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

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Message-ID: <CANcMJZAuKo9yQkErDZYtBx-Bm92hMAG3SDK-DJRPn=m=T9hcag@mail.gmail.com>
Date:	Fri, 15 Feb 2013 14:34:33 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	David Engraf <david.engraf@...go.com>
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

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.

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

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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