netdev - Re: [PATCH V7 1/8] ntp: add ADJ

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Date:	Wed, 22 Dec 2010 05:57:02 +0900
From:	"Kuwahara,T." <6vvetjsrt26xsrzlh1z0zn4d2grdah@...il.com>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	netdev@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Christoph Lameter <cl@...ux.com>,
	David Miller <davem@...emloft.net>,
	John Stultz <johnstul@...ibm.com>,
	Krzysztof Halasa <khc@...waw.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Rodolfo Giometti <giometti@...ux.it>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH V7 1/8] ntp: add ADJ_SETOFFSET mode bit

On Tue, Dec 21, 2010 at 4:56 PM, Richard Cochran
<richardcochran@...il.com> wrote:
> Can you please elaborate?

The timex.constant is defined as equal to the binary logarithm of the
reciprocal of
the natural frequency minus SHIFT_PLL.  In other words, the following
equation holds:

	log2(natural frequency) + time_constant + SHIFT_PLL = 0,

which means that decreasing time_constant increases natural frequency
exponentially.
And since a larger natural frequency gives a smaller settling time, a
sufficiently
large negative time_constant results in immediate time step, at least in theory.

> I don't see any way to use timex.constant with ADJ_OFFSET in order to
> correct a time offset.

How about this?

if (txc->modes & ADJ_OFFSET) {
	if (txc->constant == INT32_MIN) {
		/* step time */
	} else {
		/* slew time */
	}
}

> The 'time_constant' in kernel/time/ntp.c is
> restricted to the interval [0..MAXTC], and MAXTC is 10 in timex.h.

Then let's just ignore the restriction.  (It's possible by setting the
timex.constant
without setting the ADJ_TIMECONST flag.)

That said, I'm somehow against the idea of using the adjtimex syscall
for that purpose.
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