linux/init/calibrate.c
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   1// SPDX-License-Identifier: GPL-2.0
   2/* calibrate.c: default delay calibration
   3 *
   4 * Excised from init/main.c
   5 *  Copyright (C) 1991, 1992  Linus Torvalds
   6 */
   7
   8#include <linux/jiffies.h>
   9#include <linux/delay.h>
  10#include <linux/init.h>
  11#include <linux/timex.h>
  12#include <linux/smp.h>
  13#include <linux/percpu.h>
  14
  15unsigned long lpj_fine;
  16unsigned long preset_lpj;
  17static int __init lpj_setup(char *str)
  18{
  19        preset_lpj = simple_strtoul(str,NULL,0);
  20        return 1;
  21}
  22
  23__setup("lpj=", lpj_setup);
  24
  25#ifdef ARCH_HAS_READ_CURRENT_TIMER
  26
  27/* This routine uses the read_current_timer() routine and gets the
  28 * loops per jiffy directly, instead of guessing it using delay().
  29 * Also, this code tries to handle non-maskable asynchronous events
  30 * (like SMIs)
  31 */
  32#define DELAY_CALIBRATION_TICKS                 ((HZ < 100) ? 1 : (HZ/100))
  33#define MAX_DIRECT_CALIBRATION_RETRIES          5
  34
  35static unsigned long calibrate_delay_direct(void)
  36{
  37        unsigned long pre_start, start, post_start;
  38        unsigned long pre_end, end, post_end;
  39        unsigned long start_jiffies;
  40        unsigned long timer_rate_min, timer_rate_max;
  41        unsigned long good_timer_sum = 0;
  42        unsigned long good_timer_count = 0;
  43        unsigned long measured_times[MAX_DIRECT_CALIBRATION_RETRIES];
  44        int max = -1; /* index of measured_times with max/min values or not set */
  45        int min = -1;
  46        int i;
  47
  48        if (read_current_timer(&pre_start) < 0 )
  49                return 0;
  50
  51        /*
  52         * A simple loop like
  53         *      while ( jiffies < start_jiffies+1)
  54         *              start = read_current_timer();
  55         * will not do. As we don't really know whether jiffy switch
  56         * happened first or timer_value was read first. And some asynchronous
  57         * event can happen between these two events introducing errors in lpj.
  58         *
  59         * So, we do
  60         * 1. pre_start <- When we are sure that jiffy switch hasn't happened
  61         * 2. check jiffy switch
  62         * 3. start <- timer value before or after jiffy switch
  63         * 4. post_start <- When we are sure that jiffy switch has happened
  64         *
  65         * Note, we don't know anything about order of 2 and 3.
  66         * Now, by looking at post_start and pre_start difference, we can
  67         * check whether any asynchronous event happened or not
  68         */
  69
  70        for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
  71                pre_start = 0;
  72                read_current_timer(&start);
  73                start_jiffies = jiffies;
  74                while (time_before_eq(jiffies, start_jiffies + 1)) {
  75                        pre_start = start;
  76                        read_current_timer(&start);
  77                }
  78                read_current_timer(&post_start);
  79
  80                pre_end = 0;
  81                end = post_start;
  82                while (time_before_eq(jiffies, start_jiffies + 1 +
  83                                               DELAY_CALIBRATION_TICKS)) {
  84                        pre_end = end;
  85                        read_current_timer(&end);
  86                }
  87                read_current_timer(&post_end);
  88
  89                timer_rate_max = (post_end - pre_start) /
  90                                        DELAY_CALIBRATION_TICKS;
  91                timer_rate_min = (pre_end - post_start) /
  92                                        DELAY_CALIBRATION_TICKS;
  93
  94                /*
  95                 * If the upper limit and lower limit of the timer_rate is
  96                 * >= 12.5% apart, redo calibration.
  97                 */
  98                if (start >= post_end)
  99                        printk(KERN_NOTICE "calibrate_delay_direct() ignoring "
 100                                        "timer_rate as we had a TSC wrap around"
 101                                        " start=%lu >=post_end=%lu\n",
 102                                start, post_end);
 103                if (start < post_end && pre_start != 0 && pre_end != 0 &&
 104                    (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
 105                        good_timer_count++;
 106                        good_timer_sum += timer_rate_max;
 107                        measured_times[i] = timer_rate_max;
 108                        if (max < 0 || timer_rate_max > measured_times[max])
 109                                max = i;
 110                        if (min < 0 || timer_rate_max < measured_times[min])
 111                                min = i;
 112                } else
 113                        measured_times[i] = 0;
 114
 115        }
 116
 117        /*
 118         * Find the maximum & minimum - if they differ too much throw out the
 119         * one with the largest difference from the mean and try again...
