path: root/kernel/time/clocksource.c

// SPDX-License-Identifier: GPL-2.0+
/*
 * This file contains the functions which manage clocksource drivers.
 *
 * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/clocksource.h>
#include <linux/cpu.h>
#include <linux/delay.h>
#include <linux/device.h>
#include <linux/init.h>
#include <linux/kthread.h>
#include <linux/module.h>
#include <linux/prandom.h>
#include <linux/sched.h>
#include <linux/tick.h>
#include <linux/topology.h>

#include "tick-internal.h"
#include "timekeeping_internal.h"

static void clocksource_enqueue(struct clocksource *cs);

static noinline u64 cycles_to_nsec_safe(struct clocksource *cs, u64 start, u64 end)
{
	u64 delta = clocksource_delta(end, start, cs->mask, cs->max_raw_delta);

	if (likely(delta < cs->max_cycles))
		return clocksource_cyc2ns(delta, cs->mult, cs->shift);

	return mul_u64_u32_shr(delta, cs->mult, cs->shift);
}

/**
 * clocks_calc_mult_shift - calculate mult/shift factors for scaled math of clocks
 * @mult:	pointer to mult variable
 * @shift:	pointer to shift variable
 * @from:	frequency to convert from
 * @to:		frequency to convert to
 * @maxsec:	guaranteed runtime conversion range in seconds
 *
 * The function evaluates the shift/mult pair for the scaled math
 * operations of clocksources and clockevents.
 *
 * @to and @from are frequency values in HZ. For clock sources @to is
 * NSEC_PER_SEC == 1GHz and @from is the counter frequency. For clock
 * event @to is the counter frequency and @from is NSEC_PER_SEC.
 *
 * The @maxsec conversion range argument controls the time frame in
 * seconds which must be covered by the runtime conversion with the
 * calculated mult and shift factors. This guarantees that no 64bit
 * overflow happens when the input value of the conversion is
 * multiplied with the calculated mult factor. Larger ranges may
 * reduce the conversion accuracy by choosing smaller mult and shift
 * factors.
 */
void
clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 maxsec)
{
	u64 tmp;
	u32 sft, sftacc= 32;

	/*
	 * Calculate the shift factor which is limiting the conversion
	 * range:
	 */
	tmp = ((u64)maxsec * from) >> 32;
	while (tmp) {
		tmp >>=1;
		sftacc--;
	}

	/*
	 * Find the conversion shift/mult pair which has the best
	 * accuracy and fits the maxsec conversion range:
	 */
	for (sft = 32; sft > 0; sft--) {
		tmp = (u64) to << sft;
		tmp += from / 2;
		do_div(tmp, from);
		if ((tmp >> sftacc) == 0)
			break;
	}
	*mult = tmp;
	*shift = sft;
}
EXPORT_SYMBOL_GPL(clocks_calc_mult_shift);

/*[Clocksource internal variables]---------
 * curr_clocksource:
 *	currently selected clocksource.
 * suspend_clocksource:
 *	used to calculate the suspend time.
 * clocksource_list:
 *	linked list with the registered clocksources
 * clocksource_mutex:
 *	protects manipulations to curr_clocksource and the clocksource_list
 * override_name:
 *	Name of the user-specified clocksource.
 */
static struct clocksource *curr_clocksource;
static struct clocksource *suspend_clocksource;
static LIST_HEAD(clocksource_list);
static DEFINE_MUTEX(clocksource_mutex);
static char override_name[CS_NAME_LEN];
static int finished_booting;
static u64 suspend_start;

#ifdef CONFIG_CLOCKSOURCE_WATCHDOG
static void clocksource_watchdog_work(struct work_struct *work);
static void clocksource_select(void);

static LIST_HEAD(watchdog_list);
static struct clocksource *watchdog;
static struct timer_list watchdog_timer;
static DECLARE_WORK(watchdog_work, clocksource_watchdog_work);
static DEFINE_SPINLOCK(watchdog_lock);
static int watchdog_running;
static atomic_t watchdog_reset_pending;

/* Watchdog interval: 0.5sec. */
#define WATCHDOG_INTERVAL		(HZ >> 1)
#define WATCHDOG_INTERVAL_NS		(WATCHDOG_INTERVAL * (NSEC_PER_SEC / HZ))

/* Maximum time between two reference watchdog readouts */
#define WATCHDOG_READOUT_MAX_NS		(50U * NSEC_PER_USEC)

/*
 * Maximum time between two remote readouts for NUMA=n. On NUMA enabled systems
 * the timeout is calculated from the numa distance.
 */
#define WATCHDOG_DEFAULT_TIMEOUT_NS	(50U * NSEC_PER_USEC)

/*
 * Remote timeout NUMA distance multiplier. The local distance is 10. The
 * default remote distance is 20. ACPI tables provide more accurate numbers
 * which are guaranteed to be greater than the local distance.
 *
 * This results in a 5us base value, which is equivalent to the above !NUMA
 * default.
 */
#define WATCHDOG_NUMA_MULTIPLIER_NS	((u64)(WATCHDOG_DEFAULT_TIMEOUT_NS / LOCAL_DISTANCE))

/* Limit the NUMA timeout in case the distance values are insanely big */
#define WATCHDOG_NUMA_MAX_TIMEOUT_NS	((u64)(500U * NSEC_PER_USEC))

/* Shift values to calculate the approximate $N ppm of a given delta. */
#define SHIFT_500PPM			11
#define SHIFT_4000PPM			8

/* Number of attempts to read the watchdog */
#define WATCHDOG_FREQ_RETRIES		3

/* Five reads local and r