arch/arm64/lib/delay.c

Source file repositories/reference/linux-study-clean/arch/arm64/lib/delay.c

File Facts

System
Linux kernel
Corpus path
arch/arm64/lib/delay.c
Extension
.c
Size
2130 bytes
Lines
85
Domain
Architecture Layer
Bucket
arch/arm64
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/delay.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/timex.h>

#include <clocksource/arm_arch_timer.h>

#define USECS_TO_CYCLES(time_usecs)			\
	xloops_to_cycles((time_usecs) * 0x10C7UL)

static inline unsigned long xloops_to_cycles(unsigned long xloops)
{
	return (xloops * loops_per_jiffy * HZ) >> 32;
}

/*
 * Force the use of CNTVCT_EL0 in order to have the same base as WFxT.
 * This avoids some annoying issues when CNTVOFF_EL2 is not reset 0 on a
 * KVM host running at EL1 until we do a vcpu_put() on the vcpu. When
 * running at EL2, the effective offset is always 0.
 *
 * Note that userspace cannot change the offset behind our back either,
 * as the vcpu mutex is held as long as KVM_RUN is in progress.
 */
static cycles_t notrace __delay_cycles(void)
{
	guard(preempt_notrace)();
	return __arch_counter_get_cntvct_stable();
}

void __delay(unsigned long cycles)
{
	cycles_t start = __delay_cycles();

	if (alternative_has_cap_unlikely(ARM64_HAS_WFXT)) {
		u64 end = start + cycles;

		/*
		 * Start with WFIT. If an interrupt makes us resume
		 * early, use a WFET loop to complete the delay.
		 */
		wfit(end);
		while ((__delay_cycles() - start) < cycles)
			wfet(end);
	} else 	if (arch_timer_evtstrm_available()) {
		const cycles_t timer_evt_period =
			USECS_TO_CYCLES(ARCH_TIMER_EVT_STREAM_PERIOD_US);

		while ((__delay_cycles() - start + timer_evt_period) < cycles)
			wfe();
	}

	while ((__delay_cycles() - start) < cycles)
		cpu_relax();
}
EXPORT_SYMBOL(__delay);

inline void __const_udelay(unsigned long xloops)
{
	__delay(xloops_to_cycles(xloops));
}
EXPORT_SYMBOL(__const_udelay);

void __udelay(unsigned long usecs)
{
	__const_udelay(usecs * 0x10C7UL); /* 2**32 / 1000000 (rounded up) */
}
EXPORT_SYMBOL(__udelay);

void __ndelay(unsigned long nsecs)
{
	__const_udelay(nsecs * 0x5UL); /* 2**32 / 1000000000 (rounded up) */
}
EXPORT_SYMBOL(__ndelay);

Annotation

Implementation Notes