linux/arch/x86/include/asm/timer.h
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   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
  10unsigned long long native_sched_clock(void);
  11extern int recalibrate_cpu_khz(void);
  12
  13extern 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
  53DECLARE_PER_CPU(unsigned long, cyc2ns);
  54DECLARE_PER_CPU(unsigned long long, cyc2ns_offset);
  55
  56#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */
  57
  58static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
  59{
  60        unsigned long long quot;
  61        unsigned long long rem;
  62        int cpu = smp_processor_id();
  63        unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
  64        quot = (cyc >> CYC2NS_SCALE_FACTOR);
  65        rem = cyc & ((1ULL << CYC2NS_SCALE_FACTOR) - 1);
  66        ns += quot * per_cpu(cyc2ns, cpu) +
  67                ((rem * per_cpu(cyc2ns, cpu)) >> CYC2NS_SCALE_FACTOR);
  68        return ns;
  69}
  70
  71static inline unsigned long long cycles_2_ns(unsigned long long cyc)
  72{
  73        unsigned long long ns;
  74        unsigned long flags;
  75
  76        local_irq_save(flags);
  77        ns = __cycles_2_ns(cyc);
  78        local_irq_restore(flags);
  79
  80        return ns;
  81}
  82
  83#endif /* _ASM_X86_TIMER_H */
  84
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