diff options
Diffstat (limited to 'arch/i386/kernel/timers/timer_hpet.c')
| -rw-r--r-- | arch/i386/kernel/timers/timer_hpet.c | 17 |
1 files changed, 11 insertions, 6 deletions
diff --git a/arch/i386/kernel/timers/timer_hpet.c b/arch/i386/kernel/timers/timer_hpet.c index d973a8b681fd..be242723c339 100644 --- a/arch/i386/kernel/timers/timer_hpet.c +++ b/arch/i386/kernel/timers/timer_hpet.c @@ -30,23 +30,28 @@ static seqlock_t monotonic_lock = SEQLOCK_UNLOCKED; * basic equation: * ns = cycles / (freq / ns_per_sec) * ns = cycles * (ns_per_sec / freq) - * ns = cycles * (10^9 / (cpu_mhz * 10^6)) - * ns = cycles * (10^3 / cpu_mhz) + * 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 * (10^3 * SC / cpu_mhz) / SC + * 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!" */ static unsigned long cyc2ns_scale; #define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */ -static inline void set_cyc2ns_scale(unsigned long cpu_mhz) +static inline void set_cyc2ns_scale(unsigned long cpu_khz) { - cyc2ns_scale = (1000 << CYC2NS_SCALE_FACTOR)/cpu_mhz; + cyc2ns_scale = (1000000 << CYC2NS_SCALE_FACTOR)/cpu_khz; } static inline unsigned long long cycles_2_ns(unsigned long long cyc) @@ -163,7 +168,7 @@ static int __init init_hpet(char* override) printk("Detected %u.%03u MHz processor.\n", cpu_khz / 1000, cpu_khz % 1000); } - set_cyc2ns_scale(cpu_khz/1000); + set_cyc2ns_scale(cpu_khz); } /* set this only when cpu_has_tsc */ timer_hpet.read_timer = read_timer_tsc; |