linux-kernel - Re: [v4 10/11] riscv: dts: fu740: Add pmu node

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Message-ID: <YXsMtrmuavGAHk8S@Jessicas-MacBook-Pro.local>
Date:   Thu, 28 Oct 2021 21:48:54 +0100
From:   Jessica Clarke <jrtc27@...c27.com>
To:     Atish Patra <atish.patra@....com>
Cc:     linux-kernel@...r.kernel.org, Anup Patel <anup.patel@....com>,
        David Abdurachmanov <david.abdurachmanov@...ive.com>,
        devicetree@...r.kernel.org, Greentime Hu <greentime.hu@...ive.com>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Heinrich Schuchardt <xypron.glpk@....de>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-perf-users@...r.kernel.org, linux-riscv@...ts.infradead.org,
        Nick Kossifidis <mick@....forth.gr>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>,
        Vincent Chen <vincent.chen@...ive.com>
Subject: Re: [v4 10/11] riscv: dts: fu740: Add pmu node

On Mon, Oct 25, 2021 at 12:53:49PM -0700, Atish Patra wrote:
> HiFive unmatched supports HPMCounters but does not implement mcountinhibit
> or sscof extension. Thus, perf monitoring can be used on the unmatched
> board without sampling.
> 
> Add the PMU node with compatible string so that Linux perf driver can
> utilize this to enable PMU.
> 
> Signed-off-by: Atish Patra <atish.patra@....com>
> ---
>  arch/riscv/boot/dts/sifive/fu740-c000.dtsi | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/riscv/boot/dts/sifive/fu740-c000.dtsi b/arch/riscv/boot/dts/sifive/fu740-c000.dtsi
> index abbb960f90a0..b35b96b58820 100644
> --- a/arch/riscv/boot/dts/sifive/fu740-c000.dtsi
> +++ b/arch/riscv/boot/dts/sifive/fu740-c000.dtsi
> @@ -140,6 +140,9 @@ soc {
>  		#size-cells = <2>;
>  		compatible = "simple-bus";
>  		ranges;
> +		pmu {
> +			compatible = "riscv,pmu";
> +		};

This is a property of the user-replaceable firmware, not a property of
the hardware, so having this in the device tree under /soc, let alone
hard-coded in Linux, is utterly wrong. Why can this not just be probed
like any other SBI interface? The "Probe SBI extension" interface is
precisely for this kind of thing.

Jess