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/*
* sched_clock.c: support for extending counters to full 64-bit ns counter
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*/
#include <linux/clocksource.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/timer.h>
#include <asm/sched_clock.h>
struct clock_data {
u64 epoch_ns;
u32 epoch_cyc;
u32 epoch_cyc_copy;
u32 mult;
u32 shift;
};
static void sched_clock_poll(unsigned long wrap_ticks);
static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0);
static struct clock_data cd = {
.mult = NSEC_PER_SEC / HZ,
};
static u32 __read_mostly sched_clock_mask = 0xffffffff;
static u32 notrace jiffy_sched_clock_read(void)
{
return (u32)(jiffies - INITIAL_JIFFIES);
}
static u32 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;
static inline u64 cyc_to_ns(u64 cyc, u32 mult, u32 shift)
{
return (cyc * mult) >> shift;
}
static unsigned long long cyc_to_sched_clock(u32 cyc, u32 mask)
{
u64 epoch_ns;
u32 epoch_cyc;
/*
* Load the epoch_cyc and epoch_ns atomically. We do this by
* ensuring that we always write epoch_cyc, epoch_ns and
* epoch_cyc_copy in strict order, and read them in strict order.
* If epoch_cyc and epoch_cyc_copy are not equal, then we're in
* the middle of an update, and we should repeat the load.
*/
do {
epoch_cyc = cd.epoch_cyc;
smp_rmb();
epoch_ns = cd.epoch_ns;
smp_rmb();
} while (epoch_cyc != cd.epoch_cyc_copy);
return epoch_ns + cyc_to_ns((cyc - epoch_cyc) & mask, cd.mult, cd.shift);
}
/*
* Atomically update the sched_clock epoch.
*/
static void notrace update_sched_clock(void)
{
unsigned long flags;
u32 cyc;
u64 ns;
cyc = read_sched_clock();
ns = cd.epoch_ns +
cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
cd.mult, cd.shift);
/*
* Write epoch_cyc and epoch_ns in a way that the update is
* detectable in cyc_to_fixed_sched_clock().
*/
raw_local_irq_save(flags);
cd.epoch_cyc = cyc;
smp_wmb();
cd.epoch_ns = ns;
smp_wmb();
cd.epoch_cyc_copy = cyc;
raw_local_irq_restore(flags);
}
static void sched_clock_poll(unsigned long wrap_ticks)
{
mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks));
update_sched_clock();
}
void __init setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate)
{
unsigned long r, w;
u64 res, wrap;
char r_unit;
BUG_ON(bits > 32);
WARN_ON(!irqs_disabled());
WARN_ON(read_sched_clock != jiffy_sched_clock_read);
read_sched_clock = read;
sched_clock_mask = (1 << bits) - 1;
/* calculate the mult/shift to convert counter ticks to ns. */
clocks_calc_mult_shift(&cd.mult, &cd.shift, rate, NSEC_PER_SEC, 0);
r = rate;
if (r >= 4000000) {
r /= 1000000;
r_unit = 'M';
} else if (r >= 1000) {
r /= 1000;
r_unit = 'k';
} else
r_unit = ' ';
/* calculate how many ns until we wrap */
wrap = cyc_to_ns((1ULL << bits) - 1, cd.mult, cd.shift);
do_div(wrap, NSEC_PER_MSEC);
w = wrap;
/* calculate the ns resolution of this counter */
res = cyc_to_ns(1ULL, cd.mult, cd.shift);
pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lums\n",
bits, r, r_unit, res, w);
/*
* Start the timer to keep sched_clock() properly updated and
* sets the initial epoch.
*/
sched_clock_timer.data = msecs_to_jiffies(w - (w / 10));
update_sched_clock();
/*
* Ensure that sched_clock() starts off at 0ns
*/
cd.epoch_ns = 0;
pr_debug("Registered %pF as sched_clock source\n", read);
}
unsigned long long notrace sched_clock(void)
{
u32 cyc = read_sched_clock();
return cyc_to_sched_clock(cyc, sched_clock_mask);
}
void __init sched_clock_postinit(void)
{
/*
* If no sched_clock function has been provided at that point,
* make it the final one one.
*/
if (read_sched_clock == jiffy_sched_clock_read)
setup_sched_clock(jiffy_sched_clock_read, 32, HZ);
sched_clock_poll(sched_clock_timer.data);
}
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