linux-kernel - Re: [RFC][patch 11/12] timekeeper read clock helper functions

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Date:	Fri, 31 Jul 2009 09:45:49 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Walker <dwalker@...o99.com>
Subject: Re: [RFC][patch 11/12] timekeeper read clock helper functions

On Thu, 30 Jul 2009 14:39:54 -0700
john stultz <johnstul@...ibm.com> wrote:

> On Wed, 2009-07-29 at 15:41 +0200, Martin Schwidefsky wrote:
> > plain text document attachment (timekeeper-helper.diff)
> > From: Martin Schwidefsky <schwidefsky@...ibm.com>
> > 
> > Add timekeeper_read_clock_ntp and timekeeper_read_clock_raw and use
> > them for getnstimeofday, ktime_get, ktime_get_ts and getrawmonotonic.
> >
> > Cc: Ingo Molnar <mingo@...e.hu>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: john stultz <johnstul@...ibm.com>
> > Cc: Daniel Walker <dwalker@...o99.com>
> > Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
> > ---
> >  kernel/time/timekeeping.c |   91 +++++++++++++++++++---------------------------
> >  1 file changed, 38 insertions(+), 53 deletions(-)
> > 
> > Index: linux-2.6/kernel/time/timekeeping.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/time/timekeeping.c
> > +++ linux-2.6/kernel/time/timekeeping.c
> > @@ -84,6 +84,40 @@ static void timekeeper_setup_internals(s
> >  	timekeeper.shift = clock->shift;
> >  }
> > 
> > +/* Timekeeper helper functions. */
> > +static inline s64 timekeeper_read_clock_ntp(void)
> > +{
> > +	cycle_t cycle_now, cycle_delta;
> > +	struct clocksource *clock;
> > +
> > +	/* read clocksource: */
> > +	clock = timekeeper.clock;
> > +	cycle_now = clock->read(clock);
> > +
> 
> I know it seems nice to have it here, but I think these helpers would be
> more reusable in other contexts if they took the cycle_now value as an
> argument. Also I'd drop the ntp bit, just to avoid confusing it with
> some ntp specific function. So:
> 
>  timekeeping_get_ns(cycle_t now);
>  timekeeping_get_ns_raw(cycle_t now);
> 
> That way in some situations we don't have to make two accesses to the
> hardware if we want to get both values at the same point.
> 
> Seem reasonable?

The new names are fine but if we pull out the ->read call to the
caller we again have a rather strange mix. The caller gets the cycle
value using some clock, the helper uses the value of the timekeeper
clock or the timerkeeper mult/shift. I would like to keep the ->read
call in the helper. Is there a situation where we need both
calculations for the same cycles value? There is none in the current
code as far as I can see.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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