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 * This program is free software; you can redistribute it and/or modify 8 * it under the terms of the GNU General Public License as published by 9 * the Free Software Foundation; either version 2 of the License, or 10 * (at your option) any later version. 11 * 12 * This program is distributed in the hope that it will be useful, 13 * but WITHOUT ANY WARRANTY; without even the implied warranty of 14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 15 * GNU General Public License for more details. 16 * 17 * You should have received a copy of the GNU General Public License 18 * along with this program; if not, write to the Free Software 19 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. 20 * 21 ************************************************************************/ 22 #include <linux/clocksource.h> 23 #include <linux/jiffies.h> 24 #include <linux/module.h> 25 #include <linux/init.h> 26 27 #include "timekeeping.h" 28 29 30 /* Since jiffies uses a simple TICK_NSEC multiplier 31 * conversion, the .shift value could be zero. However 32 * this would make NTP adjustments impossible as they are 33 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to 34 * shift both the nominator and denominator the same 35 * amount, and give ntp adjustments in units of 1/2^8 36 * 37 * The value 8 is somewhat carefully chosen, as anything 38 * larger can result in overflows. TICK_NSEC grows as HZ 39 * shrinks, so values greater than 8 overflow 32bits when 40 * HZ=100. 41 */ 42 #if HZ < 34 43 #define JIFFIES_SHIFT 6 44 #elif HZ < 67 45 #define JIFFIES_SHIFT 7 46 #else 47 #define JIFFIES_SHIFT 8 48 #endif 49 50 static u64 jiffies_read(struct clocksource *cs) 51 { 52 return (u64) jiffies; 53 } 54 55 /* 56 * The Jiffies based clocksource is the lowest common 57 * denominator clock source which should function on 58 * all systems. It has the same coarse resolution as 59 * the timer interrupt frequency HZ and it suffers 60 * inaccuracies caused by missed or lost timer 61 * interrupts and the inability for the timer 62 * interrupt hardware to accuratly tick at the 63 * requested HZ value. It is also not recommended 64 * for "tick-less" systems. 65 */ 66 static struct clocksource clocksource_jiffies = { 67 .name = "jiffies", 68 .rating = 1, /* lowest valid rating*/ 69 .read = jiffies_read, 70 .mask = CLOCKSOURCE_MASK(32), 71 .mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */ 72 .shift = JIFFIES_SHIFT, 73 .max_cycles = 10, 74 }; 75 76 __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock); 77 78 #if (BITS_PER_LONG < 64) 79 u64 get_jiffies_64(void) 80 { 81 unsigned long seq; 82 u64 ret; 83 84 do { 85 seq = read_seqbegin(&jiffies_lock); 86 ret = jiffies_64; 87 } while (read_seqretry(&jiffies_lock, seq)); 88 return ret; 89 } 90 EXPORT_SYMBOL(get_jiffies_64); 91 #endif 92 93 EXPORT_SYMBOL(jiffies); 94 95 static int __init init_jiffies_clocksource(void) 96 { 97 return __clocksource_register(&clocksource_jiffies); 98 } 99 100 core_initcall(init_jiffies_clocksource); 101 102 struct clocksource * __init __weak clocksource_default_clock(void) 103 { 104 return &clocksource_jiffies; 105 } 106 107 struct clocksource refined_jiffies; 108 109 int register_refined_jiffies(long cycles_per_second) 110 { 111 u64 nsec_per_tick, shift_hz; 112 long cycles_per_tick; 113 114 115 116 refined_jiffies = clocksource_jiffies; 117 refined_jiffies.name = "refined-jiffies"; 118 refined_jiffies.rating++; 119 120 /* Calc cycles per tick */ 121 cycles_per_tick = (cycles_per_second + HZ/2)/HZ; 122 /* shift_hz stores hz<<8 for extra accuracy */ 123 shift_hz = (u64)cycles_per_second << 8; 124 shift_hz += cycles_per_tick/2; 125 do_div(shift_hz, cycles_per_tick); 126 /* Calculate nsec_per_tick using shift_hz */ 127 nsec_per_tick = (u64)NSEC_PER_SEC << 8; 128 nsec_per_tick += (u32)shift_hz/2; 129 do_div(nsec_per_tick, (u32)shift_hz); 130 131 refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT; 132 133 __clocksource_register(&refined_jiffies); 134 return 0; 135 } 136