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

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Date:	Wed, 04 Aug 2010 15:49:47 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Alexander Gordeev <lasaine@....cs.msu.su>
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

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.

> +/* 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
-john


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