x15/kern/clock.h

/*
 * Copyright (c) 2017 Richard Braun.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 *
 * Timekeeping module.
 */

#ifndef KERN_CLOCK_H
#define KERN_CLOCK_H

#include <stdalign.h>
#include <stdbool.h>
#include <stdint.h>

#include <kern/atomic.h>
#include <kern/init.h>
#include <kern/macros.h>

#include <machine/cpu.h>

union clock_global_time
{
  __cacheline_aligned uint64_t ticks;

#ifndef __LP64__
  struct
    {
#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
      uint32_t high1;
      uint32_t low;
#else
      uint32_t low;
      uint32_t high1;
#endif
      uint32_t high2;
    };
#endif
};

// Clock frequency.
#define CLOCK_FREQ   CONFIG_CLOCK_FREQ

#if CLOCK_FREQ < 100 || CLOCK_FREQ > 1000 || (1000 % CLOCK_FREQ) != 0
  #error "invalid clock frequency"
#endif

/*
 * Arbitrary value used to determine if a time is in the past or the future.
 *
 * Time is represented as 64-bits unsigned integers counting ticks. The
 * global time currently starts from 0 but this isn't a strong assumption
 * users should rely on. Instead, all time checks involve a time reference
 * against which to compare. The result of that comparison, done by
 * substraction, is either in the future, i.e. the difference is less
 * than the expire threshold, or in the past, i.e. the difference is
 * greater (keep in mind the result is unsigned). The threshold must be
 * large enough to allow both a wide range of possible times in the future,
 * but also enough time in the past for reliable timeout detection. Note
 * that using signed integers would be equivalent to dividing the range
 * in two (almost) equal past and future halves.
 */
#define CLOCK_EXPIRE_THRESHOLD   (-(1ULL << 60))

static inline uint64_t
clock_get_time (void)
{
  extern union clock_global_time clock_global_time;

#ifdef __LP64__

  /*
   * Don't enforce a stronger memory order, since :
   *  1/ it's useless as long as the reader remains on the same processor
   *  2/ thread migration enforces sequential consistency
   */
  return (atomic_load_rlx (&clock_global_time.ticks));

#else   // ATOMIC_HAVE_64B_OPS.

  uint32_t high1, low, high2;

  /*
   * For 32-bit machines, this implementation uses a variant of the two-digit
   * monotonic-clock algorithm, described in the paper "Concurrent Reading and
   * Writing of Clocks" by Leslie Lamport.
   */

  do
    {
      high1 = atomic_load_acq (&clock_global_time.high1);
      low = atomic_load_acq (&clock_global_time.low);
      high2 = atomic_load_rlx (&clock_global_time.high2);
    }
  while (high1 != high2);

  return (((uint64_t) high2 << 32) | low);

#endif
}

static inline uint64_t
clock_ticks_to_ms (uint64_t ticks)
{
  return (ticks * (1000 / CLOCK_FREQ));
}

static inline uint64_t
clock_ticks_from_ms (uint64_t ms)
{
  return (DIV_CEIL (ms, (1000 / CLOCK_FREQ)));
}

static inline bool
clock_time_expired (uint64_t t, uint64_t ref)
{
  return (t - ref > CLOCK_EXPIRE_THRESHOLD);
}

static inline bool
clock_time_occurred (uint64_t t, uint64_t ref)
{
  return (t == ref || clock_time_expired (t, ref));
}

void clock_tick_intr (void);

#endif