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

[David Daney]

On 02/08/2016 02:12 PM, Bjorn Helgaas wrote:
> On Mon, Feb 08, 2016 at 01:39:21PM -0800, David Daney wrote:
> > On 02/08/2016 01:12 PM, Rob Herring wrote:
> > > On Mon, Feb 8, 2016 at 2:47 PM, David Daney <ddaney@xxxxxxxxxxxxxxxxxx> wrote:
> > > > On 02/08/2016 11:56 AM, Rob Herring wrote:
> > > > > On Fri, Feb 05, 2016 at 03:41:15PM -0800, David Daney wrote:
> > > > > > From: David Daney <david.daney@xxxxxxxxxx>
> > >
> > > [...]
> > >
> > > > > > +Properties of the host controller node that differ from
> > > > > > +host-generic-pci.txt:
> > > > > > +
> > > > > > +- compatible     : Must be "cavium,pci-host-thunder-ecam"
> > > > > > +
> > > > > > +Example:
> > > > > > +
> > > > > > +       pci@84b0,00000000 {
> > > > >
> > > > >
> > > > > Drop the comma,
> > > >
> > > >
> > > > OK...
> > > >
> > > > > and the node name should be "pcie".
> > > >
> > > > Why pcie?
> > > >
> > > > There are no PCIe devices or buses reachable from this type of root complex.
> > > > There are however many PCI devices.
> > >
> > > I thought ECAM is a PCIe thing. If not, then nevermind.
>
> The "ECAM" confusion bites again :)
>
> > Well, Enhanced Configuration Access Mechanism (ECAM) is defined  the
> > the PCI Express(R) base Specification, but it just defines a
> > standard layout of address bits to memory map config space
> > operations.  Since the PCI config space is a sub set of the PCIe
> > config space, ECAM can also be used in PCI systems.
> >
> > Really, it is a bit of a gray area here as we don't have any bridges
> > to PCIe buses and there are multiple devices residing on each bus,
> > so from that point of view it cannot be PCIe.  There are, however,
> > devices that implement the PCI Express Capability structure, so does
> > that make it PCIe?  It is not clear what the specifications demand
> > here.
>
> The PCI core doesn't care about the node name in the device tree.  But
> it *does* care about some details of PCI/PCIe topology.  We consider
> anything with a PCIe capability to be PCIe.  For example,
>
>    - pci_cfg_space_size() thinks PCIe devices have 4K of config space
>
>    - only_one_child() thinks a PCIe bus, i.e., a link, only has a
>      single device on it
>
>    - a PCIe device should have a PCIe Root Port or PCIe Downstream Port
>      upstream from it (we did remove some of these restrictions with
>      b35b1df5e6c2 ("PCI: Tolerate hierarchies with no Root Port"), but
>      it's possible we didn't get them all)
>
> 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.

For the purpose of naming the device tree node, I would like to stick 
with the name "pci@..." as it is somewhat accurate, a value contemplated 
by the device tree specifications, ignored by the kernel code, and 
already implemented.

David Daney


> Bjorn