linux-kernel - Re: [PATCHv3 12/16] ntp: add hardpps implementation

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Date:	Thu, 5 Aug 2010 16:00:45 +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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Kacur <jkacur@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCHv3 12/16] ntp: add hardpps implementation

В Wed, 04 Aug 2010 15:49:47 -0700
john stultz <johnstul@...ibm.com> пишет:

> On Thu, 2010-08-05 at 01:06 +0400, Alexander Gordeev wrote:
> > This commit adds hardpps() implementation based upon the original one
> > from the NTPv4 reference kernel code from David Mills. However, it is
> > highly optimized towards very fast syncronization and maximum stickness
> > to PPS signal. The typical error is less then a microsecond.
> > To make it sync faster I had to throw away exponential phase filter so
> > that the full phase offset is corrected immediately. Then I also had to
> > throw away median phase filter because it gives a bigger error itself
> > if used without exponential filter.
> > Maybe we will find an appropriate filtering scheme in the future but
> > it's not necessary if the signal quality is ok.
> > 
> > Signed-off-by: Alexander Gordeev <lasaine@....cs.msu.su>
> 
> [snip]
> 
> > +#ifdef	CONFIG_NTP_PPS
> > +
> > +struct pps_normtime {
> > +	__kernel_time_t	sec;	/* seconds */
> > +	long		nsec;	/* nanoseconds */
> > +};
> 
> I don't quite remember the history here (it may be I suggested you use
> this instead of overloading a timespec? I honestly don't recall), but
> could you add some extra context in a comment here for what a
> pps_normtime structure represents and why its used instead of a
> timespec? The comment below sort of hints at it, but it would be useful
> if it was more explicit.

Yes, you asked me to do this. :)
Sure, I'll add an explicit comment.

> > +/* normalize the timestamp so that nsec is in the
> > +   ( -NSEC_PER_SEC / 2, NSEC_PER_SEC / 2 ] interval */
> > +static inline struct pps_normtime pps_normalize_ts(struct timespec ts)
> > +{
> > +	struct pps_normtime norm = {
> > +		.sec = ts.tv_sec,
> > +		.nsec = ts.tv_nsec
> > +	};
> > +
> > +	if (norm.nsec > (NSEC_PER_SEC >> 1)) {
> > +		norm.nsec -= NSEC_PER_SEC;
> > +		norm.sec++;
> > +	}
> > +
> > +	return norm;
> > +}
> 
> Otherwise the code looks pretty good to me.
> 
> Acked-by: John Stultz <johnstul@...ibm.com>

Thanks!

-- 
  Alexander

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