include/linux/timecounter.h

   1 /* SPDX-License-Identifier: GPL-2.0-or-later */
   2 /*
   3  * linux/include/linux/timecounter.h
   4  *
   5  * based on code that migrated away from
   6  * linux/include/linux/clocksource.h
   7  */
   8 #ifndef _LINUX_TIMECOUNTER_H
   9 #define _LINUX_TIMECOUNTER_H
  10
  11 #include <linux/types.h>
  12
  13 /* simplify initialization of mask field */
  14 #define CYCLECOUNTER_MASK(bits) (u64)((bits) < 64 ? ((1ULL<<(bits))-1) : -1)
  15
  16 /**
  17  * struct cyclecounter - hardware abstraction for a free running counter
  18  *      Provides completely state-free accessors to the underlying hardware.
  19  *      Depending on which hardware it reads, the cycle counter may wrap
  20  *      around quickly. Locking rules (if necessary) have to be defined
  21  *      by the implementor and user of specific instances of this API.
  22  *
  23  * @read:               returns the current cycle value
  24  * @mask:               bitmask for two's complement
  25  *                      subtraction of non-64-bit counters,
  26  *                      see CYCLECOUNTER_MASK() helper macro
  27  * @mult:               cycle to nanosecond multiplier
  28  * @shift:              cycle to nanosecond divisor (power of two)
  29  */
  30 struct cyclecounter {
  31         u64 (*read)(const struct cyclecounter *cc);
  32         u64 mask;
  33         u32 mult;
  34         u32 shift;
  35 };
  36
  37 /**
  38  * struct timecounter - layer above a &struct cyclecounter which counts nanoseconds
  39  *      Contains the state needed by timecounter_read() to detect
  40  *      cycle counter wrap around. Initialize with
  41  *      timecounter_init(). Also used to convert cycle counts into the
  42  *      corresponding nanosecond counts with timecounter_cyc2time(). Users
  43  *      of this code are responsible for initializing the underlying
  44  *      cycle counter hardware, locking issues and reading the time
  45  *      more often than the cycle counter wraps around. The nanosecond
  46  *      counter will only wrap around after ~585 years.
  47  *
  48  * @cc:                 the cycle counter used by this instance
  49  * @cycle_last:         most recent cycle counter value seen by
  50  *                      timecounter_read()
  51  * @nsec:               continuously increasing count
  52  * @mask:               bit mask for maintaining the 'frac' field
  53  * @frac:               accumulated fractional nanoseconds
  54  */
  55 struct timecounter {
  56         const struct cyclecounter *cc;
  57         u64 cycle_last;
  58         u64 nsec;
  59         u64 mask;
  60         u64 frac;
  61 };
  62
  63 /**
  64  * cyclecounter_cyc2ns - converts cycle counter cycles to nanoseconds
  65  * @cc:         Pointer to cycle counter.
  66  * @cycles:     Cycles
  67  * @mask:       bit mask for maintaining the 'frac' field
  68  * @frac:       pointer to storage for the fractional nanoseconds.
  69  *
  70  * Returns: cycle counter cycles converted to nanoseconds
  71  */
  72 static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,
  73                                       u64 cycles, u64 mask, u64 *frac)
  74 {
  75         u64 ns = (u64) cycles;
  76
  77         ns = (ns * cc->mult) + *frac;
  78         *frac = ns & mask;
  79         return ns >> cc->shift;
  80 }
  81
  82 /**
  83  * timecounter_adjtime - Shifts the time of the clock.
  84  * @tc:         The &struct timecounter to adjust
  85  * @delta:      Desired change in nanoseconds.
  86  */
  87 static inline void timecounter_adjtime(struct timecounter *tc, s64 delta)
  88 {
  89         tc->nsec += delta;
  90 }
  91
  92 /**
  93  * timecounter_init - initialize a time counter
  94  * @tc:                 Pointer to time counter which is to be initialized/reset
  95  * @cc:                 A cycle counter, ready to be used.
  96  * @start_tstamp:       Arbitrary initial time stamp.
  97  *
  98  * After this call the current cycle register (roughly) corresponds to
  99  * the initial time stamp. Every call to timecounter_read() increments
 100  * the time stamp counter by the number of elapsed nanoseconds.
 101  */
 102 extern void timecounter_init(struct timecounter *tc,
 103                              const struct cyclecounter *cc,
 104                              u64 start_tstamp);
 105
 106 /**
 107  * timecounter_read - return nanoseconds elapsed since timecounter_init()
 108  *                    plus the initial time stamp
 109  * @tc:          Pointer to time counter.
 110  *
 111  * In other words, keeps track of time since the same epoch as
 112  * the function which generated the initial time stamp.
 113  *
 114  * Returns: nanoseconds since the initial time stamp
 115  */
 116 extern u64 timecounter_read(struct timecounter *tc);
 117
 118 /**
 119  * timecounter_cyc2time - convert a cycle counter to same
 120  *                        time base as values returned by
 121  *                        timecounter_read()
 122  * @tc:         Pointer to time counter.
 123  * @cycle_tstamp:       a value returned by tc->cc->read()
 124  *
 125  * Cycle counts that are converted correctly as long as they
 126  * fall into the interval [-1/2 max cycle count, +1/2 max cycle count],
 127  * with "max cycle count" == cs->mask+1.
 128  *
 129  * This allows conversion of cycle counter values which were generated
 130  * in the past.
 131  *
 132  * Returns: cycle counter converted to nanoseconds since the initial time stamp
 133  */
 134 extern u64 timecounter_cyc2time(const struct timecounter *tc,
 135                                 u64 cycle_tstamp);
 136
 137 #endif