linux-kernel - Re: [PATCH v2] watchdog: s3c2410_wdt: Handle rounding a little better for timeout

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Date:	Wed, 8 Jan 2014 21:03:46 +0100
From:	Wim Van Sebroeck <wim@...ana.be>
To:	Doug Anderson <dianders@...omium.org>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	Leela Krishna Amudala <l.krishna@...sung.com>,
	Olof Johansson <olof@...om.net>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Ben Dooks <ben-linux@...ff.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, linux-watchdog@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] watchdog: s3c2410_wdt: Handle rounding a little better for timeout

Hi Doug,

> The existing watchdog timeout worked OK but didn't deal with
> rounding in an ideal way when dividing out all of its clocks.
> 
> Specifically if you had a timeout of 32 seconds and an input clock of
> 66666666, you'd end up setting a timeout of 31.9998 seconds and
> reporting a timeout of 31 seconds.
> 
> Specifically DBG printouts showed:
>   s3c2410wdt_set_heartbeat: count=16666656, timeout=32, freq=520833
>   s3c2410wdt_set_heartbeat: timeout=32, divisor=255, count=16666656 (0000ff4f)
> and the final timeout reported to the user was:
>   ((count / divisor) * divisor) / freq
>   (0xff4f * 255) / 520833 = 31 (truncated from 31.9998)
> the technically "correct" value is:
>   (0xff4f * 255) / (66666666.0 / 128) = 31.9998
> 
> By using "DIV_ROUND_UP" we can be a little more correct.
>   s3c2410wdt_set_heartbeat: count=16666688, timeout=32, freq=520834
>   s3c2410wdt_set_heartbeat: timeout=32, divisor=255, count=16666688 (0000ff50)
> and the final timeout reported to the user:
>   (0xff50 * 255) / 520834 = 32
> the technically "correct" value is:
>   (0xff50 * 255) / (66666666.0 / 128) = 32.0003
> 
> We'll use a DIV_ROUND_UP to solve this, generally erroring on the side
> of reporting shorter values to the user and setting the watchdog to
> slightly longer than requested:
> * Round input frequency up to assume watchdog is counting faster.
> * Round divisions by divisor up to give us extra time.
> 
> At the same time we can avoid a for loop by just doing the right math.
> 
> Signed-off-by: Doug Anderson <dianders@...omium.org>
> ---
> Changes in v2:
> - Avoid a for loop as per Guenter.

Patch added to linux-watchdog-next.

Kind regards,
Wim.

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