linux-kernel - Re: [RFC][patch 10/12] move NTP adjusted clock multiplier to struct timekeeper

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Message-ID: <20090731095254.3ed0cb7d@skybase>
Date:	Fri, 31 Jul 2009 09:52:54 +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 10/12] move NTP adjusted clock multiplier to struct
 timekeeper

On Thu, 30 Jul 2009 15:04:48 -0700
john stultz <johnstul@...ibm.com> wrote:

> On Wed, 2009-07-29 at 15:41 +0200, Martin Schwidefsky wrote:
> > plain text document attachment (timekeeper-mult.diff)
> > From: Martin Schwidefsky <schwidefsky@...ibm.com>
> > 
> > The clocksource structure has two multipliers, the unmodified multiplier
> > clock->mult_orig and the NTP corrected multiplier clock->mult. The NTP
> > multiplier is misplaced in the struct clocksource, this is private
> > information of the timekeeping code. Add the mult field to the struct
> > timekeeper to contain the NTP corrected value, keep the unmodifed
> > multiplier in clock->mult and remove clock->mult_orig. For consistency
> > add the shift value associated with the NTP corrected mult value to
> > struct timekeeper as well.
> > 
> > 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>
> > ---
> >  arch/arm/plat-omap/common.c |    2 +-
> >  include/linux/clocksource.h |    4 +---
> >  kernel/time/timekeeping.c   |   43 +++++++++++++++++++++++++------------------
> >  3 files changed, 27 insertions(+), 22 deletions(-)
> > 
> > Index: linux-2.6/include/linux/clocksource.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/clocksource.h
> > +++ linux-2.6/include/linux/clocksource.h
> > @@ -149,8 +149,7 @@ extern u64 timecounter_cyc2time(struct t
> >   * @disable:		optional function to disable the clocksource
> >   * @mask:		bitmask for two's complement
> >   *			subtraction of non 64 bit counters
> > - * @mult:		cycle to nanosecond multiplier (adjusted by NTP)
> > - * @mult_orig:		cycle to nanosecond multiplier (unadjusted by NTP)
> > + * @mult:		cycle to nanosecond multiplier
> >   * @shift:		cycle to nanosecond divisor (power of two)
> >   * @flags:		flags describing special properties
> >   * @vread:		vsyscall based read
> > @@ -168,7 +167,6 @@ struct clocksource {
> >  	void (*disable)(struct clocksource *cs);
> >  	cycle_t mask;
> >  	u32 mult;
> > -	u32 mult_orig;
> >  	u32 shift;
> >  	unsigned long flags;
> >  	cycle_t (*vread)(void);
> > Index: linux-2.6/kernel/time/timekeeping.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/time/timekeeping.c
> > +++ linux-2.6/kernel/time/timekeeping.c
> > @@ -29,6 +29,8 @@ struct timekeeper {
> >  	s64	ntp_error;
> >  	int	xtime_shift;
> >  	int	ntp_error_shift;
> > +	u32	mult;			/* NTP adjusted clock multiplier */
> > +	u32	shift;			/* NTP adjusted clock shift */
> 
> I'm a little confused here. NTP doesn't adjust the shift value. This
> seems redundant with the xtime_shift value introduced in [patch 9/12].

True the NTP code does not adjust the shift value. But mult and shift
build a pair, no? I think it is more readable if we use timekeeper.mult 
and timekeeper.shift instead of timekeeper.mult and clock->shift.
And the xtime_shift does not need to be the same value as the clock
shift - at least in theory. It is just a shift to increase precision. 
Therefore I would like to use the xtime_shift and ntp_error_shift values
exclusively as the shifts for the respective xtime_nsec and ntp_error
values. Makes sense ?

-- 
blue skies,
   Martin.

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

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