kernel/time/timecounter.c

   1 /*
   2  * linux/kernel/time/timecounter.c
   3  *
   4  * based on code that migrated away from
   5  * linux/kernel/time/clocksource.c
   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
  18 #include <linux/export.h>
  19 #include <linux/timecounter.h>
  20
  21 void timecounter_init(struct timecounter *tc,
  22                       const struct cyclecounter *cc,
  23                       u64 start_tstamp)
  24 {
  25         tc->cc = cc;
  26         tc->cycle_last = cc->read(cc);
  27         tc->nsec = start_tstamp;
  28         tc->mask = (1ULL << cc->shift) - 1;
  29         tc->frac = 0;
  30 }
  31 EXPORT_SYMBOL_GPL(timecounter_init);
  32
  33 /**
  34  * timecounter_read_delta - get nanoseconds since last call of this function
  35  * @tc:         Pointer to time counter
  36  *
  37  * When the underlying cycle counter runs over, this will be handled
  38  * correctly as long as it does not run over more than once between
  39  * calls.
  40  *
  41  * The first call to this function for a new time counter initializes
  42  * the time tracking and returns an undefined result.
  43  */
  44 static u64 timecounter_read_delta(struct timecounter *tc)
  45 {
  46         cycle_t cycle_now, cycle_delta;
  47         u64 ns_offset;
  48
  49         /* read cycle counter: */
  50         cycle_now = tc->cc->read(tc->cc);
  51
  52         /* calculate the delta since the last timecounter_read_delta(): */
  53         cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;
  54
  55         /* convert to nanoseconds: */
  56         ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta,
  57                                         tc->mask, &tc->frac);
  58
  59         /* update time stamp of timecounter_read_delta() call: */
  60         tc->cycle_last = cycle_now;
  61
  62         return ns_offset;
  63 }
  64
  65 u64 timecounter_read(struct timecounter *tc)
  66 {
  67         u64 nsec;
  68
  69         /* increment time by nanoseconds since last call */
  70         nsec = timecounter_read_delta(tc);
  71         nsec += tc->nsec;
  72         tc->nsec = nsec;
  73
  74         return nsec;
  75 }
  76 EXPORT_SYMBOL_GPL(timecounter_read);
  77
  78 /*
  79  * This is like cyclecounter_cyc2ns(), but it is used for computing a
  80  * time previous to the time stored in the cycle counter.
  81  */
  82 static u64 cc_cyc2ns_backwards(const struct cyclecounter *cc,
  83                                cycle_t cycles, u64 mask, u64 frac)
  84 {
  85         u64 ns = (u64) cycles;
  86
  87         ns = ((ns * cc->mult) - frac) >> cc->shift;
  88
  89         return ns;
  90 }
  91
  92 u64 timecounter_cyc2time(struct timecounter *tc,
  93                          cycle_t cycle_tstamp)
  94 {
  95         u64 delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;
  96         u64 nsec = tc->nsec, frac = tc->frac;
  97
  98         /*
  99          * Instead of always treating cycle_tstamp as more recent
 100          * than tc->cycle_last, detect when it is too far in the
 101          * future and treat it as old time stamp instead.
 102          */
 103         if (delta > tc->cc->mask / 2) {
 104                 delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask;
 105                 nsec -= cc_cyc2ns_backwards(tc->cc, delta, tc->mask, frac);
 106         } else {
 107                 nsec += cyclecounter_cyc2ns(tc->cc, delta, tc->mask, &frac);
 108         }
 109
 110         return nsec;
 111 }
 112 EXPORT_SYMBOL_GPL(timecounter_cyc2time);