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

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Date:   Mon, 5 Nov 2018 14:10:36 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Palmer Dabbelt <palmer@...ive.com>
Cc:     Atish Patra <atish.patra@....com>, linux-riscv@...ts.infradead.org,
        Anup Patel <anup@...infault.org>,
        Christoph Hellwig <hch@...radead.org>, 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, alankao@...estech.com,
        Zong Li <zong@...estech.com>
Subject: Re: [RFC 1/2] dt-bindings: topology: Add RISC-V cpu topology.

On Mon, Nov 5, 2018 at 1:39 PM Palmer Dabbelt <palmer@...ive.com> wrote:
>
> 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"?

Yes, but ".../bindings/cpu/cpu-topology.txt".

And if we need $arch extensions, they can be moved there. (Really, I'd
like to get rid of /bindings/$arch/* except for maybe a few things.)

> 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.