linux-kernel - Re: [PATCHv3 13/16] pps: capture MONOTONIC

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Date:	Thu, 5 Aug 2010 16:16:37 +0400
From:	Alexander Gordeev <lasaine@....cs.msu.su>
To:	john stultz <johnstul@...ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	"Nikita V. Youshchenko" <yoush@...msu.su>,
	linuxpps@...enneenne.com, Rodolfo Giometti <giometti@...eenne.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jon Hunter <jon-hunter@...com>, Ingo Molnar <mingo@...e.hu>,
	John Kacur <jkacur@...hat.com>
Subject: Re: [PATCHv3 13/16] pps: capture MONOTONIC_RAW timestamps as well

В Wed, 04 Aug 2010 16:03:57 -0700
john stultz <johnstul@...ibm.com> пишет:

> On Thu, 2010-08-05 at 01:06 +0400, Alexander Gordeev wrote:
> > MONOTONIC_RAW clock timestamps are ideally suited for frequency
> > calculation and also fit well into the original NTP hardpps design. Now
> > phase and frequency can be adjusted separately: the former based on
> > REALTIME clock and the latter based on MONOTONIC_RAW clock.
> > A new function getnstime_raw_and_real is added to timekeeping subsystem
> > to capture both timestamps at the same time and atomically.
> > 
> > Signed-off-by: Alexander Gordeev <lasaine@....cs.msu.su>
> > ---
> >  include/linux/pps_kernel.h |    3 ++-
> >  include/linux/time.h       |    2 ++
> >  kernel/time/timekeeping.c  |   34 ++++++++++++++++++++++++++++++++++
> >  3 files changed, 38 insertions(+), 1 deletions(-)
> > 
> > diff --git a/include/linux/pps_kernel.h b/include/linux/pps_kernel.h
> > index 5af0498..39fc712 100644
> > --- a/include/linux/pps_kernel.h
> > +++ b/include/linux/pps_kernel.h
> > @@ -48,6 +48,7 @@ struct pps_source_info {
> >  };
> > 
> >  struct pps_event_time {
> > +	struct timespec ts_raw;
> >  	struct timespec ts_real;
> >  };
> > 
> > @@ -111,7 +112,7 @@ static inline void timespec_to_pps_ktime(struct pps_ktime *kt,
> > 
> >  static inline void pps_get_ts(struct pps_event_time *ts)
> >  {
> > -	getnstimeofday(&ts->ts_real);
> > +	getnstime_raw_and_real(&ts->ts_raw, &ts->ts_real);
> >  }
> > 
> >  #endif /* LINUX_PPS_KERNEL_H */
> > diff --git a/include/linux/time.h b/include/linux/time.h
> > index ea3559f..1da8a7b 100644
> > --- a/include/linux/time.h
> > +++ b/include/linux/time.h
> > @@ -143,6 +143,8 @@ extern unsigned int alarm_setitimer(unsigned int seconds);
> >  extern int do_getitimer(int which, struct itimerval *value);
> >  extern void getnstimeofday(struct timespec *tv);
> >  extern void getrawmonotonic(struct timespec *ts);
> > +extern void getnstime_raw_and_real(struct timespec *ts_raw,
> > +		struct timespec *ts_real);
> >  extern void getboottime(struct timespec *ts);
> >  extern void monotonic_to_bootbased(struct timespec *ts);
> > 
> > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> > index caf8d4d..ef37b14 100644
> > --- a/kernel/time/timekeeping.c
> > +++ b/kernel/time/timekeeping.c
> > @@ -286,6 +286,40 @@ void ktime_get_ts(struct timespec *ts)
> >  EXPORT_SYMBOL_GPL(ktime_get_ts);
> > 
> >  /**
> > + * getnstime_raw_and_real - Returns both the time of day an raw
> > + * monotonic time in a timespec format
> > + * @ts_mono_raw:	pointer to the timespec to be set to raw
> > + *			monotonic time
> > + * @ts_real:		pointer to the timespec to be set to the time
> > + *			of day
> > + */
> > +void getnstime_raw_and_real(struct timespec *ts_raw, struct timespec *ts_real)
> > +{
> > +	unsigned long seq;
> > +	s64 nsecs_raw, nsecs_real;
> > +
> > +	WARN_ON(timekeeping_suspended);
> > +
> > +	do {
> > +		seq = read_seqbegin(&xtime_lock);
> > +
> > +		*ts_raw = raw_time;
> > +		*ts_real = xtime;
> > +
> > +		nsecs_raw = timekeeping_get_ns_raw();
> > +		nsecs_real = timekeeping_get_ns();
> > +
> > +		/* If arch requires, add in gettimeoffset() */
> > +		nsecs_real += arch_gettimeoffset();
> 
> Hmm. So this brings up an interesting point. If the system is an one
> which uses arch_gettimeoffset() (which means it doesn't have
> clocksources), I'm not sure if this function would return what you'd
> expect.
> 
> The issue is that jiffies is the basic clocksource that is in use on
> these systems, but the arch_gettimeoffset() function returns an
> inter-tick time value (usually by reading the interrupt generating
> decrementer or whatnot). So in this case, you'd get a
> CLOCK_MONOTONIC_RAW time that is based off the raw jiffies value at the
> last tick, and CLOCK_REALTIME time value at the exact time of the call.
> 
> That offset between the two would then vary depending on when in the
> inter-tick period the call was made.
> 
> 
> The two solutions would be:
> 1) Add the arch_gettimeoffset() value to nsecs_raw as well. Should be
> safe, as any freq variance from the raw freq will be limited to 1 tick
> length and won't accumulate.
> 
> 2) Don't add arch_gettimeoffset to either value, providing the raw and
> real time values at the last tick.
> 
> I'd probably go with #1.
> 
> Otherwise, the code looks good.

Ok, thanks for the point! I'll use #1 then.

-- 
  Alexander

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