linux-kernel - Re: Shifts for clocksources and clockevents

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Date:	Tue, 27 May 2008 18:25:12 -0700
From:	"john stultz" <johnstul@...ibm.com>
To:	"Linus Walleij" <linus.ml.walleij@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Shifts for clocksources and clockevents

On Tue, May 27, 2008 at 7:54 AM, Linus Walleij
<linus.ml.walleij@...il.com> wrote:
> I asked this on the ARM Linux mailing list a while back, but
> it appears noone there really has a clue. (Or they didn't see
> it.)
>
> Can someone give me a rationale behind the values assigned to
> the .shift field in clock_event_device and clocksource in the current
> ARM platforms?
>
> When I read the documentation in Documentation/hrtimers/highres.txt
> it points me to the PDF
> http://www.linuxsymposium.org/2006/linuxsymposium_procv1.pdf
> from which I get the impression that shift shall be assigned so
> that
>
> (time in ns for 1 cycle)
> --------------------------     = 1
>           shift
>         2
>
> So for a timer running cycles at 1MHz shift would be something like
> 10 so that 1000/1024 ~= 1.
>
> However the current assignments doesn't look like that at all. I get
> the impression that .shift for clock_event_device seem to be set
> to 32 at all times in order to blow 32bit counters up to 64 bit,
> and clocksource .shift is set to something like 18, 20 or 24 for
> unclear reasons.
>
> Any hints here?

The goal is to have the highest shift-value possible (providing
improved slewing accuracy for ntp), while not overflowing on the
multiply if the cycles are not accumulated over an extended period of
time (usually 2-5 seconds).

See the comments in kernel/time/jiffies.c for an example.

thanks
-john
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