Arnd Bergmann wrote:

On Monday 08 February 2016 17:24:30 Bjorn Helgaas wrote:

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I assume your system conforms to expectations like these; I'm just
pointing them out because you mentioned buses with multiple devices on
them, which is definitely something one doesn't expect in PCIe.

The topology we have is currently working with the kernel's core PCI
code.  I don't really want to get into discussing what the
definition of PCIe is.  We have multiple devices (more than 32) on a
single bus, and they have PCI Express and ARI Capabilities.  Is that
PCIe?  I don't know.

I don't need to know the details of your topology.  As long as it
conforms to the PCIe spec, it should be fine.  If it *doesn't* conform
to the spec, but things currently seem to work, that's less fine,
because a future Linux change is liable to break something for you.

I was a little concerned about your statement that "there are multiple
devices residing on each bus, so from that point of view it cannot be
PCIe."  That made it sound like you're doing something outside the
spec.  If you're just using regular multi-function devices or ARI,
then I don't see any issue (or any reason to say it can't be PCIe).

It doesn't conform to the PCIe port spec, because there are no external
ports but just integrated devices in the host bridge. 

Is there a spec section you have in mind?  Based on sec 1.3.1, I don't
think there's a requirement to have PCI Express Ports (is that what
you mean by "external ports"?)

Root Complex Integrated Endpoints (sec 1.3.2.3) are clearly supported
and they would not be behind a Root Port.  If you're using those, I
hope they're correctly identified via the PCIe capability Device/Port
Type (sec 7.8.2) because we rely on that type to figure out whether
the link-related registers are implemented.

The spec does include rules related to peer-to-peer transactions, MPS,
ASPM, error reporting, etc., and Linux relies on those, so I think it
would be important to get those right.

For this special
case, I don't think it matters at all from the point of view of the DT
binding whether we call the node name "pci" or "pcie".

And the PCI core doesn't even know the node name, it doesn't matter
there either.

Bjorn