/*
   * sched_clock for unstable cpu clocks
   *
   *  Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
   *

   *  Updates and enhancements:
   *    Copyright (C) 2008 Red Hat, Inc. Steven Rostedt <srostedt@redhat.com>
   *

   * Based on code by:
   *   Ingo Molnar <mingo@redhat.com>
   *   Guillaume Chazarain <guichaz@gmail.com>
   *

   *
   * What:
   *
   * cpu_clock(i) provides a fast (execution time) high resolution
   * clock with bounded drift between CPUs. The value of cpu_clock(i)
   * is monotonic for constant i. The timestamp returned is in nanoseconds.
   *
   * ######################### BIG FAT WARNING ##########################
   * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
   * # go backwards !!                                                  #
   * ####################################################################
   *
   * There is no strict promise about the base, although it tends to start
   * at 0 on boot (but people really shouldn't rely on that).
   *
   * cpu_clock(i)       -- can be used from any context, including NMI.
   * sched_clock_cpu(i) -- must be used with local IRQs disabled (implied by NMI)
   * local_clock()      -- is cpu_clock() on the current cpu.
   *
   * How:
   *
   * The implementation either uses sched_clock() when
   * !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the
   * sched_clock() is assumed to provide these properties (mostly it means
   * the architecture provides a globally synchronized highres time source).
   *
   * Otherwise it tries to create a semi stable clock from a mixture of other
   * clocks, including:
   *
   *  - GTOD (clock monotomic)

   *  - sched_clock()
   *  - explicit idle events
   *

   * We use GTOD as base and use sched_clock() deltas to improve resolution. The
   * deltas are filtered to provide monotonicity and keeping it within an
   * expected window.

   *
   * Furthermore, explicit sleep and wakeup hooks allow us to account for time
   * that is otherwise invisible (TSC gets stopped).
   *

   *
   * Notes:
   *
   * The !IRQ-safetly of sched_clock() and sched_clock_cpu() comes from things
   * like cpufreq interrupts that can change the base clock (TSC) multiplier
   * and cause funny jumps in time -- although the filtering provided by
   * sched_clock_cpu() should mitigate serious artifacts we cannot rely on it
   * in general since for !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK we fully rely on
   * sched_clock().

   */

  #include <linux/spinlock.h>

  #include <linux/hardirq.h>

  #include <linux/module.h>

  #include <linux/percpu.h>
  #include <linux/ktime.h>
  #include <linux/sched.h>

  /*
   * Scheduler clock - returns current time in nanosec units.
   * This is default implementation.
   * Architectures and sub-architectures can override this.
   */
  unsigned long long __attribute__((weak)) sched_clock(void)
  {

  	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
  					* (NSEC_PER_SEC / HZ);

  }

  EXPORT_SYMBOL_GPL(sched_clock);

  static __read_mostly int sched_clock_running;

  #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK

  __read_mostly int sched_clock_stable;

  
  struct sched_clock_data {

  	u64			tick_raw;
  	u64			tick_gtod;
  	u64			clock;
  };
  
  static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
  
  static inline struct sched_clock_data *this_scd(void)
  {
  	return &__get_cpu_var(sched_clock_data);
  }
  
  static inline struct sched_clock_data *cpu_sdc(int cpu)
  {
  	return &per_cpu(sched_clock_data, cpu);
  }
  
  void sched_clock_init(void)
  {
  	u64 ktime_now = ktime_to_ns(ktime_get());

  	int cpu;
  
  	for_each_possible_cpu(cpu) {
  		struct sched_clock_data *scd = cpu_sdc(cpu);

  		scd->tick_raw = 0;

  		scd->tick_gtod = ktime_now;
  		scd->clock = ktime_now;
  	}

  
  	sched_clock_running = 1;

  }
  
  /*

   * min, max except they take wrapping into account

   */
  
  static inline u64 wrap_min(u64 x, u64 y)
  {
  	return (s64)(x - y) < 0 ? x : y;
  }
  
  static inline u64 wrap_max(u64 x, u64 y)
  {
  	return (s64)(x - y) > 0 ? x : y;
  }
  
  /*

   * update the percpu scd from the raw @now value
   *
   *  - filter out backward motion

   *  - use the GTOD tick value to create a window to filter crazy TSC values

   */

  static u64 sched_clock_local(struct sched_clock_data *scd)

  {

  	u64 now, clock, old_clock, min_clock, max_clock;
  	s64 delta;

  again:
  	now = sched_clock();
  	delta = now - scd->tick_raw;

  	if (unlikely(delta < 0))
  		delta = 0;

  	old_clock = scd->clock;

  	/*
  	 * scd->clock = clamp(scd->tick_gtod + delta,

  	 *		      max(scd->tick_gtod, scd->clock),
  	 *		      scd->tick_gtod + TICK_NSEC);

