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From: Lin Mac on 5 Mar 2010 16:30 hi, While working with the clocksource, I found that the relation of cycle and nanosecond of div_sc and clocksource_cyc2ns is different. linux-2.6.33 div_sc in include/linux/clockchips.h suggests that clock_ticks = (nanoseconds * factor) >> shift. 98/* 99 * Calculate a multiplication factor for scaled math, which is used to convert 100 * nanoseconds based values to clock ticks: 101 * 102 * clock_ticks = (nanoseconds * factor) >> shift. 103 * 104 * div_sc is the rearranged equation to calculate a factor from a given clock 105 * ticks / nanoseconds ratio: 106 * 107 * factor = (clock_ticks << shift) / nanoseconds 108 */ 109static inline unsigned long div_sc(unsigned long ticks, unsigned long nsec, 110 int shift) 111{ 112 uint64_t tmp = ((uint64_t)ticks) << shift; 113 114 do_div(tmp, nsec); 115 return (unsigned long) tmp; 116} However, clocksource_cyc2ns in include/linux/clocksource.h suggest nanoseconds=(cycles*mult)>>shift. 268static inline s64 clocksource_cyc2ns(cycle_t cycles, u32 mult, u32 shift) 269{ 270 return ((u64) cycles * mult) >> shift; 271} But there are many others using div_sc, I don't think they got issues with it. So what am I missing? Best Regards, Mac Lin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: john stultz on 5 Mar 2010 16:30
On Fri, Mar 5, 2010 at 1:14 PM, Lin Mac <mkl0301(a)gmail.com> wrote: > While working with the clocksource, I found that the relation of cycle > and nanosecond of div_sc and clocksource_cyc2ns is different. > > linux-2.6.33 > div_sc in include/linux/clockchips.h suggests that clock_ticks = > (nanoseconds * factor) >> shift. > However, clocksource_cyc2ns in include/linux/clocksource.h suggest > nanoseconds=(cycles*mult)>>shift. You're comparing similar helper conversion functions from two different subsystems. Both use similar shifted math, but the div_sc() is for clockchips.h, which is used by the clockevents code to trigger interrupts at a specified time, while clocksource_cyc2ns() is for the clocksource code, which converts clockosurce cycles into nanoseconds for timekeeping. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |