linux/init/calibrate.c

// SPDX-License-Identifier: GPL-2.0
/* calibrate.c: default delay calibration
 *
 * Excised from init/main.c
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

#include <linux/jiffies.h>
#include <linux/delay.h>
#include <linux/init.h>
#include <linux/timex.h>
#include <linux/smp.h>
#include <linux/percpu.h>

unsigned long lpj_fine;
unsigned long preset_lpj;
static int __init lpj_setup(char *str)
{ … }

__setup(…);

#ifdef ARCH_HAS_READ_CURRENT_TIMER

/* This routine uses the read_current_timer() routine and gets the
 * loops per jiffy directly, instead of guessing it using delay().
 * Also, this code tries to handle non-maskable asynchronous events
 * (like SMIs)
 */
#define DELAY_CALIBRATION_TICKS …
#define MAX_DIRECT_CALIBRATION_RETRIES …

static unsigned long calibrate_delay_direct(void)
{ … }
#else
static unsigned long calibrate_delay_direct(void)
{
	return 0;
}
#endif

/*
 * This is the number of bits of precision for the loops_per_jiffy.  Each
 * time we refine our estimate after the first takes 1.5/HZ seconds, so try
 * to start with a good estimate.
 * For the boot cpu we can skip the delay calibration and assign it a value
 * calculated based on the timer frequency.
 * For the rest of the CPUs we cannot assume that the timer frequency is same as
 * the cpu frequency, hence do the calibration for those.
 */
#define LPS_PREC …

static unsigned long calibrate_delay_converge(void)
{ … }

static DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = …;

/*
 * Check if cpu calibration delay is already known. For example,
 * some processors with multi-core sockets may have all cores
 * with the same calibration delay.
 *
 * Architectures should override this function if a faster calibration
 * method is available.
 */
unsigned long __attribute__((weak)) calibrate_delay_is_known(void)
{ … }

/*
 * Indicate the cpu delay calibration is done. This can be used by
 * architectures to stop accepting delay timer registrations after this point.
 */

void __attribute__((weak)) calibration_delay_done(void)
{ … }

void calibrate_delay(void)
{ … }