arch/arm/lib/delay.c

Source file repositories/reference/linux-study-clean/arch/arm/lib/delay.c

File Facts

System
Linux kernel
Corpus path
arch/arm/lib/delay.c
Extension
.c
Size
2452 bytes
Lines
106
Domain
Architecture Layer
Bucket
arch/arm
Inferred role
Architecture Layer: exported/initcall integration point
Status
integration implementation candidate

Why This File Exists

CPU and platform-specific kernel glue: boot entry, traps, syscall entry, interrupts, page tables, context switch, and low-level barriers.

Dependency Surface

Detected Declarations

Annotated Snippet

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Delay loops based on the OpenRISC implementation.
 *
 * Copyright (C) 2012 ARM Limited
 *
 * Author: Will Deacon <will.deacon@arm.com>
 */

#include <linux/clocksource.h>
#include <linux/delay.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/timex.h>

/*
 * Default to the loop-based delay implementation.
 */
struct arm_delay_ops arm_delay_ops __ro_after_init = {
	.delay		= __loop_delay,
	.const_udelay	= __loop_const_udelay,
	.udelay		= __loop_udelay,
};

static const struct delay_timer *delay_timer;
static bool delay_calibrated;
static u64 delay_res;

int read_current_timer(unsigned long *timer_val)
{
	if (!delay_timer)
		return -ENXIO;

	*timer_val = delay_timer->read_current_timer();
	return 0;
}
EXPORT_SYMBOL_GPL(read_current_timer);

static inline u64 cyc_to_ns(u64 cyc, u32 mult, u32 shift)
{
	return (cyc * mult) >> shift;
}

static void __timer_delay(unsigned long cycles)
{
	cycles_t start = get_cycles();

	while ((get_cycles() - start) < cycles)
		cpu_relax();
}

static void __timer_const_udelay(unsigned long xloops)
{
	unsigned long long loops = xloops;
	loops *= arm_delay_ops.ticks_per_jiffy;
	__timer_delay(loops >> UDELAY_SHIFT);
}

static void __timer_udelay(unsigned long usecs)
{
	__timer_const_udelay(usecs * UDELAY_MULT);
}

void __init register_current_timer_delay(const struct delay_timer *timer)
{
	u32 new_mult, new_shift;
	u64 res;

	clocks_calc_mult_shift(&new_mult, &new_shift, timer->freq,
			       NSEC_PER_SEC, 3600);
	res = cyc_to_ns(1ULL, new_mult, new_shift);

	if (res > 1000) {
		pr_err("Ignoring delay timer %ps, which has insufficient resolution of %lluns\n",
			timer, res);
		return;
	}

	if (!delay_calibrated && (!delay_res || (res < delay_res))) {
		pr_info("Switching to timer-based delay loop, resolution %lluns\n", res);
		delay_timer			= timer;
		lpj_fine			= timer->freq / HZ;
		delay_res			= res;

		/* cpufreq may scale loops_per_jiffy, so keep a private copy */
		arm_delay_ops.ticks_per_jiffy	= lpj_fine;
		arm_delay_ops.delay		= __timer_delay;
		arm_delay_ops.const_udelay	= __timer_const_udelay;
		arm_delay_ops.udelay		= __timer_udelay;

Annotation

Implementation Notes