calibrate.c

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/* 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)
{
    preset_lpj = simple_strtoul(str,NULL,0);
    return 1;
}

__setup("lpj=", lpj_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         ((HZ < 100) ? 1 : (HZ/100))
#define MAX_DIRECT_CALIBRATION_RETRIES      5

static unsigned long __cpuinit calibrate_delay_direct(void)
{
    unsigned long pre_start, start, post_start;
    unsigned long pre_end, end, post_end;
    unsigned long start_jiffies;
    unsigned long timer_rate_min, timer_rate_max;
    unsigned long good_timer_sum = 0;
    unsigned long good_timer_count = 0;
    unsigned long measured_times[MAX_DIRECT_CALIBRATION_RETRIES];
    int max = -1; /* index of measured_times with max/min values or not set */
    int min = -1;
    int i;

    if (read_current_timer(&pre_start) < 0 )
        return 0;

    /*
     * A simple loop like
     *  while ( jiffies < start_jiffies+1)
     *      start = read_current_timer();
     * will not do. As we don't really know whether jiffy switch
     * happened first or timer_value was read first. And some asynchronous
     * event can happen between these two events introducing errors in lpj.
     *
     * So, we do
     * 1. pre_start <- When we are sure that jiffy switch hasn't happened
     * 2. check jiffy switch
     * 3. start <- timer value before or after jiffy switch
     * 4. post_start <- When we are sure that jiffy switch has happened
     *
     * Note, we don't know anything about order of 2 and 3.
     * Now, by looking at post_start and pre_start difference, we can
     * check whether any asynchronous event happened or not
     */

    for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
        pre_start = 0;
        read_current_timer(&start);
        start_jiffies = jiffies;
        while (time_before_eq(jiffies, start_jiffies + 1)) {
            pre_start = start;
            read_current_timer(&start);
        }
        read_current_timer(&post_start);

        pre_end = 0;
        end = post_start;
        while (time_before_eq(jiffies, start_jiffies + 1 +
                           DELAY_CALIBRATION_TICKS)) {
            pre_end = end;
            read_current_timer(&end);
        }
        read_current_timer(&post_end);

        timer_rate_max = (post_end - pre_start) /
                    DELAY_CALIBRATION_TICKS;
        timer_rate_min = (pre_end - post_start) /
                    DELAY_CALIBRATION_TICKS;

        /*
         * If the upper limit and lower limit of the timer_rate is
         * >= 12.5% apart, redo calibration.
         */
        if (start >= post_end)
            printk(KERN_NOTICE "calibrate_delay_direct() ignoring "
                    "timer_rate as we had a TSC wrap around"
                    " start=%lu >=post_end=%lu\n",
                start, post_end);
        if (start < post_end && pre_start != 0 && pre_end != 0 &&
            (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
            good_timer_count++;
            good_timer_sum += timer_rate_max;
            measured_times[i] = timer_rate_max;
            if (max < 0 || timer_rate_max > measured_times[max])
                max = i;
            if (min < 0 || timer_rate_max < measured_times[min])
                min = i;
        } else
            measured_times[i] = 0;

    }

    /*
     * Find the maximum & minimum - if they differ too much throw out the
     * one with the largest difference from the mean and try again...
     */
    while (good_timer_count > 1) {
        unsigned long estimate;
        unsigned long maxdiff;

        /* compute the estimate */
        estimate = (good_timer_sum/good_timer_count);
        maxdiff = estimate >> 3;

        /* if range is within 12% let's take it */
        if ((measured_times[max] - measured_times[min]) < maxdiff)
            return estimate;

