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From:   "Lad, Prabhakar" <prabhakar.csengg@gmail.com>
Date:   Thu, 28 Sep 2023 17:18:12 +0100
Message-ID: <CA+V-a8sSmy-UNwsB3b_yuETTRGS2fBWvS4sTMaVghTWm6oUb+Q@mail.gmail.com>
Subject: Re: [PATCH] clocksource/drivers/riscv: Increase the clock_event rating
To:     Samuel Holland <samuel.holland@sifive.com>
Cc:     Daniel Lezcano <daniel.lezcano@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Samuel Holland <samuel@sholland.org>,
        Anup Patel <anup@brainfault.org>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Conor Dooley <conor.dooley@microchip.com>,
        Biju Das <biju.das.jz@bp.renesas.com>,
        linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
        linux-renesas-soc@vger.kernel.org,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
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Hi Samuel,

On Thu, Sep 28, 2023 at 5:04=E2=80=AFPM Samuel Holland
<samuel.holland@sifive.com> wrote:
>
> On 2023-09-28 5:45 AM, Prabhakar wrote:
> > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> >
> > Renesas RZ/Five SoC has OSTM blocks which can be used for clock_event a=
nd
> > clocksource [0]. The clock_event rating for the OSTM is set 300 but
> > whereas the rating for riscv-timer clock_event is set to 100 due to whi=
ch
> > the kernel is choosing OSTM for clock_event.
> >
> > As riscv-timer is much more efficient than MMIO clock_event, increase t=
he
> > rating to 400 so that the kernel prefers riscv-timer over the MMIO base=
d
> > clock_event.
>
> This is only true if you have the Sstc extension and can set stimecmp dir=
ectly.
> Otherwise you have the overhead of an SBI call, which is going to be much=
 higher
> than an MMIO write. So the rating should depend on Sstc, as in this patch=
:
>
> https://lore.kernel.org/linux-riscv/20230710131902.1459180-3-apatel@venta=
namicro.com/
>
Thank you for the pointer. Do you know any tool/util which I can use
to make comparisons?

Cheers,
Prabhakar