linux-kernel - Re: [PATCH 2/2 (v2)] kselftest: timers: Add adjtick test to validate adjtimex() tick adjustments

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Date:	Fri, 2 Oct 2015 13:49:14 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Miroslav Lichvar <mlichvar@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Nuno Gonçalves <nunojpg@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Shuah Khan <shuahkh@....samsung.com>
Subject: Re: [PATCH 2/2 (v2)] kselftest: timers: Add adjtick test to validate
 adjtimex() tick adjustments

On Mon, Sep 14, 2015 at 7:48 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> Is there a cheap way to calculate this?
> xtime_remainder * (ntp_tick >> ntp_error_shift) / NTP_INTERVAL_LENGTH

So as I was trying to figure out before I prematurely hit send...

I was thinking...

 xtime_remainder = (NTP_INTERVAL_LENGTH << tk->tkr_mono.clock->shift)
- (tk->cycle_interval * tk->tkr_mono.clock->mult)

or for simplification:

xtime_remainder = NTP_INTERVAL_SHIFTED - ORIG_MULT_INTERVAL_SHIFTED

And we want to scale it as you pointed out above (though slightly
fixed here) by:
(tk->ntp_tick >> tk->ntp_error_shift) / (NTP_INTERVAL_LENGTH <<
tk->tkr_mono.clock->shift)

Which we'll simplify a touch as:
(tk->ntp_tick >> tk->ntp_error_shift) / NTP_INTERVAL_SHIFTED

So multiplying these comes out to:

(tk->ntp_tick >> tk->ntp_error_shift) - ((tk->ntp_tick >>
tk->ntp_error_shift)  * ORIG_MULT_INTERVAL_SHIFTED /
NTP_INTERVAL_SHIFTED)

Which simplifies a bit to:

(tk->ntp_tick - (tk->ntp_tick *  ORIG_MULT_INTERVAL_SHIFTED /
NTP_INTERVAL_SHIFTED)) >> tk->ntp_error_shift

(tk->ntp_tick * ( 1 - ORIG_MULT_INTERVAL_SHIFTED /
NTP_INTERVAL_SHIFTED)) >> tk->ntp_error_shift;

So this looks like it would be cheap to calculate,  but since we're
doing integer math, the problem is that  ORIG_MULT_INTERVAL_SHIFTED /
NTP_INTERVAL_SHIFTED isn't an integer value, so if we want any
precision we'd still have the costly tk->ntp_tick/NTP_INTERVAL_SHIFTED
to do each time.

So.. that's a long winded way to say I can't think of a cheap way.... :P

thanks
-john
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