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

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Date:   Thu, 28 Sep 2023 11:04:43 -0500
From:   Samuel Holland <samuel.holland@...ive.com>
To:     Prabhakar <prabhakar.csengg@...il.com>,
        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>
Cc:     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

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/

Regards,
Samuel

> 
> [0] drivers/clocksource/renesas-ostm.c
> 
> Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> ---
> Note, Ive set the rating similar to RISC-V clocksource, on ARM architecture
> the rating for clk_event is set to 450.
> ---
>  drivers/clocksource/timer-riscv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> index da3071b387eb..e4fc5da119a2 100644
> --- a/drivers/clocksource/timer-riscv.c
> +++ b/drivers/clocksource/timer-riscv.c
> @@ -54,7 +54,7 @@ static unsigned int riscv_clock_event_irq;
>  static DEFINE_PER_CPU(struct clock_event_device, riscv_clock_event) = {
>  	.name			= "riscv_timer_clockevent",
>  	.features		= CLOCK_EVT_FEAT_ONESHOT,
> -	.rating			= 100,
> +	.rating			= 400,
>  	.set_next_event		= riscv_clock_next_event,
>  };
>