b3c869d35b
CLOCK_TICK_RATE is used to accurately caclulate exactly how a tick will be at a given HZ. This is useful, because while we'd expect NSEC_PER_SEC/HZ, the underlying hardware will have some granularity limit, so we won't be able to have exactly HZ ticks per second. This slight error can cause timekeeping quality problems when using the jiffies or other jiffies driven clocksources. Thus we currently use compile time CLOCK_TICK_RATE value to generate SHIFTED_HZ and NSEC_PER_JIFFIES, which we then use to adjust the jiffies clocksource to correct this error. Unfortunately though, since CLOCK_TICK_RATE is a compile time value, and the jiffies clocksource is registered very early during boot, there are a number of cases where there are different possible hardware timers that have different tick rates. This causes problems in cases like ARM where there are numerous different types of hardware, each having their own compile-time CLOCK_TICK_RATE, making it hard to accurately support different hardware with a single kernel. For the most part, this doesn't matter all that much, as not too many systems actually utilize the jiffies or jiffies driven clocksource. Usually there are other highres clocksources who's granularity error is negligable. Even so, we have some complicated calcualtions that we do everywhere to handle these edge cases. This patch removes the compile time SHIFTED_HZ value, and introduces a register_refined_jiffies() function. This results in the default jiffies clock as being assumed a perfect HZ freq, and allows archtectures that care about jiffies accuracy to call register_refined_jiffies() with the tick rate, specified dynamically at boot. This allows us, where necessary, to not have a compile time CLOCK_TICK_RATE constant, simplifies the jiffies code, and still provides a way to have an accurate jiffies clock. NOTE: Since this patch does not add register_refinied_jiffies() calls for every arch, it may cause time quality regressions in some cases. Its likely these will not be noticable, but if they are an issue, adding the following to the end of setup_arch() should resolve the regression: register_refinied_jiffies(CLOCK_TICK_RATE) Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Richard Cochran <richardcochran@gmail.com> Cc: Prarit Bhargava <prarit@redhat.com> Cc: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: John Stultz <john.stultz@linaro.org>
127 lines
3.7 KiB
C
127 lines
3.7 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/module.h>
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#include <linux/init.h>
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#include "tick-internal.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 recommended
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* for "tick-less" systems.
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*/
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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
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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 than 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(struct clocksource *cs)
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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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#if (BITS_PER_LONG < 64)
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u64 get_jiffies_64(void)
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{
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unsigned long seq;
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u64 ret;
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do {
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seq = read_seqbegin(&xtime_lock);
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ret = jiffies_64;
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} while (read_seqretry(&xtime_lock, seq));
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return ret;
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}
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EXPORT_SYMBOL(get_jiffies_64);
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#endif
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EXPORT_SYMBOL(jiffies);
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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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struct clocksource * __init __weak clocksource_default_clock(void)
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{
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return &clocksource_jiffies;
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}
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struct clocksource refined_jiffies;
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int register_refined_jiffies(long cycles_per_second)
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{
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u64 nsec_per_tick, shift_hz;
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long cycles_per_tick;
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refined_jiffies = clocksource_jiffies;
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refined_jiffies.name = "refined-jiffies";
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refined_jiffies.rating++;
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/* Calc cycles per tick */
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cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
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/* shift_hz stores hz<<8 for extra accuracy */
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shift_hz = (u64)cycles_per_second << 8;
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shift_hz += cycles_per_tick/2;
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do_div(shift_hz, cycles_per_tick);
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/* Calculate nsec_per_tick using shift_hz */
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nsec_per_tick = (u64)NSEC_PER_SEC << 8;
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nsec_per_tick += (u32)shift_hz/2;
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do_div(nsec_per_tick, (u32)shift_hz);
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refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
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clocksource_register(&refined_jiffies);
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return 0;
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}
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