// SPDX-License-Identifier: GPL-2.0+
  /*
   * This file contains the jiffies based clocksource.
   *
   * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
   */
  #include <linux/clocksource.h>
  #include <linux/jiffies.h>
  #include <linux/module.h>
  #include <linux/init.h>
  
  #include "timekeeping.h"
  
  
  /* Since jiffies uses a simple TICK_NSEC multiplier
   * conversion, the .shift value could be zero. However
   * this would make NTP adjustments impossible as they are
   * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
   * shift both the nominator and denominator the same
   * amount, and give ntp adjustments in units of 1/2^8
   *
   * The value 8 is somewhat carefully chosen, as anything
   * larger can result in overflows. TICK_NSEC grows as HZ
   * shrinks, so values greater than 8 overflow 32bits when
   * HZ=100.
   */
  #if HZ < 34
  #define JIFFIES_SHIFT	6
  #elif HZ < 67
  #define JIFFIES_SHIFT	7
  #else
  #define JIFFIES_SHIFT	8
  #endif
  
  static u64 jiffies_read(struct clocksource *cs)
  {
  	return (u64) jiffies;
  }
  
  /*
   * The Jiffies based clocksource is the lowest common
   * denominator clock source which should function on
   * all systems. It has the same coarse resolution as
   * the timer interrupt frequency HZ and it suffers
   * inaccuracies caused by missed or lost timer
   * interrupts and the inability for the timer
   * interrupt hardware to accuratly tick at the
   * requested HZ value. It is also not recommended
   * for "tick-less" systems.
   */
  static struct clocksource clocksource_jiffies = {
  	.name		= "jiffies",
  	.rating		= 1, /* lowest valid rating*/
  	.read		= jiffies_read,
  	.mask		= CLOCKSOURCE_MASK(32),
  	.mult		= TICK_NSEC << JIFFIES_SHIFT, /* details above */
  	.shift		= JIFFIES_SHIFT,
  	.max_cycles	= 10,
  };
  
  __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
  
  #if (BITS_PER_LONG < 64)
  u64 get_jiffies_64(void)
  {
  	unsigned int seq;
  	u64 ret;
  
  	do {
  		seq = read_seqbegin(&jiffies_lock);
  		ret = jiffies_64;
  	} while (read_seqretry(&jiffies_lock, seq));
  	return ret;
  }
  EXPORT_SYMBOL(get_jiffies_64);
  #endif
  
  EXPORT_SYMBOL(jiffies);
  
  static int __init init_jiffies_clocksource(void)
  {
  	return __clocksource_register(&clocksource_jiffies);
  }
  
  core_initcall(init_jiffies_clocksource);
  
  struct clocksource * __init __weak clocksource_default_clock(void)
  {
  	return &clocksource_jiffies;
  }
  
  static struct clocksource refined_jiffies;
  
  int register_refined_jiffies(long cycles_per_second)
  {
  	u64 nsec_per_tick, shift_hz;
  	long cycles_per_tick;
  
  
  
  	refined_jiffies = clocksource_jiffies;
  	refined_jiffies.name = "refined-jiffies";
  	refined_jiffies.rating++;
  
  	/* Calc cycles per tick */
  	cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
  	/* shift_hz stores hz<<8 for extra accuracy */
  	shift_hz = (u64)cycles_per_second << 8;
  	shift_hz += cycles_per_tick/2;
  	do_div(shift_hz, cycles_per_tick);
  	/* Calculate nsec_per_tick using shift_hz */
  	nsec_per_tick = (u64)NSEC_PER_SEC << 8;
  	nsec_per_tick += (u32)shift_hz/2;
  	do_div(nsec_per_tick, (u32)shift_hz);
  
  	refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
  
  	__clocksource_register(&refined_jiffies);
  	return 0;
  }