// SPDX-License-Identifier: GPL-2.0
/* calibrate.c: default delay calibration
 *
 * Excised from init/main.c
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

#include <linux/delay.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/kstrtox.h>
#include <linux/percpu.h>
#include <linux/printk.h>
#include <linux/smp.h>
#include <linux/stddef.h>
#include <linux/timex.h>

unsigned long lpj_fine;
unsigned long preset_lpj;

static int __init lpj_setup(char *str)
{
        return kstrtoul(str, 0, &preset_lpj) == 0;
}

__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 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 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 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)) calibrate_delay_is_known(void)
{
        return 0;
}

/*
 * Indicate the cpu delay calibration is done. This can be used by
 * architectures to stop accepting delay timer registrations after this point.
 */

void __attribute__((weak)) calibration_delay_done(void)
{
}

void 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;

        calibration_delay_done();
}