```   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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