        /* ok - drop the worse value and try again... */
        good_timer_sum = 0;
        good_timer_count = 0;
        if ((measured_times[max] - estimate) <
                (estimate - measured_times[min])) {
            printk(KERN_NOTICE "calibrate_delay_direct() dropping "
                    "min bogoMips estimate %d = %lu\n",
                min, measured_times[min]);
            measured_times[min] = 0;
            min = max;
        } else {
            printk(KERN_NOTICE "calibrate_delay_direct() dropping "
                    "max bogoMips estimate %d = %lu\n",
                max, measured_times[max]);
            measured_times[max] = 0;
            max = min;
        }

        for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
            if (measured_times[i] == 0)
                continue;
            good_timer_count++;
            good_timer_sum += measured_times[i];
            if (measured_times[i] < measured_times[min])
                min = i;
            if (measured_times[i] > measured_times[max])
                max = i;
        }

    }

    printk(KERN_NOTICE "calibrate_delay_direct() failed to get a good "
           "estimate for loops_per_jiffy.\nProbably due to long platform "
        "interrupts. Consider using \"lpj=\" boot option.\n");
    return 0;
}
#else
static unsigned long __cpuinit 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 8

static unsigned long __cpuinit calibrate_delay_converge(void)
{
    /* First stage - slowly accelerate to find initial bounds */
    unsigned long lpj, lpj_base, ticks, loopadd, loopadd_base, chop_limit;
    int trials = 0, band = 0, trial_in_band = 0;

    lpj = (1<<12);

    /* wait for "start of" clock tick */
    ticks = jiffies;
    while (ticks == jiffies)
        ; /* nothing */
    /* Go .. */
    ticks = jiffies;
    do {
        if (++trial_in_band == (1<<band)) {
            ++band;
            trial_in_band = 0;
        }
        __delay(lpj * band);
        trials += band;
    } while (ticks == jiffies);
    /*
     * We overshot, so retreat to a clear underestimate. Then estimate
     * the largest likely undershoot. This defines our chop bounds.
     */
    trials -= band;
    loopadd_base = lpj * band;
    lpj_base = lpj * trials;

recalibrate:
    lpj = lpj_base;
    loopadd = loopadd_base;

    /*
     * Do a binary approximation to get lpj set to
     * equal one clock (up to LPS_PREC bits)
     */
    chop_limit = lpj >> LPS_PREC;
    while (loopadd > chop_limit) {
        lpj += loopadd;
        ticks = jiffies;
        while (ticks == jiffies)
            ; /* nothing */
        ticks = jiffies;
        __delay(lpj);
        if (jiffies != ticks)   /* longer than 1 tick */
            lpj -= loopadd;
        loopadd >>= 1;
    }
    /*
     * If we incremented every single time possible, presume we've
     * massively underestimated initially, and retry with a higher
     * start, and larger range. (Only seen on x86_64, due to SMIs)
     */
    if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
        lpj_base = lpj;
        loopadd_base <<= 2;
        goto recalibrate;
    }

    return lpj;
}

static DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };

/*
 * 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)) __cpuinit calibrate_delay_is_known(void)
{
    return 0;
}

void __cpuinit calibrate_delay(void)
{
    unsigned long lpj;
    static bool printed;
    int this_cpu = smp_processor_id();

    if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
        lpj = per_cpu(cpu_loops_per_jiffy, this_cpu);
        if (!printed)
            pr_info("Calibrating delay loop (skipped) "
                "already calibrated this CPU");
    } else if (preset_lpj) {
        lpj = preset_lpj;
        if (!printed)
            pr_info("Calibrating delay loop (skipped) "
                "preset value.. ");
    } else if ((!printed) && lpj_fine) {
        lpj = lpj_fine;
        pr_info("Calibrating delay loop (skipped), "
            "value calculated using timer frequency.. ");
    } else if ((lpj = calibrate_delay_is_known())) {
        ;
    } else if ((lpj = calibrate_delay_direct()) != 0) {
        if (!printed)
            pr_info("Calibrating delay using timer "
                "specific routine.. ");
    } else {
        if (!printed)
            pr_info("Calibrating delay loop... ");
        lpj = calibrate_delay_converge();
    }
    per_cpu(cpu_loops_per_jiffy, this_cpu) = lpj;
    if (!printed)
        pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
            lpj/(500000/HZ),
            (lpj/(5000/HZ)) % 100, lpj);

    loops_per_jiffy = lpj;
    printed = true;
}