Re: [PATCH v5 2/2] PCI: quirks: Fix ThunderX2 dma alias handling

[Jon Masters <jcm@redhat.com>]

One additional footnote. I spent a bunch of time recently on my personal
Xeon systems walking through the PCIe topology and aligning on how to
advise the ARM server community proceed going forward. If you look at
how Intel vs AMD handle their host bridges for example, you'll see two
very different approaches to the case of cross-socket PCIe. But my
operating assumption is that anything longer term which looks boring and
x86 enough is probably fine from an ARM server point of view.

On 04/19/2017 07:38 PM, Jon Masters wrote:
> Hi Bjorn, JC,
> 
> On 04/13/2017 08:19 PM, Bjorn Helgaas wrote:
> 
>> I tentatively applied both patches to pci/host-thunder for v4.12.
> 
> Thanks for that :)
> 
>> However, I am concerned about the topology here:
> 
> Various feedback has been provided on this one over the past $time. In
> addition, I have requested that this serve as an example of why we need
> a more complete PCIe integration guide for ARMv8. It's on the list of
> things for my intended opus magnum on the topic ;)
> 
>> On Thu, Apr 13, 2017 at 08:30:45PM +0000, Jayachandran C wrote:
>>> On Cavium ThunderX2 arm64 SoCs (called Broadcom Vulcan earlier), the
>>> PCI topology is slightly unusual.  For a multi-node system, it looks
>>> like:
>>>
>>>     00:00.0 [PCI] bridge to [bus 01-1e]
>>>     01:0a.0 [PCI-PCIe bridge, type 8] bridge to [bus 02-04]
>>>     02:00.0 [PCIe root port, type 4] bridge to [bus 03-04] (XLATE_ROOT)
>>>     03:00.0 PCIe Endpoint
>>
>> A root port normally has a single PCIe link leading downstream.
> 
> In integration terms, there's always a bus on the other side of the RC.
> It's just usually a processor local bus of some kind on a proprietary
> interconnect, but there's always something there. The issue is when you
> can see it as PCIe and it's not through a transparent glue bridge.
> 
> I had originally hoped that the ECAM could be hacked up so that we would
> first walk the topology at the 02:00.0 as a root and not see what's
> above it BUT there are other device attachments up there for the on-SoC
> devices and I think we really intend to use those.
> 
> Bottom line in my opinion is document this case, use it as a learning
> example, and move forward. This has been useful in justifying why we
> need better integration documentation from the server community. And in
> particular from the OS vendors, of which I guess we can allude to their
> being more than Linux interested in ARM server these days ;)
> 
> Jon.
> 

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