linux-kernel - Re: [patch 4/6] timekeeping: Use mul_u64_u32

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Date:   Fri, 9 Dec 2016 10:41:12 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Liav Rehana <liavr@...lanox.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Richard Cochran <richardcochran@...il.com>,
        Parit Bhargava <prarit@...hat.com>,
        Laurent Vivier <lvivier@...hat.com>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>
Subject: Re: [patch 4/6] timekeeping: Use mul_u64_u32_shr() instead of open
 coding it

On Thu, Dec 08, 2016 at 08:49:38PM -0000, Thomas Gleixner wrote:
> The resume code must deal with a clocksource delta which is potentially big
> enough to overflow the 64bit mult.
> 
> Replace the open coded handling with the proper function.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Reviewed-by: David Gibson <david@...son.dropbear.id.au>

> ---
>  kernel/time/timekeeping.c |   26 +++++---------------------
>  1 file changed, 5 insertions(+), 21 deletions(-)
> 
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1615,7 +1615,7 @@ void timekeeping_resume(void)
>  	struct clocksource *clock = tk->tkr_mono.clock;
>  	unsigned long flags;
>  	struct timespec64 ts_new, ts_delta;
> -	cycle_t cycle_now, cycle_delta;
> +	cycle_t cycle_now;
>  
>  	sleeptime_injected = false;
>  	read_persistent_clock64(&ts_new);
> @@ -1641,27 +1641,11 @@ void timekeeping_resume(void)
>  	cycle_now = tk->tkr_mono.read(clock);
>  	if ((clock->flags & CLOCK_SOURCE_SUSPEND_NONSTOP) &&
>  		cycle_now > tk->tkr_mono.cycle_last) {
> -		u64 num, max = ULLONG_MAX;
> -		u32 mult = clock->mult;
> -		u32 shift = clock->shift;
> -		s64 nsec = 0;
> -
> -		cycle_delta = clocksource_delta(cycle_now, tk->tkr_mono.cycle_last,
> -						tk->tkr_mono.mask);
> -
> -		/*
> -		 * "cycle_delta * mutl" may cause 64 bits overflow, if the
> -		 * suspended time is too long. In that case we need do the
> -		 * 64 bits math carefully
> -		 */
> -		do_div(max, mult);
> -		if (cycle_delta > max) {
> -			num = div64_u64(cycle_delta, max);
> -			nsec = (((u64) max * mult) >> shift) * num;
> -			cycle_delta -= num * max;
> -		}
> -		nsec += ((u64) cycle_delta * mult) >> shift;
> +		u64 nsec, cyc_delta;
>  
> +		cyc_delta = clocksource_delta(cycle_now, tk->tkr_mono.cycle_last,
> +					      tk->tkr_mono.mask);
> +		nsec = mul_u64_u32_shr(cyc_delta, clock->mult, clock->shift);
>  		ts_delta = ns_to_timespec64(nsec);
>  		sleeptime_injected = true;
>  	} else if (timespec64_compare(&ts_new, &timekeeping_suspend_time) > 0) {
> 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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