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

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Date:	Tue, 9 Feb 2016 10:26:28 -0600
From:	Bjorn Helgaas <helgaas@...nel.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	David Daney <ddaney@...iumnetworks.com>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	David Daney <david.daney@...ium.com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	David Daney <ddaney.cavm@...il.com>,
	Kumar Gala <galak@...eaurora.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [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:
> > > >
> > > >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