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Linux/include/linux/timecounter.h

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  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 /* simplify initialization of mask field */
 23 #define CYCLECOUNTER_MASK(bits) (u64)((bits) < 64 ? ((1ULL<<(bits))-1) : -1)
 24 
 25 /**
 26  * struct cyclecounter - hardware abstraction for a free running counter
 27  *      Provides completely state-free accessors to the underlying hardware.
 28  *      Depending on which hardware it reads, the cycle counter may wrap
 29  *      around quickly. Locking rules (if necessary) have to be defined
 30  *      by the implementor and user of specific instances of this API.
 31  *
 32  * @read:               returns the current cycle value
 33  * @mask:               bitmask for two's complement
 34  *                      subtraction of non 64 bit counters,
 35  *                      see CYCLECOUNTER_MASK() helper macro
 36  * @mult:               cycle to nanosecond multiplier
 37  * @shift:              cycle to nanosecond divisor (power of two)
 38  */
 39 struct cyclecounter {
 40         u64 (*read)(const struct cyclecounter *cc);
 41         u64 mask;
 42         u32 mult;
 43         u32 shift;
 44 };
 45 
 46 /**
 47  * struct timecounter - layer above a %struct cyclecounter which counts nanoseconds
 48  *      Contains the state needed by timecounter_read() to detect
 49  *      cycle counter wrap around. Initialize with
 50  *      timecounter_init(). Also used to convert cycle counts into the
 51  *      corresponding nanosecond counts with timecounter_cyc2time(). Users
 52  *      of this code are responsible for initializing the underlying
 53  *      cycle counter hardware, locking issues and reading the time
 54  *      more often than the cycle counter wraps around. The nanosecond
 55  *      counter will only wrap around after ~585 years.
 56  *
 57  * @cc:                 the cycle counter used by this instance
 58  * @cycle_last:         most recent cycle counter value seen by
 59  *                      timecounter_read()
 60  * @nsec:               continuously increasing count
 61  * @mask:               bit mask for maintaining the 'frac' field
 62  * @frac:               accumulated fractional nanoseconds
 63  */
 64 struct timecounter {
 65         const struct cyclecounter *cc;
 66         u64 cycle_last;
 67         u64 nsec;
 68         u64 mask;
 69         u64 frac;
 70 };
 71 
 72 /**
 73  * cyclecounter_cyc2ns - converts cycle counter cycles to nanoseconds
 74  * @cc:         Pointer to cycle counter.
 75  * @cycles:     Cycles
 76  * @mask:       bit mask for maintaining the 'frac' field
 77  * @frac:       pointer to storage for the fractional nanoseconds.
 78  */
 79 static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,
 80                                       u64 cycles, u64 mask, u64 *frac)
 81 {
 82         u64 ns = (u64) cycles;
 83 
 84         ns = (ns * cc->mult) + *frac;
 85         *frac = ns & mask;
 86         return ns >> cc->shift;
 87 }
 88 
 89 /**
 90  * timecounter_adjtime - Shifts the time of the clock.
 91  * @delta:      Desired change in nanoseconds.
 92  */
 93 static inline void timecounter_adjtime(struct timecounter *tc, s64 delta)
 94 {
 95         tc->nsec += delta;
 96 }
 97 
 98 /**
 99  * timecounter_init - initialize a time counter
100  * @tc:                 Pointer to time counter which is to be initialized/reset
101  * @cc:                 A cycle counter, ready to be used.
102  * @start_tstamp:       Arbitrary initial time stamp.
103  *
104  * After this call the current cycle register (roughly) corresponds to
105  * the initial time stamp. Every call to timecounter_read() increments
106  * the time stamp counter by the number of elapsed nanoseconds.
107  */
108 extern void timecounter_init(struct timecounter *tc,
109                              const struct cyclecounter *cc,
110                              u64 start_tstamp);
111 
112 /**
113  * timecounter_read - return nanoseconds elapsed since timecounter_init()
114  *                    plus the initial time stamp
115  * @tc:          Pointer to time counter.
116  *
117  * In other words, keeps track of time since the same epoch as
118  * the function which generated the initial time stamp.
119  */
120 extern u64 timecounter_read(struct timecounter *tc);
121 
122 /**
123  * timecounter_cyc2time - convert a cycle counter to same
124  *                        time base as values returned by
125  *                        timecounter_read()
126  * @tc:         Pointer to time counter.
127  * @cycle_tstamp:       a value returned by tc->cc->read()
128  *
129  * Cycle counts that are converted correctly as long as they
130  * fall into the interval [-1/2 max cycle count, +1/2 max cycle count],
131  * with "max cycle count" == cs->mask+1.
132  *
133  * This allows conversion of cycle counter values which were generated
134  * in the past.
135  */
136 extern u64 timecounter_cyc2time(struct timecounter *tc,
137                                 u64 cycle_tstamp);
138 
139 #endif
140 

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