On Mon, Sep 14, 2015 at 7:48 AM, Miroslav Lichvar <[email protected]> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
