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    SubjectRe: [RFC][patch 09/12] add xtime_shift and ntp_error_shift to struct timekeeper
    On Thu, 30 Jul 2009 15:15:23 -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-shift.diff)
    > > From: Martin Schwidefsky <schwidefsky@de.ibm.com>
    > >
    > > The xtime_nsec value in the timekeeper structure is shifted by a few
    > > bits to improve precision. This happens to be the same value as the
    > > clock->shift. To improve readability add xtime_shift to the timekeeper
    > > and use it instead of the clock->shift. Likewise add ntp_error_shift
    > > and replace all (NTP_SCALE_SHIFT - clock->shift) expressions.
    > >
    > > 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>
    > > ---
    > > kernel/time/timekeeping.c | 33 ++++++++++++++++++---------------
    > > 1 file changed, 18 insertions(+), 15 deletions(-)
    > >
    > > Index: linux-2.6/kernel/time/timekeeping.c
    > > ===================================================================
    > > --- linux-2.6.orig/kernel/time/timekeeping.c
    > > +++ linux-2.6/kernel/time/timekeeping.c
    > > @@ -27,6 +27,8 @@ struct timekeeper {
    > > u32 raw_interval;
    > > u64 xtime_nsec;
    > > s64 ntp_error;
    > > + int xtime_shift;
    > > + int ntp_error_shift;
    > > };
    >
    > I suspect ntp_error_shift is one of the lease intuitive values in the
    > timekeeper structure. They all probably need nice comments explaining
    > what they store, but especially this one.
    >
    > Something like:
    >
    > struct clocksource *clock; /* current clocksource used for timekeeping*/
    > cycle_t cycle_interval; /* fixed chunk of cycles used in accumulation */
    > u64 xtime_interval; /* number clock shifted nsecs accumulated per cycle_interval */
    > u32 raw_interval; /* raw nsecs accumulated per cycle_interval */
    > u64 xtime_nsec; /* clock shifted nsecs remainder not stored in xtime.tv_nsec */
    > s64 ntp_error; /* Difference between accumulated time and NTP time in ntp shifted nsecs */
    > int xtime_shift; /* The shift value of the current clocksource */
    > int ntp_error_shift; /* Shift conversion between clock shifted nsecs and ntp shifted nsecs */

    For xtime_shift I would like to use a different comment. As I think
    about the value it is the number of bits the xtime_nsec value is
    shifted to improve precision. It is the same as clock->shift but the
    semantics is different. How about:

    /* Shift conversion between the xtime_nsec remainder and nsecs */
    int xtime_shift;

    The other comments are good, I've added them.


    > Other then that this patch looks ok, but will need rigorous testing, as
    > its very complicated and difficult to understand code thats being
    > changed.

    This patch in particular is rather easy to verify, but the overall series
    will definitely need some time in linux-next.

    --
    blue skies,
    Martin.

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



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