diff options
Diffstat (limited to 'include/asm-i386/timer.h')
| -rw-r--r-- | include/asm-i386/timer.h | 32 |
1 files changed, 31 insertions, 1 deletions
diff --git a/include/asm-i386/timer.h b/include/asm-i386/timer.h index 153770e25fa..51a713e33a9 100644 --- a/include/asm-i386/timer.h +++ b/include/asm-i386/timer.h @@ -15,8 +15,38 @@ extern int no_sync_cmos_clock; extern int recalibrate_cpu_khz(void); #ifndef CONFIG_PARAVIRT -#define get_scheduled_cycles(val) rdtscll(val) #define calculate_cpu_khz() native_calculate_cpu_khz() #endif +/* Accellerators for sched_clock() + * convert from cycles(64bits) => nanoseconds (64bits) + * basic equation: + * ns = cycles / (freq / ns_per_sec) + * ns = cycles * (ns_per_sec / freq) + * ns = cycles * (10^9 / (cpu_khz * 10^3)) + * ns = cycles * (10^6 / cpu_khz) + * + * Then we use scaling math (suggested by george@mvista.com) to get: + * ns = cycles * (10^6 * SC / cpu_khz) / SC + * ns = cycles * cyc2ns_scale / SC + * + * And since SC is a constant power of two, we can convert the div + * into a shift. + * + * We can use khz divisor instead of mhz to keep a better percision, since + * cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits. + * (mathieu.desnoyers@polymtl.ca) + * + * -johnstul@us.ibm.com "math is hard, lets go shopping!" + */ +extern unsigned long cyc2ns_scale __read_mostly; + +#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */ + +static inline unsigned long long cycles_2_ns(unsigned long long cyc) +{ + return (cyc * cyc2ns_scale) >> CYC2NS_SCALE_FACTOR; +} + + #endif |