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kernel/time/jiffies.c
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/*********************************************************************** * linux/kernel/time/jiffies.c * * This file contains the jiffies based clocksource. * * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com) * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. * ************************************************************************/ #include <linux/clocksource.h> #include <linux/jiffies.h> |
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#include <linux/module.h> |
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#include <linux/init.h> |
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#include "timekeeping.h" |
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/* 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 |
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* requested HZ value. It is also not recommended |
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* for "tick-less" systems. */ |
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#define NSEC_PER_JIFFY ((NSEC_PER_SEC+HZ/2)/HZ) |
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/* Since jiffies uses a simple NSEC_PER_JIFFY 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. NSEC_PER_JIFFY grows as |
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* HZ shrinks, so values greater than 8 overflow 32bits when |
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* HZ=100. */ |
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#if HZ < 34 #define JIFFIES_SHIFT 6 #elif HZ < 67 #define JIFFIES_SHIFT 7 #else |
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#define JIFFIES_SHIFT 8 |
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#endif |
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static cycle_t jiffies_read(struct clocksource *cs) |
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{ return (cycle_t) jiffies; } |
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static struct clocksource clocksource_jiffies = { |
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.name = "jiffies", |
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.rating = 1, /* lowest valid rating*/ |
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.read = jiffies_read, .mask = 0xffffffff, /*32bits*/ .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */ .shift = JIFFIES_SHIFT, |
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.max_cycles = 10, |
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}; |
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__cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock); |
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#if (BITS_PER_LONG < 64) u64 get_jiffies_64(void) { unsigned long seq; u64 ret; do { |
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seq = read_seqbegin(&jiffies_lock); |
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ret = jiffies_64; |
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} while (read_seqretry(&jiffies_lock, seq)); |
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return ret; } EXPORT_SYMBOL(get_jiffies_64); #endif EXPORT_SYMBOL(jiffies); |
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static int __init init_jiffies_clocksource(void) { |
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return __clocksource_register(&clocksource_jiffies); |
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} |
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core_initcall(init_jiffies_clocksource); |
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struct clocksource * __init __weak clocksource_default_clock(void) { return &clocksource_jiffies; } |
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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; |
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__clocksource_register(&refined_jiffies); |
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return 0; } |