linux-kernel - Re: [PATCH] clocksource/drivers/riscv: Increase the clock

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Date:   Thu, 28 Sep 2023 17:18:12 +0100
From:   "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To:     Samuel Holland <samuel.holland@...ive.com>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Samuel Holland <samuel@...lland.org>,
        Anup Patel <anup@...infault.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Biju Das <biju.das.jz@...renesas.com>,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-renesas-soc@...r.kernel.org,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH] clocksource/drivers/riscv: Increase the clock_event rating

Hi Samuel,

On Thu, Sep 28, 2023 at 5:04 PM Samuel Holland
<samuel.holland@...ive.com> wrote:
>
> On 2023-09-28 5:45 AM, Prabhakar wrote:
> > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> >
> > Renesas RZ/Five SoC has OSTM blocks which can be used for clock_event and
> > 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 which
> > the kernel is choosing OSTM for clock_event.
> >
> > As riscv-timer is much more efficient than MMIO clock_event, increase the
> > rating to 400 so that the kernel prefers riscv-timer over the MMIO based
> > clock_event.
>
> This is only true if you have the Sstc extension and can set stimecmp directly.
> 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@ventanamicro.com/
>
Thank you for the pointer. Do you know any tool/util which I can use
to make comparisons?

Cheers,
Prabhakar