edc4d27255
The purpose of the minsec argument is to prevent 64-bit math overflow when the number of cycles is multiplied up. However, the multipler is 32-bit, and in the sched_clock() case, the cycle counter is up to 32-bit as well. So the math can never overflow. With a value of 60, and clock rates greater than 71MHz, the calculated multiplier is unnecessarily reduced in value, which reduces accuracy by maybe 70ppt. It's almost not worth bothering with as the oscillator driving the counter won't be any more than 1ppm - unless you're using a rubidium lamp or caesium fountain frequency standard. So, set the minsec argument to zero. Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
75 lines
1.9 KiB
C
75 lines
1.9 KiB
C
/*
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* sched_clock.c: support for extending counters to full 64-bit ns counter
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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 version 2 as
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* published by the Free Software Foundation.
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*/
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#include <linux/clocksource.h>
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#include <linux/init.h>
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#include <linux/jiffies.h>
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#include <linux/kernel.h>
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#include <linux/sched.h>
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#include <linux/timer.h>
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#include <asm/sched_clock.h>
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static void sched_clock_poll(unsigned long wrap_ticks);
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static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0);
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static void (*sched_clock_update_fn)(void);
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static void sched_clock_poll(unsigned long wrap_ticks)
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{
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mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks));
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sched_clock_update_fn();
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}
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void __init init_sched_clock(struct clock_data *cd, void (*update)(void),
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unsigned int clock_bits, unsigned long rate)
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{
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unsigned long r, w;
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u64 res, wrap;
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char r_unit;
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sched_clock_update_fn = update;
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/* calculate the mult/shift to convert counter ticks to ns. */
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clocks_calc_mult_shift(&cd->mult, &cd->shift, rate, NSEC_PER_SEC, 0);
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r = rate;
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if (r >= 4000000) {
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r /= 1000000;
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r_unit = 'M';
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} else {
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r /= 1000;
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r_unit = 'k';
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}
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/* calculate how many ns until we wrap */
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wrap = cyc_to_ns((1ULL << clock_bits) - 1, cd->mult, cd->shift);
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do_div(wrap, NSEC_PER_MSEC);
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w = wrap;
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/* calculate the ns resolution of this counter */
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res = cyc_to_ns(1ULL, cd->mult, cd->shift);
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pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lums\n",
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clock_bits, r, r_unit, res, w);
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/*
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* Start the timer to keep sched_clock() properly updated and
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* sets the initial epoch.
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*/
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sched_clock_timer.data = msecs_to_jiffies(w - (w / 10));
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update();
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/*
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* Ensure that sched_clock() starts off at 0ns
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*/
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cd->epoch_ns = 0;
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}
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void __init sched_clock_postinit(void)
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{
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sched_clock_poll(sched_clock_timer.data);
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}
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