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    SubjectRe: [PATCH 2/2] Introduce CLOCK_MONOTONIC_RAW
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    On Wed, 2008-04-02 at 13:50 +0200, Roman Zippel wrote:
    > Hi,
    >
    > On Tue, 18 Mar 2008, john stultz wrote:
    >
    > > My solution is to introduce CLOCK_MONOTONIC_RAW. This exposes a
    > > nanosecond based time value, that increments starting at bootup and has
    > > no frequency adjustments made to it what so ever.
    > >
    > > The time is accessed from userspace via the posix_clock_gettime()
    > > syscall, passing CLOCK_MONOTONIC_RAW as the clock_id.
    >
    > This is a reworked version of this patch based on the previous
    > clocksource_forward_now patch, since clocksource_forward_now() takes
    > care of the time offset now, it's not needed to do this at various places.
    > I also got rid of the monotonic_raw splitting, so the work done during
    > update_wall_time() is quite a bit simpler.
    >
    > bye, Roman
    >
    > Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
    >
    > ---
    > include/linux/clocksource.h | 5 ++++
    > include/linux/time.h | 2 +
    > kernel/posix-timers.c | 15 ++++++++++++++
    > kernel/time/timekeeping.c | 47 ++++++++++++++++++++++++++++++++++++++++++++
    > 4 files changed, 69 insertions(+)
    >
    > Index: linux-2.6/include/linux/clocksource.h
    > ===================================================================
    > --- linux-2.6.orig/include/linux/clocksource.h
    > +++ linux-2.6/include/linux/clocksource.h
    > @@ -79,6 +79,7 @@ struct clocksource {
    > /* timekeeping specific data, ignore */
    > cycle_t cycle_interval;
    > u64 xtime_interval;
    > + u64 raw_interval;
    > /*
    > * Second part is written at each timer interrupt
    > * Keep it in a different cache line to dirty no
    > @@ -87,6 +88,8 @@ struct clocksource {
    > cycle_t cycle_last ____cacheline_aligned_in_smp;
    > u64 xtime_nsec;
    > s64 error;
    > + u64 raw_nsec;
    > + long raw_sec;


    So, with the raw_sec being stored in the clocksource, and there not
    being a monotonic_raw value, doesn't this mean the MONOTONIC_RAW value
    will clear to zero on clocksource changes?


    > #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
    > /* Watchdog related data, used by the framework */
    > @@ -215,6 +218,8 @@ static inline void clocksource_calculate
    >
    > /* Go back from cycles -> shifted ns, this time use ntp adjused mult */
    > c->xtime_interval = (u64)c->cycle_interval * c->mult;
    > + c->raw_interval = ((u64)c->cycle_interval * c->mult_orig) <<
    > + (NTP_SCALE_SHIFT - c->shift);
    > }

    Could you explain further how this extra shift scaling is beneficial?
    (Additionally, if we're using it for more then just NTP's shift, we
    might want to change its name).

    thanks
    -john



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