linux-kernel - Re: [PATCH] time: Use div64_long() instead of do

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Date: Mon, 26 Feb 2024 11:28:41 +0800
From: "Liao, Chang" <liaochang1@...wei.com>
To: Thorsten Blum <thorsten.blum@...lux.com>, <linux-kernel@...r.kernel.org>
CC: Thomas Gleixner <tglx@...utronix.de>, Anna-Maria Behnsen
	<anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, "Peter
 Zijlstra" <peterz@...radead.org>, John Stultz <jstultz@...gle.com>
Subject: Re: [PATCH] time: Use div64_long() instead of do_div()

Thorsten,

在 2024/2/26 7:25, Thorsten Blum 写道:
> Fixes Coccinelle/coccicheck warning reported by do_div.cocci.
> 
> Compared to do_div(), div64_long() does not implicitly cast the divisor and
> does not unnecessarily calculate the remainder.
> 
> Signed-off-by: Thorsten Blum <thorsten.blum@...lux.com>
> ---
>  kernel/time/jiffies.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
> index bc4db9e5ab70..9d23178e2b6a 100644
> --- a/kernel/time/jiffies.c
> +++ b/kernel/time/jiffies.c
> @@ -91,7 +91,7 @@ int register_refined_jiffies(long cycles_per_second)
>  	/* shift_hz stores hz<<8 for extra accuracy */
>  	shift_hz = (u64)cycles_per_second << 8;
>  	shift_hz += cycles_per_tick/2;
> -	do_div(shift_hz, cycles_per_tick);
> +	shift_hz = div64_long(shift_hz, cycles_per_tick);

I am considering using div64_ul() to calculate the result. as shift_hz is
unsigned long, assume the sign bit of divisor cycles_per_tick never be set
in this context,then div64_long() will do a extra sign extension for result.
Or are there other recommendations?

Thanks.

>  	/* Calculate nsec_per_tick using shift_hz */
>  	nsec_per_tick = (u64)NSEC_PER_SEC << 8;
>  	nsec_per_tick += (u32)shift_hz/2;

-- 
BR
Liao, Chang