Line data    Source code

       1             : // SPDX-License-Identifier: GPL-2.0+
       2             : /*
       3             :  * This file contains the jiffies based clocksource.
       4             :  *
       5             :  * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
       6             :  */
       7             : #include <linux/clocksource.h>
       8             : #include <linux/jiffies.h>
       9             : #include <linux/module.h>
      10             : #include <linux/init.h>
      11             : 
      12             : #include "timekeeping.h"
      13             : 
      14             : 
      15             : /* Since jiffies uses a simple TICK_NSEC multiplier
      16             :  * conversion, the .shift value could be zero. However
      17             :  * this would make NTP adjustments impossible as they are
      18             :  * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
      19             :  * shift both the nominator and denominator the same
      20             :  * amount, and give ntp adjustments in units of 1/2^8
      21             :  *
      22             :  * The value 8 is somewhat carefully chosen, as anything
      23             :  * larger can result in overflows. TICK_NSEC grows as HZ
      24             :  * shrinks, so values greater than 8 overflow 32bits when
      25             :  * HZ=100.
      26             :  */
      27             : #if HZ < 34
      28             : #define JIFFIES_SHIFT   6
      29             : #elif HZ < 67
      30             : #define JIFFIES_SHIFT   7
      31             : #else
      32             : #define JIFFIES_SHIFT   8
      33             : #endif
      34             : 
      35         583 : static u64 jiffies_read(struct clocksource *cs)
      36             : {
      37         583 :         return (u64) jiffies;
      38             : }
      39             : 
      40             : /*
      41             :  * The Jiffies based clocksource is the lowest common
      42             :  * denominator clock source which should function on
      43             :  * all systems. It has the same coarse resolution as
      44             :  * the timer interrupt frequency HZ and it suffers
      45             :  * inaccuracies caused by missed or lost timer
      46             :  * interrupts and the inability for the timer
      47             :  * interrupt hardware to accuratly tick at the
      48             :  * requested HZ value. It is also not recommended
      49             :  * for "tick-less" systems.
      50             :  */
      51             : static struct clocksource clocksource_jiffies = {
      52             :         .name           = "jiffies",
      53             :         .rating         = 1, /* lowest valid rating*/
      54             :         .read           = jiffies_read,
      55             :         .mask           = CLOCKSOURCE_MASK(32),
      56             :         .mult           = TICK_NSEC << JIFFIES_SHIFT, /* details above */
      57             :         .shift          = JIFFIES_SHIFT,
      58             :         .max_cycles     = 10,
      59             : };
      60             : 
      61             : __cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
      62             : __cacheline_aligned_in_smp seqcount_raw_spinlock_t jiffies_seq =
      63             :         SEQCNT_RAW_SPINLOCK_ZERO(jiffies_seq, &jiffies_lock);
      64             : 
      65             : #if (BITS_PER_LONG < 64)
      66             : u64 get_jiffies_64(void)
      67             : {
      68             :         unsigned int seq;
      69             :         u64 ret;
      70             : 
      71             :         do {
      72             :                 seq = read_seqcount_begin(&jiffies_seq);
      73             :                 ret = jiffies_64;
      74             :         } while (read_seqcount_retry(&jiffies_seq, seq));
      75             :         return ret;
      76             : }
      77             : EXPORT_SYMBOL(get_jiffies_64);
      78             : #endif
      79             : 
      80             : EXPORT_SYMBOL(jiffies);
      81             : 
      82           1 : static int __init init_jiffies_clocksource(void)
      83             : {
      84           1 :         return __clocksource_register(&clocksource_jiffies);
      85             : }
      86             : 
      87             : core_initcall(init_jiffies_clocksource);
      88             : 
      89           2 : struct clocksource * __init __weak clocksource_default_clock(void)
      90             : {
      91           2 :         return &clocksource_jiffies;
      92             : }
      93             : 
      94             : static struct clocksource refined_jiffies;
      95             : 
      96           1 : int register_refined_jiffies(long cycles_per_second)
      97             : {
      98           1 :         u64 nsec_per_tick, shift_hz;
      99           1 :         long cycles_per_tick;
     100             : 
     101             : 
     102             : 
     103           1 :         refined_jiffies = clocksource_jiffies;
     104           1 :         refined_jiffies.name = "refined-jiffies";
     105           1 :         refined_jiffies.rating++;
     106             : 
     107             :         /* Calc cycles per tick */
     108           1 :         cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
     109             :         /* shift_hz stores hz<<8 for extra accuracy */
     110           1 :         shift_hz = (u64)cycles_per_second << 8;
     111           1 :         shift_hz += cycles_per_tick/2;
     112           1 :         do_div(shift_hz, cycles_per_tick);
     113             :         /* Calculate nsec_per_tick using shift_hz */
     114           1 :         nsec_per_tick = (u64)NSEC_PER_SEC << 8;
     115           1 :         nsec_per_tick += (u32)shift_hz/2;
     116           1 :         do_div(nsec_per_tick, (u32)shift_hz);
     117             : 
     118           1 :         refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
     119             : 
     120           1 :         __clocksource_register(&refined_jiffies);
     121           1 :         return 0;
     122             : }