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

Martin Schwidefsky
2009-07-31T09:00:26Z

On Fri, 31 Jul 2009 01:12:59 -0700
john stultz <johnstul@us.ibm.com> wrote:

> On Fri, 2009-07-31 at 09:52 +0200, Martin Schwidefsky wrote:
> > On Thu, 30 Jul 2009 15:04:48 -0700
> > john stultz <johnstul@us.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@de.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@elte.hu>
> > > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > > Cc: john stultz <johnstul@us.ibm.com>
> > > > Cc: Daniel Walker <dwalker@fifo99.com>
> > > > Signed-off-by: Martin Schwidefsky <schwidefsky@de.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 ?
>
> But can't we drop the xtime_shift if we have the clocksource shift kept
> internally? Unless I'm missing something more subtle in your patches,
> they should be the same at all times.

How about this definition for struct timekeeper?

/* Structure holding internal timekeeping values. */
struct timekeeper {
/* Current clocksource used for timekeeping. */
struct clocksource *clock;
/* The shift value of the current clocksource. */
int shift;

/* Number of clock cycles in one NTP interval. */
cycle_t cycle_interval;
/* Number of clock shifted nano seconds in one NTP interval. */
u64 xtime_interval;
/* Raw nano seconds accumulated per NTP interval. */
u32 raw_interval;

/* Clock shifted nano seconds remainder not stored in xtime.tv_nsec. */
u64 xtime_nsec;
/* Difference between accumulated time and NTP time in ntp
* shifted nano seconds. */
s64 ntp_error;
/* Shift conversion between clock shifted nano seconds and
* ntp shifted nano seconds. */
int ntp_error_shift;
/* NTP adjusted clock multiplier */
u32 mult;
};