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

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Message-ID: <56B919A5.5090000@caviumnetworks.com>
Date:	Mon, 8 Feb 2016 14:41:41 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Bjorn Helgaas <helgaas@...nel.org>
CC:	Rob Herring <robh@...nel.org>, David Daney <ddaney.cavm@...il.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Will Deacon <will.deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH v5 3/3] pci, pci-thunder-ecam: Add driver for ThunderX-pass1
 on-chip devices

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@...iumnetworks.com> 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@...ium.com>
>>>
>>> [...]
>>>
>>>>>> +Properties of the host controller node that differ from
>>>>>> +host-generic-pci.txt:
>>>>>> +
>>>>>> +- compatible     : Must be "cavium,pci-host-thunder-ecam"
>>>>>> +
>>>>>> +Example:
>>>>>> +
>>>>>> +       pci@...0,00000000 {
>>>>>
>>>>>
>>>>> 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
>