Re: [RFC 1/2] dt-bindings: topology: Add RISC-V cpu topology.

[Atish Patra]

On 11/2/18 8:50 AM, Sudeep Holla wrote:
> On Fri, Nov 02, 2018 at 10:11:38AM -0500, Rob Herring wrote:
> > On Fri, Nov 2, 2018 at 8:31 AM Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
> > > On Fri, Nov 02, 2018 at 08:09:39AM -0500, Rob Herring wrote:
> > > > On Thu, Nov 1, 2018 at 6:04 PM Atish Patra <atish.patra@xxxxxxx> wrote:
> > > > > Define a RISC-V cpu topology. This is based on cpu-map in ARM world.
> > > > > But it doesn't need a separate thread node for defining SMT systems.
> > > > > Multiple cpu phandle properties can be parsed to identify the sibling
> > > > > hardware threads. Moreover, we do not have cluster concept in RISC-V.
> > > > > So package is a better word choice than cluster for RISC-V.
> > > >
> > > > There was a proposal to add package info for ARM recently. Not sure
> > > > what happened to that, but we don't need 2 different ways.
> > >
> > > We still need that, I can brush it up and post what Lorenzo had previously
> > > proposed[1]. We want to keep both DT and ACPI CPU topology story aligned.
> >
> > Frankly, I don't care what the ACPI story is. I care whether each cpu
>
> Sorry I meant feature parity with ACPI and didn't refer to the mechanics.
>
> > arch does its own thing in DT or not. If a package prop works for
> > RISC-V folks and that happens to align with ACPI, then okay. Though I
> > tend to prefer a package represented as a node rather than a property
> > as I think that's more consistent.
>
> Sounds good. One of the reason for making it *optional* property is for
> backward compatibility. But we should be able to deal with that even with
> node.

If you are introducing a package node, can we make cluster node 
optional? I feel it is a redundant node for use cases where one doesn't 
have a different grouped cpus in a package.

We may have some architecture that requires cluster, it can be added to 
the DT at that time, I believe.

> > Any comments on the thread aspect (whether it has ever been used)?
> > Though I think thread as a node level is more consistent with each
> > topology level being a node (same with package).
> Not 100% sure, the only multi threaded core in the market I know is
> Cavium TX2 which is ACPI.

Any advantages of keeping thread node if it's not being used. If I am 
not wrong, we can always use multiple cpuN phandles to represent SMT 
nodes. It will result in less code and DT documentation as well.


Regards,
Atish
> --
> Regards,
> Sudeep