1 /*********************************************************************** 2 * linux/kernel/time/jiffies.c 3 * 4 * This file contains the jiffies based clocksource. 5 * 6 * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com) 7 * 8 * This program is free software; you can redistribute it and/or modify 9 * it under the terms of the GNU General Public License as published by 10 * the Free Software Foundation; either version 2 of the License, or 11 * (at your option) any later version. 12 * 13 * This program is distributed in the hope that it will be useful, 14 * but WITHOUT ANY WARRANTY; without even the implied warranty of 15 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 16 * GNU General Public License for more details. 17 * 18 * You should have received a copy of the GNU General Public License 19 * along with this program; if not, write to the Free Software 20 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. 21 * 22 ************************************************************************/ 23 #include <linux/clocksource.h> 24 #include <linux/jiffies.h> 25 #include <linux/module.h> 26 #include <linux/init.h> 27 28 #include "tick-internal.h" 29 30 /* The Jiffies based clocksource is the lowest common 31 * denominator clock source which should function on 32 * all systems. It has the same coarse resolution as 33 * the timer interrupt frequency HZ and it suffers 34 * inaccuracies caused by missed or lost timer 35 * interrupts and the inability for the timer 36 * interrupt hardware to accuratly tick at the 37 * requested HZ value. It is also not recommended 38 * for "tick-less" systems. 39 */ 40 #define NSEC_PER_JIFFY ((u32)((((u64)NSEC_PER_SEC)<<8)/SHIFTED_HZ)) 41 42 /* Since jiffies uses a simple NSEC_PER_JIFFY multiplier 43 * conversion, the .shift value could be zero. However 44 * this would make NTP adjustments impossible as they are 45 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to 46 * shift both the nominator and denominator the same 47 * amount, and give ntp adjustments in units of 1/2^8 48 * 49 * The value 8 is somewhat carefully chosen, as anything 50 * larger can result in overflows. NSEC_PER_JIFFY grows as 51 * HZ shrinks, so values greater than 8 overflow 32bits when 52 * HZ=100. 53 */ 54 #define JIFFIES_SHIFT 8 55 56 static cycle_t jiffies_read(struct clocksource *cs) 57 { 58 return (cycle_t) jiffies; 59 } 60 61 struct clocksource clocksource_jiffies = { 62 .name = "jiffies", 63 .rating = 1, /* lowest valid rating*/ 64 .read = jiffies_read, 65 .mask = 0xffffffff, /*32bits*/ 66 .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */ 67 .shift = JIFFIES_SHIFT, 68 }; 69 70 #if (BITS_PER_LONG < 64) 71 u64 get_jiffies_64(void) 72 { 73 unsigned long seq; 74 u64 ret; 75 76 do { 77 seq = read_seqbegin(&xtime_lock); 78 ret = jiffies_64; 79 } while (read_seqretry(&xtime_lock, seq)); 80 return ret; 81 } 82 EXPORT_SYMBOL(get_jiffies_64); 83 #endif 84 85 EXPORT_SYMBOL(jiffies); 86 87 static int __init init_jiffies_clocksource(void) 88 { 89 return clocksource_register(&clocksource_jiffies); 90 } 91 92 core_initcall(init_jiffies_clocksource); 93 94 struct clocksource * __init __weak clocksource_default_clock(void) 95 { 96 return &clocksource_jiffies; 97 } 98