  	 */

  	clock = scd->tick_gtod + delta;

  	min_clock = wrap_max(scd->tick_gtod, old_clock);
  	max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);

  	clock = wrap_max(clock, min_clock);
  	clock = wrap_min(clock, max_clock);

  	if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)

  		goto again;

  	return clock;

  }

  static u64 sched_clock_remote(struct sched_clock_data *scd)

  {

  	struct sched_clock_data *my_scd = this_scd();
  	u64 this_clock, remote_clock;
  	u64 *ptr, old_val, val;
  
  	sched_clock_local(my_scd);
  again:
  	this_clock = my_scd->clock;
  	remote_clock = scd->clock;
  
  	/*
  	 * Use the opportunity that we have both locks
  	 * taken to couple the two clocks: we take the
  	 * larger time as the latest time for both
  	 * runqueues. (this creates monotonic movement)
  	 */
  	if (likely((s64)(remote_clock - this_clock) < 0)) {
  		ptr = &scd->clock;
  		old_val = remote_clock;
  		val = this_clock;

  	} else {

  		/*
  		 * Should be rare, but possible:
  		 */
  		ptr = &my_scd->clock;
  		old_val = this_clock;
  		val = remote_clock;

  	}

  	if (cmpxchg64(ptr, old_val, val) != old_val)

  		goto again;
  
  	return val;

  }

  /*
   * Similar to cpu_clock(), but requires local IRQs to be disabled.
   *
   * See cpu_clock().
   */

  u64 sched_clock_cpu(int cpu)
  {

  	struct sched_clock_data *scd;

  	u64 clock;
  
  	WARN_ON_ONCE(!irqs_disabled());

  	if (sched_clock_stable)
  		return sched_clock();

  	if (unlikely(!sched_clock_running))
  		return 0ull;

  	scd = cpu_sdc(cpu);

  	if (cpu != smp_processor_id())
  		clock = sched_clock_remote(scd);
  	else
  		clock = sched_clock_local(scd);

  	return clock;
  }
  
  void sched_clock_tick(void)
  {

  	struct sched_clock_data *scd;

  	u64 now, now_gtod;

  	if (sched_clock_stable)
  		return;

  	if (unlikely(!sched_clock_running))
  		return;

  	WARN_ON_ONCE(!irqs_disabled());

  	scd = this_scd();

  	now_gtod = ktime_to_ns(ktime_get());

  	now = sched_clock();

  	scd->tick_raw = now;
  	scd->tick_gtod = now_gtod;

  	sched_clock_local(scd);

  }
  
  /*
   * We are going deep-idle (irqs are disabled):
   */
  void sched_clock_idle_sleep_event(void)
  {
  	sched_clock_cpu(smp_processor_id());
  }
  EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
  
  /*
   * We just idled delta nanoseconds (called with irqs disabled):
   */
  void sched_clock_idle_wakeup_event(u64 delta_ns)
  {

  	if (timekeeping_suspended)
  		return;

  	sched_clock_tick();

  	touch_softlockup_watchdog();
  }
  EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);

  /*
   * As outlined at the top, provides a fast, high resolution, nanosecond
   * time source that is monotonic per cpu argument and has bounded drift
   * between cpus.
   *
   * ######################### BIG FAT WARNING ##########################
   * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
   * # go backwards !!                                                  #
   * ####################################################################
   */
  u64 cpu_clock(int cpu)

  {

  	u64 clock;

  	unsigned long flags;
  
  	local_irq_save(flags);
  	clock = sched_clock_cpu(cpu);
  	local_irq_restore(flags);
  
  	return clock;
  }

  /*
   * Similar to cpu_clock() for the current cpu. Time will only be observed
   * to be monotonic if care is taken to only compare timestampt taken on the
   * same CPU.
   *
   * See cpu_clock().
   */
  u64 local_clock(void)
  {
  	u64 clock;
  	unsigned long flags;
  
  	local_irq_save(flags);
  	clock = sched_clock_cpu(smp_processor_id());
  	local_irq_restore(flags);
  
  	return clock;
  }

  #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
  
  void sched_clock_init(void)
  {
  	sched_clock_running = 1;
  }
  
  u64 sched_clock_cpu(int cpu)
  {
  	if (unlikely(!sched_clock_running))
  		return 0;
  
  	return sched_clock();
  }

  u64 cpu_clock(int cpu)

  {

  	return sched_clock_cpu(cpu);
  }

  u64 local_clock(void)
  {
  	return sched_clock_cpu(0);
  }

  #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */

  EXPORT_SYMBOL_GPL(cpu_clock);

  EXPORT_SYMBOL_GPL(local_clock);