LXR linux/kernel/time/timecounter.c

   1// SPDX-License-Identifier: GPL-2.0+
   2/*
   3 * Based on clocksource code. See commit 74d23cc704d1
   4 */
   5#include <linux/export.h>
   6#include <linux/timecounter.h>
   7
   8void timecounter_init(struct timecounter *tc,
   9                      const struct cyclecounter *cc,
  10                      u64 start_tstamp)
  11{
  12        tc->cc = cc;
  13        tc->cycle_last = cc->read(cc);
  14        tc->nsec = start_tstamp;
  15        tc->mask = (1ULL << cc->shift) - 1;
  16        tc->frac = 0;
  17}
  18EXPORT_SYMBOL_GPL(timecounter_init);
  19
  20/**
  21 * timecounter_read_delta - get nanoseconds since last call of this function
  22 * @tc:         Pointer to time counter
  23 *
  24 * When the underlying cycle counter runs over, this will be handled
  25 * correctly as long as it does not run over more than once between
  26 * calls.
  27 *
  28 * The first call to this function for a new time counter initializes
  29 * the time tracking and returns an undefined result.
  30 */
  31static u64 timecounter_read_delta(struct timecounter *tc)
  32{
  33        u64 cycle_now, cycle_delta;
  34        u64 ns_offset;
  35
  36        /* read cycle counter: */
  37        cycle_now = tc->cc->read(tc->cc);
  38
  39        /* calculate the delta since the last timecounter_read_delta(): */
  40        cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;
  41
  42        /* convert to nanoseconds: */
  43        ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta,
  44                                        tc->mask, &tc->frac);
  45
  46        /* update time stamp of timecounter_read_delta() call: */
  47        tc->cycle_last = cycle_now;
  48
  49        return ns_offset;
  50}
  51
  52u64 timecounter_read(struct timecounter *tc)
  53{
  54        u64 nsec;
  55
  56        /* increment time by nanoseconds since last call */
  57        nsec = timecounter_read_delta(tc);
  58        nsec += tc->nsec;
  59        tc->nsec = nsec;
  60
  61        return nsec;
  62}
  63EXPORT_SYMBOL_GPL(timecounter_read);
  64
  65/*
  66 * This is like cyclecounter_cyc2ns(), but it is used for computing a
  67 * time previous to the time stored in the cycle counter.
  68 */
  69static u64 cc_cyc2ns_backwards(const struct cyclecounter *cc,
  70                               u64 cycles, u64 mask, u64 frac)
  71{
  72        u64 ns = (u64) cycles;
  73
  74        ns = ((ns * cc->mult) - frac) >> cc->shift;
  75
  76        return ns;
  77}
  78
  79u64 timecounter_cyc2time(const struct timecounter *tc,
  80                         u64 cycle_tstamp)
  81{
  82        u64 delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;
  83        u64 nsec = tc->nsec, frac = tc->frac;
  84
  85        /*
  86         * Instead of always treating cycle_tstamp as more recent
  87         * than tc->cycle_last, detect when it is too far in the
  88         * future and treat it as old time stamp instead.
  89         */
  90        if (delta > tc->cc->mask / 2) {
  91                delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask;
  92                nsec -= cc_cyc2ns_backwards(tc->cc, delta, tc->mask, frac);
  93        } else {
  94                nsec += cyclecounter_cyc2ns(tc->cc, delta, tc->mask, &frac);
  95        }
  96
  97        return nsec;
  98}
  99EXPORT_SYMBOL_GPL(timecounter_cyc2time);
 100