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

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Message-ID: <9e77989d6b868d914bc328401cd64557@mailhost.ics.forth.gr>
Date:   Tue, 06 Nov 2018 12:03:17 +0200
From:   Nick Kossifidis <mick@....forth.gr>
To:     Palmer Dabbelt <palmer@...ive.com>
Cc:     robh+dt@...nel.org, mark.rutland@....com,
        devicetree@...r.kernel.org, Damien.LeMoal@....com,
        alankao@...estech.com, zong@...estech.com, anup@...infault.org,
        linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>, atish.patra@....com,
        linux-riscv@...ts.infradead.org, tglx@...utronix.de
Subject: Re: [RFC 1/2] dt-bindings: topology: Add RISC-V cpu topology.

Στις 2018-11-05 21:38, Palmer Dabbelt έγραψε:
> On Fri, 02 Nov 2018 06:09:39 PDT (-0700), robh+dt@...nel.org wrote:
>> On Thu, Nov 1, 2018 at 6:04 PM Atish Patra <atish.patra@....com> 
>> 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.
>> 
>> There's never going to be clusters for RISC-V? What prevents that?
>> Seems shortsighted to me.
>> 
>>> 
>>> Signed-off-by: Atish Patra <atish.patra@....com>
>>> ---
>>>  .../devicetree/bindings/riscv/topology.txt         | 154 
>>> +++++++++++++++++++++
>>>  1 file changed, 154 insertions(+)
>>>  create mode 100644 
>>> Documentation/devicetree/bindings/riscv/topology.txt
>>> 
>>> diff --git a/Documentation/devicetree/bindings/riscv/topology.txt 
>>> b/Documentation/devicetree/bindings/riscv/topology.txt
>>> new file mode 100644
>>> index 00000000..96039ed3
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/riscv/topology.txt
>>> @@ -0,0 +1,154 @@
>>> +===========================================
>>> +RISC-V cpu topology binding description
>>> +===========================================
>>> +
>>> +===========================================
>>> +1 - Introduction
>>> +===========================================
>>> +
>>> +In a RISC-V system, the hierarchy of CPUs can be defined through 
>>> following nodes that
>>> +are used to describe the layout of physical CPUs in the system:
>>> +
>>> +- packages
>>> +- core
>>> +
>>> +The cpu nodes (bindings defined in [1]) represent the devices that
>>> +correspond to physical CPUs and are to be mapped to the hierarchy 
>>> levels.
>>> +Simultaneous multi-threading (SMT) systems can also represent their 
>>> topology
>>> +by defining multiple cpu phandles inside core node. The details are 
>>> explained
>>> +in paragraph 3.
>> 
>> I don't see a reason to do this differently than ARM. That said, I
>> don't think the thread part is in use on ARM, so it could possibly be
>> changed.
>> 
>>> +
>>> +The remainder of this document provides the topology bindings for 
>>> ARM, based
>> 
>> for ARM?
>> 
>>> +on the Devicetree Specification, available from:
>>> +
>>> +https://www.devicetree.org/specifications/
>>> +
>>> +If not stated otherwise, whenever a reference to a cpu node phandle 
>>> is made its
>>> +value must point to a cpu node compliant with the cpu node bindings 
>>> as
>>> +documented in [1].
>>> +A topology description containing phandles to cpu nodes that are not 
>>> compliant
>>> +with bindings standardized in [1] is therefore considered invalid.
>>> +
>>> +This cpu topology binding description is mostly based on the 
>>> topology defined
>>> +in ARM [2].
>>> +===========================================
>>> +2 - cpu-topology node
>> 
>> cpu-map. Why change this?
>> 
>> What I would like to see is the ARM topology binding reworked to be
>> common or some good reasons why it doesn't work for RISC-V as-is.
> 
> I think it would be great if CPU topologies were not a RISC-V specific
> thing.  We don't really do anything different than anyone else, so
> it'd be great if we could all share the same spec and code.  Looking
> quickly at the ARM cpu-map bindings, I don't see any reason why we
> can't just use the same thing on RISC-V -- it's not quite how I'd do
> it, but I don't think the differences are worth having another
> implementation.  Mechanically I'm not sure how to do this: should
> there just be a "Documentation/devicetree/bindings/cpu-map.txt"?
> 
> If everyone is OK with that then I vote we just go ahead and
> genericise the ARM "cpu-map" stuff for CPU topology.  Sharing the
> implementation looks fairly straight-forward as well.
> 

Please check this out...
https://lkml.org/lkml/2018/11/3/99

It's also non arch-dependent and it can handle the scheduler's 
capabilities
better than cpu-map.