diff options
| author | Tony Lindgren <tony@atomide.com> | 2012-02-17 15:12:36 -0800 |
|---|---|---|
| committer | Tony Lindgren <tony@atomide.com> | 2012-02-17 15:12:36 -0800 |
| commit | 4d68c05ce11f4cdf6a6392f3a18dc6a985b4d0c4 (patch) | |
| tree | 72e500b97c4bdcb9631209f3121f42b05ae30fea /arch/x86/include/asm/timer.h | |
| parent | 40c0591f0a349ec074357e05c6ab1a3bc951807c (diff) | |
| parent | 19bfb76ca32f8e4fa80746608ff4a77707f40520 (diff) | |
| download | linux-4d68c05ce11f4cdf6a6392f3a18dc6a985b4d0c4.tar.bz2 | |
Merge branch 'for_3.4/dt_base' of git://git.kernel.org/pub/scm/linux/kernel/git/bcousson/linux-omap-dt into dt
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 fa7b9176b76c..431793e5d484 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; } |