linux-kernel - Re: timekeeping_adjust may set mult to 0

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Message-ID: <512CB301.8020208@sysgo.com>
Date:	Tue, 26 Feb 2013 14:05:05 +0100
From:	David Engraf <david.engraf@...go.com>
To:	John Stultz <john.stultz@...aro.org>
CC:	John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: timekeeping_adjust may set mult to 0

Am 15.02.2013 23:34, schrieb John Stultz:
> On Mon, Feb 11, 2013 at 7:48 AM, David Engraf <david.engraf@...go.com> wrote:
>> I have encountered a problem when a linux system uses a clocksource with
>> mult = 1 and shift = 0 (clocksource cycle = nanoseconds). It may happen that
>> the function timekeeping_adjust reduces the value of mult to 0 when error is
>> lower than the interval [1].
>> As soon as timekeeper.mult is 0, ktime_get will no longer work because it
>> uses timekeeping_get_ns which converts the cycle to nanoseconds with mult as
>> 0 and the system clocksource returns always 0.
>
> So you *don't* want to use shift=0, since that kills the ability for
> the frequency adjustment code to do anything, as you've found.

The problem is not shift=0 it's mult=1. The frequency adjustment code 
may increase/decrease mult and mult=0 will not work.

> Instead of calculating the clocksource mult/shift pair yourself,
> please use clocksource_register_hz/khz().

Thanks, I will try it.

Best regards
- David

> I'm hoping to kill off the open clocksource_register() call soon, to
> avoid this sort of confusion. Sorry for the trouble.
>
> thanks
> -john
>

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