arch/x86/include/asm/timer.h

   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 */