include/linux/timecounter.h

   1 /*
   2  * linux/include/linux/timecounter.h
   3  *
   4  * based on code that migrated away from
   5  * linux/include/linux/clocksource.h
   6  *
   7  * This program is free software; you can redistribute it and/or modify
   8  * it under the terms of the GNU General Public License as published by
   9  * the Free Software Foundation; either version 2 of the License, or
  10  * (at your option) any later version.
  11  *
  12  * This program is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15  * GNU General Public License for more details.
  16  */
  17 #ifndef _LINUX_TIMECOUNTER_H
  18 #define _LINUX_TIMECOUNTER_H
  19
  20 #include <linux/types.h>
  21
  22 /**
  23  * struct cyclecounter - hardware abstraction for a free running counter
  24  *      Provides completely state-free accessors to the underlying hardware.
  25  *      Depending on which hardware it reads, the cycle counter may wrap
  26  *      around quickly. Locking rules (if necessary) have to be defined
  27  *      by the implementor and user of specific instances of this API.
  28  *
  29  * @read:               returns the current cycle value
  30  * @mask:               bitmask for two's complement
  31  *                      subtraction of non 64 bit counters,
  32  *                      see CLOCKSOURCE_MASK() helper macro
  33  * @mult:               cycle to nanosecond multiplier
  34  * @shift:              cycle to nanosecond divisor (power of two)
  35  */
  36 struct cyclecounter {
  37         cycle_t (*read)(const struct cyclecounter *cc);
  38         cycle_t mask;
  39         u32 mult;
  40         u32 shift;
  41 };
  42
  43 /**
  44  * struct timecounter - layer above a %struct cyclecounter which counts nanoseconds
  45  *      Contains the state needed by timecounter_read() to detect
  46  *      cycle counter wrap around. Initialize with
  47  *      timecounter_init(). Also used to convert cycle counts into the
  48  *      corresponding nanosecond counts with timecounter_cyc2time(). Users
  49  *      of this code are responsible for initializing the underlying
  50  *      cycle counter hardware, locking issues and reading the time
  51  *      more often than the cycle counter wraps around. The nanosecond
  52  *      counter will only wrap around after ~585 years.
  53  *
  54  * @cc:                 the cycle counter used by this instance
  55  * @cycle_last:         most recent cycle counter value seen by
  56  *                      timecounter_read()
  57  * @nsec:               continuously increasing count
  58  * @mask:               bit mask for maintaining the 'frac' field
  59  * @frac:               accumulated fractional nanoseconds
  60  */
  61 struct timecounter {
  62         const struct cyclecounter *cc;
  63         cycle_t cycle_last;
  64         u64 nsec;
  65         u64 mask;
  66         u64 frac;
  67 };
  68
  69 /**
  70  * cyclecounter_cyc2ns - converts cycle counter cycles to nanoseconds
  71  * @cc:         Pointer to cycle counter.
  72  * @cycles:     Cycles
  73  * @mask:       bit mask for maintaining the 'frac' field
  74  * @frac:       pointer to storage for the fractional nanoseconds.
  75  */
  76 static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,
  77                                       cycle_t cycles, u64 mask, u64 *frac)
  78 {
  79         u64 ns = (u64) cycles;
  80
  81         ns = (ns * cc->mult) + *frac;
  82         *frac = ns & mask;
  83         return ns >> cc->shift;
  84 }
  85
  86 /**
  87  * timecounter_adjtime - Shifts the time of the clock.
  88  * @delta:      Desired change in nanoseconds.
  89  */
  90 static inline void timecounter_adjtime(struct timecounter *tc, s64 delta)
  91 {
  92         tc->nsec += delta;
  93 }
  94
  95 /**
  96  * timecounter_init - initialize a time counter
  97  * @tc:                 Pointer to time counter which is to be initialized/reset
  98  * @cc:                 A cycle counter, ready to be used.
  99  * @start_tstamp:       Arbitrary initial time stamp.
 100  *
 101  * After this call the current cycle register (roughly) corresponds to
 102  * the initial time stamp. Every call to timecounter_read() increments
 103  * the time stamp counter by the number of elapsed nanoseconds.
 104  */
 105 extern void timecounter_init(struct timecounter *tc,
 106                              const struct cyclecounter *cc,
 107                              u64 start_tstamp);
 108
 109 /**
 110  * timecounter_read - return nanoseconds elapsed since timecounter_init()
 111  *                    plus the initial time stamp
 112  * @tc:          Pointer to time counter.
 113  *
 114  * In other words, keeps track of time since the same epoch as
 115  * the function which generated the initial time stamp.
 116  */
 117 extern u64 timecounter_read(struct timecounter *tc);
 118
 119 /**
 120  * timecounter_cyc2time - convert a cycle counter to same
 121  *                        time base as values returned by
 122  *                        timecounter_read()
 123  * @tc:         Pointer to time counter.
 124  * @cycle_tstamp:       a value returned by tc->cc->read()
 125  *
 126  * Cycle counts that are converted correctly as long as they
 127  * fall into the interval [-1/2 max cycle count, +1/2 max cycle count],
 128  * with "max cycle count" == cs->mask+1.
 129  *
 130  * This allows conversion of cycle counter values which were generated
 131  * in the past.
 132  */
 133 extern u64 timecounter_cyc2time(struct timecounter *tc,
 134                                 cycle_t cycle_tstamp);
 135
 136 #endif