LXR linux/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
  21void 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}
  31EXPORT_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 */
  44static u64 timecounter_read_delta(struct timecounter *tc)
  45{
  46        u64 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
  65u64 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}
  76EXPORT_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 */
  82static u64 cc_cyc2ns_backwards(const struct cyclecounter *cc,
  83                               u64 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
  92u64 timecounter_cyc2time(struct timecounter *tc,
  93                         u64 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}
 112EXPORT_SYMBOL_GPL(timecounter_cyc2time);
 113