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)(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 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 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 * This is like cyclecounter_cyc2ns(), but it is used for computing a 120 * time previous to the time stored in the cycle counter. 121 */ 122 static inline u64 cc_cyc2ns_backwards(const struct cyclecounter *cc, u64 cycles, u64 frac) 123 { 124 return ((cycles * cc->mult) - frac) >> cc->shift; 125 } 126 127 /** 128 * timecounter_cyc2time - convert a cycle counter to same 129 * time base as values returned by 130 * timecounter_read() 131 * @tc: Pointer to time counter. 132 * @cycle_tstamp: a value returned by tc->cc->read() 133 * 134 * Cycle counts that are converted correctly as long as they 135 * fall into the interval [-1/2 max cycle count, +1/2 max cycle count], 136 * with "max cycle count" == cs->mask+1. 137 * 138 * This allows conversion of cycle counter values which were generated 139 * in the past. 140 * 141 * Returns: cycle counter converted to nanoseconds since the initial time stamp 142 */ 143 static inline u64 timecounter_cyc2time(const struct timecounter *tc, u64 cycle_tstamp) 144 { 145 const struct cyclecounter *cc = tc->cc; 146 u64 delta = (cycle_tstamp - tc->cycle_last) & cc->mask; 147 u64 nsec = tc->nsec, frac = tc->frac; 148 149 /* 150 * Instead of always treating cycle_tstamp as more recent than 151 * tc->cycle_last, detect when it is too far in the future and 152 * treat it as old time stamp instead. 153 */ 154 if (unlikely(delta > cc->mask / 2)) { 155 delta = (tc->cycle_last - cycle_tstamp) & cc->mask; 156 nsec -= cc_cyc2ns_backwards(cc, delta, frac); 157 } else { 158 nsec += cyclecounter_cyc2ns(cc, delta, tc->mask, &frac); 159 } 160 161 return nsec; 162 } 163 164 #endif 165