linux-kernel - Re: [RFT PATCH v1 0/4] Unify CPU topology across ARM64 & RISC-V

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Message-ID: <20181207134509.GA5913@e105550-lin.cambridge.arm.com>
Date:   Fri, 7 Dec 2018 13:45:21 +0000
From:   Morten Rasmussen <morten.rasmussen@....com>
To:     Atish Patra <atish.patra@....com>
Cc:     linux-kernel@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        devicetree@...r.kernel.org,
        Dmitriy Cherkasov <dmitriy@...-tech.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Jeremy Linton <jeremy.linton@....com>,
        Juri Lelli <juri.lelli@....com>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" 
        <linux-arm-kernel@...ts.infradead.org>,
        linux-riscv@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will.deacon@....com>
Subject: Re: [RFT PATCH v1 0/4] Unify CPU topology across ARM64 & RISC-V

Hi,

On Thu, Nov 29, 2018 at 03:28:16PM -0800, Atish Patra wrote:
> The cpu-map DT entry in ARM64 can describe the CPU topology in
> much better way compared to other existing approaches. RISC-V can
> easily adopt this binding to represent it's own CPU topology.
> Thus, both cpu-map DT binding and topology parsing code can be
> moved to a common location so that RISC-V or any other
> architecture can leverage that.
> 
> The relevant discussion regarding unifying cpu topology can be
> found in [1].
> 
> arch_topology seems to be a perfect place to move the common
> code. I have not introduced any functional changes in the moved
> code. The only downside in this approach is that the capacity
> code will be executed for RISC-V as well. But, it will exit
> immediately after not able to find the appropriate DT node. If
> the overhead is considered too much, we can always compile out
> capacity related functions under a different config for the
> architectures that do not support them.
> 
> The patches have been tested for RISC-V and compile tested for
> ARM64 & x86.

The cpu-map bindings are used for arch/arm too, and so is
arch_topology.c. In fact, it was introduced to allow code-sharing
between arm and arm64. Applying patch three breaks arm.

Moving the DT parsing to arch_topology.c we have to unify all three
architectures. Be aware that arm and arm64 have some differences in how
they detect cpu capacities. I think we might have to look at the split
of code between arch/* and arch_topology.c again :-/

Morten