1 #ifndef _ASM_X86_TIMER_H 2 #define _ASM_X86_TIMER_H 3 #include <linux/init.h> 4 #include <linux/pm.h> 5 #include <linux/percpu.h> 6 #include <linux/interrupt.h> 7 8 #define TICK_SIZE (tick_nsec / 1000) 9 10 unsigned long long native_sched_clock(void); 11 extern int recalibrate_cpu_khz(void); 12 13 extern int no_timer_check; 14 15 /* Accelerators for sched_clock() 16 * convert from cycles(64bits) => nanoseconds (64bits) 17 * basic equation: 18 * ns = cycles / (freq / ns_per_sec) 19 * ns = cycles * (ns_per_sec / freq) 20 * ns = cycles * (10^9 / (cpu_khz * 10^3)) 21 * ns = cycles * (10^6 / cpu_khz) 22 * 23 * Then we use scaling math (suggested by george@mvista.com) to get: 24 * ns = cycles * (10^6 * SC / cpu_khz) / SC 25 * ns = cycles * cyc2ns_scale / SC 26 * 27 * And since SC is a constant power of two, we can convert the div 28 * into a shift. 29 * 30 * We can use khz divisor instead of mhz to keep a better precision, since 31 * cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits. 32 * (mathieu.desnoyers@polymtl.ca) 33 * 34 * -johnstul@us.ibm.com "math is hard, lets go shopping!" 35 * 36 * In: 37 * 38 * ns = cycles * cyc2ns_scale / SC 39 * 40 * Although we may still have enough bits to store the value of ns, 41 * in some cases, we may not have enough bits to store cycles * cyc2ns_scale, 42 * leading to an incorrect result. 43 * 44 * To avoid this, we can decompose 'cycles' into quotient and remainder 45 * of division by SC. Then, 46 * 47 * ns = (quot * SC + rem) * cyc2ns_scale / SC 48 * = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC 49 * 50 * - sqazi@google.com 51 */ 52 53 DECLARE_PER_CPU(unsigned long, cyc2ns); 54 DECLARE_PER_CPU(unsigned long long, cyc2ns_offset); 55 56 #define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */ 57 58 static inline unsigned long long __cycles_2_ns(unsigned long long cyc) 59 { 60 int cpu = smp_processor_id(); 61 unsigned long long ns = per_cpu(cyc2ns_offset, cpu); 62 ns += mult_frac(cyc, per_cpu(cyc2ns, cpu), 63 (1UL << CYC2NS_SCALE_FACTOR)); 64 return ns; 65 } 66 67 static inline unsigned long long cycles_2_ns(unsigned long long cyc) 68 { 69 unsigned long long ns; 70 unsigned long flags; 71 72 local_irq_save(flags); 73 ns = __cycles_2_ns(cyc); 74 local_irq_restore(flags); 75 76 return ns; 77 } 78 79 #endif /* _ASM_X86_TIMER_H */ 80