73 lines
2.5 KiB
C
73 lines
2.5 KiB
C
/***********************************************************************
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* linux/kernel/time/jiffies.c
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*
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* This file contains the jiffies based clocksource.
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*
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* Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*
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************************************************************************/
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#include <linux/clocksource.h>
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#include <linux/jiffies.h>
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#include <linux/init.h>
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/* The Jiffies based clocksource is the lowest common
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* denominator clock source which should function on
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* all systems. It has the same coarse resolution as
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* the timer interrupt frequency HZ and it suffers
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* inaccuracies caused by missed or lost timer
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* interrupts and the inability for the timer
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* interrupt hardware to accuratly tick at the
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* requested HZ value. It is also not reccomended
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* for "tick-less" systems.
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*/
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#define NSEC_PER_JIFFY ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ))
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/* Since jiffies uses a simple NSEC_PER_JIFFY multiplier
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* conversion, the .shift value could be zero. However
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* this would make NTP adjustments impossible as they are
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* in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
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* shift both the nominator and denominator the same
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* amount, and give ntp adjustments in units of 1/2^8
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*
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* The value 8 is somewhat carefully chosen, as anything
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* larger can result in overflows. NSEC_PER_JIFFY grows as
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* HZ shrinks, so values greater then 8 overflow 32bits when
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* HZ=100.
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*/
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#define JIFFIES_SHIFT 8
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static cycle_t jiffies_read(void)
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{
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return (cycle_t) jiffies;
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}
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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,
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.mask = 0xffffffff, /*32bits*/
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.mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
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.shift = JIFFIES_SHIFT,
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};
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static int __init init_jiffies_clocksource(void)
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{
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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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