linux/arch/x86/lib/delay_64.c

/*
 *	Precise Delay Loops for x86-64
 *
 *	Copyright (C) 1993 Linus Torvalds
 *	Copyright (C) 1997 Martin Mares <mj@atrey.karlin.mff.cuni.cz>
 *
 *	The __delay function must _NOT_ be inlined as its execution time
 *	depends wildly on alignment on many x86 processors. 
 */

#include <linux/module.h>
#include <linux/sched.h>
#include <linux/timex.h>
#include <linux/preempt.h>
#include <linux/delay.h>
#include <linux/init.h>

#include <asm/delay.h>
#include <asm/msr.h>

#ifdef CONFIG_SMP
#include <asm/smp.h>
#endif

int __devinit read_current_timer(unsigned long *timer_value)
{
	rdtscll(*timer_value);
	return 0;
}

void __delay(unsigned long loops)
{
	unsigned bclock, now;
	int cpu;

	preempt_disable();
	cpu = smp_processor_id();
	rdtscl(bclock);
	for (;;) {
		rdtscl(now);
		if ((now - bclock) >= loops)
			break;

		/* Allow RT tasks to run */
		preempt_enable();
		rep_nop();
		preempt_disable();

		/*
		 * It is possible that we moved to another CPU, and
		 * since TSC's are per-cpu we need to calculate
		 * that. The delay must guarantee that we wait "at
		 * least" the amount of time. Being moved to another
		 * CPU could make the wait longer but we just need to
		 * make sure we waited long enough. Rebalance the
		 * counter for this CPU.
		 */
		if (unlikely(cpu != smp_processor_id())) {
			loops -= (now - bclock);
			cpu = smp_processor_id();
			rdtscl(bclock);
		}
	}
	preempt_enable();
}
EXPORT_SYMBOL(__delay);

inline void __const_udelay(unsigned long xloops)
{
	__delay(((xloops * HZ *
		cpu_data(raw_smp_processor_id()).loops_per_jiffy) >> 32) + 1);
}
EXPORT_SYMBOL(__const_udelay);

void __udelay(unsigned long usecs)
{
	__const_udelay(usecs * 0x000010c7);  /* 2**32 / 1000000 (rounded up) */
}
EXPORT_SYMBOL(__udelay);

void __ndelay(unsigned long nsecs)
{
	__const_udelay(nsecs * 0x00005);  /* 2**32 / 1000000000 (rounded up) */
}
EXPORT_SYMBOL(__ndelay);