delay.c

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/*
 *  Precise Delay Loops for i386
 *
 *  Copyright (C) 1993 Linus Torvalds
 *  Copyright (C) 1997 Martin Mares <mj@atrey.karlin.mff.cuni.cz>
 *  Copyright (C) 2008 Jiri Hladky <hladky _dot_ jiri _at_ gmail _dot_ com>
 *
 *  The __delay function must _NOT_ be inlined as its execution time
 *  depends wildly on alignment on many x86 processors. The additional
 *  jump magic is needed to get the timing stable on all the CPU's
 *  we have to worry about.
 */

#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/processor.h>
#include <asm/delay.h>
#include <asm/timer.h>

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

/* simple loop based delay: */
static void delay_loop(unsigned long loops)
{
    asm volatile(
        "   test %0,%0  \n"
        "   jz 3f       \n"
        "   jmp 1f      \n"

        ".align 16      \n"
        "1: jmp 2f      \n"

        ".align 16      \n"
        "2: dec %0      \n"
        "   jnz 2b      \n"
        "3: dec %0      \n"

        : /* we don't need output */
        :"a" (loops)
    );
}

/* TSC based delay: */
static void delay_tsc(unsigned long __loops)
{
    u32 bclock, now, loops = __loops;
    int cpu;

    preempt_disable();
    cpu = smp_processor_id();
    rdtsc_barrier();
    rdtscl(bclock);
    for (;;) {
        rdtsc_barrier();
        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();
            rdtsc_barrier();
            rdtscl(bclock);
        }
    }
    preempt_enable();
}

/*
 * Since we calibrate only once at boot, this
 * function should be set once at boot and not changed
 */
static void (*delay_fn)(unsigned long) = delay_loop;

void use_tsc_delay(void)
{
    delay_fn = delay_tsc;
}

int __devinit read_current_timer(unsigned long *timer_val)
{
    if (delay_fn == delay_tsc) {
        rdtscll(*timer_val);
        return 0;
    }
    return -1;
}

void __delay(unsigned long loops)
{
    delay_fn(loops);
}
EXPORT_SYMBOL(__delay);

inline void __const_udelay(unsigned long xloops)
{
    int d0;

    xloops *= 4;
    asm("mull %%edx"
        :"=d" (xloops), "=&a" (d0)
        :"1" (xloops), "0"
        (this_cpu_read(cpu_info.loops_per_jiffy) * (HZ/4)));

    __delay(++xloops);
}
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);