Messages in this thread | | | Date | Fri, 2 Oct 2015 13:49:14 -0700 | Subject | Re: [PATCH 2/2 (v2)] kselftest: timers: Add adjtick test to validate adjtimex() tick adjustments | From | John Stultz <> |
| |
On Mon, Sep 14, 2015 at 7:48 AM, Miroslav Lichvar <mlichvar@redhat.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
| |