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

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Date:   Fri, 2 Nov 2018 13:34:06 -0700
From:   Atish Patra <atish.patra@....com>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Anup Patel <anup@...infault.org>,
        Christoph Hellwig <hch@...radead.org>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mark Rutland <mark.rutland@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "alankao@...estech.com" <alankao@...estech.com>,
        Zong Li <zong@...estech.com>
Subject: Re: [RFC 1/2] dt-bindings: topology: Add RISC-V cpu topology.

On 11/2/18 6:09 AM, Rob Herring 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.
> 

Agreed. My intention was to keep it simple at first go.
If package node is added, that would work for us as well.

>>
>> 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?
> 

Sorry for the typo.

>> +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?
> 
To my ears, it felt a better name. But I don't mind dropping it in favor 
of cpu-map if we are going to standardize cpu-map for both ARM & RISC-V.

> 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.
> 

IMHO, ARM topology can be reworked and put it in a common place so that 
RISC-V can leverage that.

My intention for this RFC patch was to start the ball rolling on cpu 
topology in RISC-V and see if DT approach is fine with everybody. I 
would be happy to see ARM code to move it to a common code base where 
RISC-V can reuse it.


Regards,
Atish
> Rob
>