Re: [RFC PATCH 4/5] clocksource: Enlarge the maxim time interval when configuring the scale and shift

[Chen Gong <gong.chen@linux.intel.com>]

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On Mon, Jan 21, 2013 at 02:38:44PM +0800, Feng Tang wrote:
> Date:	Mon, 21 Jan 2013 14:38:44 +0800
> From: Feng Tang <feng.tang@intel.com>
> To: Thomas Gleixner <tglx@linutronix.de>, John Stultz
>  <john.stultz@linaro.org>, Ingo Molnar <mingo@elte.hu>, "H. Peter Anvin"
>  <hpa@linux.intel.com>, x86@kernel.org, Len Brown <lenb@kernel.org>,
>  "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
>  linux-kernel@vger.kernel.org
> Cc: Feng Tang <feng.tang@intel.com>
> Subject: [RFC PATCH 4/5] clocksource: Enlarge the maxim time interval when
>  configuring the scale and shift
> X-Mailer: git-send-email 1.7.9.5
> 
> On our x86 platform, we see a failure case of calling clocksource_cyc2ns(),
> which return a negative value. The reason is the time interval was large
> (more than 1000 seconds), while its TSC frequency is 2GHz, so the following
> fomular overflowed:
> 	((u64) cycles * mult) >> shift
> 
> So enlarge the time interval from 10 mins to 40 mins to fix the bug.
> 
> Another solution may be adding a "max_interval" in struct clocksource, and
> use a default value (like current 10 minutes) when clocksource driver
> doesn't set it.
> 
As you said, it looks like it is a littleb it arbitrary from 10m -> 40m, I
think max_interval is a better choice, if timer guys not minding too many
control knobs :-).

> Signed-off-by: Feng Tang <feng.tang@intel.com>
> ---
>  kernel/time/clocksource.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index c958338..48fbfcb 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -663,7 +663,7 @@ void __clocksource_updatefreq_scale(struct clocksource *cs, u32 scale, u32 freq)
>  	 * Calc the maximum number of seconds which we can run before
>  	 * wrapping around. For clocksources which have a mask > 32bit
>  	 * we need to limit the max sleep time to have a good
> -	 * conversion precision. 10 minutes is still a reasonable
> +	 * conversion precision. 40 minutes is still a reasonable
>  	 * amount. That results in a shift value of 24 for a
>  	 * clocksource with mask >= 40bit and f >= 4GHz. That maps to
>  	 * ~ 0.06ppm granularity for NTP. We apply the same 12.5%
> @@ -674,8 +674,8 @@ void __clocksource_updatefreq_scale(struct clocksource *cs, u32 scale, u32 freq)
>  	do_div(sec, scale);
>  	if (!sec)
>  		sec = 1;
> -	else if (sec > 600 && cs->mask > UINT_MAX)
> -		sec = 600;
> +	else if (sec > 2400 && cs->mask > UINT_MAX)
> +		sec = 2400;
>  
>  	clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
>  			       NSEC_PER_SEC / scale, sec * scale);
> -- 
> 1.7.9.5
> 
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