netdev - Re: [PATCH V8 02/13] ntp: add ADJ

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Date:	Mon, 10 Jan 2011 13:06:37 -0800
From:	john stultz <johnstul@...ibm.com>
To:	"Kuwahara,T." <6vvetjsrt26xsrzlh1z0zn4d2grdah@...il.com>
Cc:	Richard Cochran <richardcochran@...il.com>,
	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>,
	Krzysztof Halasa <khc@...waw.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Rodolfo Giometti <giometti@...ux.it>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH V8 02/13] ntp: add ADJ_SETOFFSET mode bit

On Tue, 2011-01-11 at 05:45 +0900, Kuwahara,T. wrote:
> On Tue, Jan 11, 2011 at 1:49 AM, john stultz <johnstul@...ibm.com> wrote:
> > Leapsecond processing is done via an absolute hrtimer. Thus when the
> > time offset is set, the hrtimers that should have expired will fire
> > (just like with settimeofday) and the adjustment will then be made.
> 
> How do you convert relative time to absolute time?  It's not trivial
> because TAI offset is also a variable.

I don't believe I understand what you're getting at.

The proposed interface is almost identical in functionality to a
userland application doing the following:

	offset = my_calculate_offset();
	clock_gettime(CLOCK_REALTIME, &now);
	newtime = my_add_ts(now, offset);
	settimeofday(&newtime, 0);

The only difference is that you avoid the error from the delay between
the gettime call and the settime call. It just adds the offset directly
to the CLOCK_REALTIME.


thanks
-john



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