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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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