diff options
| author | Pekka Enberg <penberg@kernel.org> | 2012-01-11 21:11:29 +0200 |
|---|---|---|
| committer | Pekka Enberg <penberg@kernel.org> | 2012-01-11 21:11:29 +0200 |
| commit | 5878cf431ca7233a56819ca6970153ac0b129599 (patch) | |
| tree | e5d21d04a0b468d2dabbe3a3824f23b5537fc6a7 /arch/x86/include/asm/timer.h | |
| parent | 74ee4ef1f901fbb014bdcdc9171d126490ce2b62 (diff) | |
| parent | b13683d1cc14d1dd30b8e20f3ebea3f814ad029f (diff) | |
Merge branch 'slab/urgent' into slab/for-linus
Diffstat (limited to 'arch/x86/include/asm/timer.h')
| -rw-r--r-- | arch/x86/include/asm/timer.h | 23 |
1 files changed, 22 insertions, 1 deletions
diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h index fa7b9176b76..431793e5d48 100644 --- a/arch/x86/include/asm/timer.h +++ b/arch/x86/include/asm/timer.h @@ -32,6 +32,22 @@ extern int no_timer_check; * (mathieu.desnoyers@polymtl.ca) * * -johnstul@us.ibm.com "math is hard, lets go shopping!" + * + * In: + * + * ns = cycles * cyc2ns_scale / SC + * + * Although we may still have enough bits to store the value of ns, + * in some cases, we may not have enough bits to store cycles * cyc2ns_scale, + * leading to an incorrect result. + * + * To avoid this, we can decompose 'cycles' into quotient and remainder + * of division by SC. Then, + * + * ns = (quot * SC + rem) * cyc2ns_scale / SC + * = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC + * + * - sqazi@google.com */ DECLARE_PER_CPU(unsigned long, cyc2ns); @@ -41,9 +57,14 @@ DECLARE_PER_CPU(unsigned long long, cyc2ns_offset); static inline unsigned long long __cycles_2_ns(unsigned long long cyc) { + unsigned long long quot; + unsigned long long rem; int cpu = smp_processor_id(); unsigned long long ns = per_cpu(cyc2ns_offset, cpu); - ns += cyc * per_cpu(cyc2ns, cpu) >> CYC2NS_SCALE_FACTOR; + quot = (cyc >> CYC2NS_SCALE_FACTOR); + rem = cyc & ((1ULL << CYC2NS_SCALE_FACTOR) - 1); + ns += quot * per_cpu(cyc2ns, cpu) + + ((rem * per_cpu(cyc2ns, cpu)) >> CYC2NS_SCALE_FACTOR); return ns; } |