linux-kernel - Re: [PATCH] clockevents: Sanitize ticks to nsec conversion

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Message-ID: <5238D6BE.6040603@pengutronix.de>
Date:	Wed, 18 Sep 2013 00:25:02 +0200
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Ludovic Desroches <ludovic.desroches@...el.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	nicolas.ferre@...el.com, LKML <linux-kernel@...r.kernel.org>,
	Marc Pignat <marc.pignat@...s.ch>, john.stultz@...aro.org,
	kernel@...gutronix.de, Ronald Wahl <ronald.wahl@...itan.com>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH] clockevents: Sanitize ticks to nsec conversion

On 09/17/2013 11:15 PM, Thomas Gleixner wrote:
> Marc Kleine-Budde pointed out, that commit 77cc982 "clocksource: use
> clockevents_config_and_register() where possible" caused a regression
> for some of the converted subarchs.
> 
> The reason is, that the clockevents core code converts the minimal
> hardware tick delta to a nanosecond value for core internal
> usage. This conversion is affected by integer math rounding loss, so
> the backwards conversion to hardware ticks will likely result in a
> value which is less than the configured hardware limitation. The
> affected subarchs used their own workaround (SIGH!) which got lost in
> the conversion.
> 
> Now instead of fixing the underlying core code problem, Marcs patch
> tried to work around the core code issue by increasing the minimal
> tick delta at clockevents registration time so the resulting limit in
> the core code backwards conversion did not violate the hardware
> limits. More SIGH!

It was not easy getting your attention with this problem :)

Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
Industrial Linux Solutions        | Phone: +49-231-2826-924     |
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