 120         */
 121        while (good_timer_count > 1) {
 122                unsigned long estimate;
 123                unsigned long maxdiff;
 124
 125                /* compute the estimate */
 126                estimate = (good_timer_sum/good_timer_count);
 127                maxdiff = estimate >> 3;
 128
 129                /* if range is within 12% let's take it */
 130                if ((measured_times[max] - measured_times[min]) < maxdiff)
 131                        return estimate;
 132
 133                /* ok - drop the worse value and try again... */
 134                good_timer_sum = 0;
 135                good_timer_count = 0;
 136                if ((measured_times[max] - estimate) <
 137                                (estimate - measured_times[min])) {
 138                        printk(KERN_NOTICE "calibrate_delay_direct() dropping "
 139                                        "min bogoMips estimate %d = %lu\n",
 140                                min, measured_times[min]);
 141                        measured_times[min] = 0;
 142                        min = max;
 143                } else {
 144                        printk(KERN_NOTICE "calibrate_delay_direct() dropping "
 145                                        "max bogoMips estimate %d = %lu\n",
 146                                max, measured_times[max]);
 147                        measured_times[max] = 0;
 148                        max = min;
 149                }
 150
 151                for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
 152                        if (measured_times[i] == 0)
 153                                continue;
 154                        good_timer_count++;
 155                        good_timer_sum += measured_times[i];
 156                        if (measured_times[i] < measured_times[min])
 157                                min = i;
 158                        if (measured_times[i] > measured_times[max])
 159                                max = i;
 160                }
 161
 162        }
 163
 164        printk(KERN_NOTICE "calibrate_delay_direct() failed to get a good "
 165               "estimate for loops_per_jiffy.\nProbably due to long platform "
 166                "interrupts. Consider using \"lpj=\" boot option.\n");
 167        return 0;
 168}
 169#else
 170static unsigned long calibrate_delay_direct(void)
 171{
 172        return 0;
 173}
 174#endif
 175
 176/*
 177 * This is the number of bits of precision for the loops_per_jiffy.  Each
 178 * time we refine our estimate after the first takes 1.5/HZ seconds, so try
 179 * to start with a good estimate.
 180 * For the boot cpu we can skip the delay calibration and assign it a value
 181 * calculated based on the timer frequency.
 182 * For the rest of the CPUs we cannot assume that the timer frequency is same as
 183 * the cpu frequency, hence do the calibration for those.
 184 */
 185#define LPS_PREC 8
 186
 187static unsigned long calibrate_delay_converge(void)
 188{
 189        /* First stage - slowly accelerate to find initial bounds */
 190        unsigned long lpj, lpj_base, ticks, loopadd, loopadd_base, chop_limit;
 191        int trials = 0, band = 0, trial_in_band = 0;
 192
 193        lpj = (1<<12);
 194
 195        /* wait for "start of" clock tick */
 196        ticks = jiffies;
 197        while (ticks == jiffies)
 198                ; /* nothing */
 199        /* Go .. */
 200        ticks = jiffies;
 201        do {
 202                if (++trial_in_band == (1<<band)) {
 203                        ++band;
 204                        trial_in_band = 0;
 205                }
 206                __delay(lpj * band);
 207                trials += band;
 208        } while (ticks == jiffies);
 209        /*
 210         * We overshot, so retreat to a clear underestimate. Then estimate
 211         * the largest likely undershoot. This defines our chop bounds.
 212         */
 213        trials -= band;
 214        loopadd_base = lpj * band;
 215        lpj_base = lpj * trials;
 216
 217recalibrate:
 218        lpj = lpj_base;
 219        loopadd = loopadd_base;
 220
 221        /*
 222         * Do a binary approximation to get lpj set to
 223         * equal one clock (up to LPS_PREC bits)
 224         */
 225        chop_limit = lpj >> LPS_PREC;
 226        while (loopadd > chop_limit) {
 227                lpj += loopadd;
 228                ticks = jiffies;
 229                while (ticks == jiffies)
 230                        ; /* nothing */
 231                ticks = jiffies;
 232                __delay(lpj);
 233                if (jiffies != ticks)   /* longer than 1 tick */
 234                        lpj -= loopadd;
 235                loopadd >>= 1;
 236        }
 237        /*
 238         * If we incremented every single time possible, presume we've
 239         * massively underestimated initially, and retry with a higher
 240         * start, and larger range. (Only seen on x86_64, due to SMIs)
 241         */
 242        if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
 243                lpj_base = lpj;
 244                loopadd_base <<= 2;
 245                goto recalibrate;
 246        }
 247
 248        return lpj;
 249}
 250
 251static DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };
 252
 253/*
 254 * Check if cpu calibration delay is already known. For example,
 255 * some processors with multi-core sockets may have all cores
 256 * with the same calibration delay.
 257 *
 258 * Architectures should override this function if a faster calibration
 259 * method is available.
 260 */
 261unsigned long __attribute__((weak)) calibrate_delay_is_known(void)
 262{
 263        return 0;
 264}
 265
 266/*
 267 * Indicate the cpu delay calibration is done. This can be used by
 268 * architectures to stop accepting delay timer registrations after this point.
 269 */
 270
 271void __attribute__((weak)) calibration_delay_done(void)
 272{
 273}
 274
 275void calibrate_delay(void)
 276{
 277        unsigned long lpj;
 278        static bool printed;
 279        int this_cpu = smp_processor_id();
 280
 281        if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
 282                lpj = per_cpu(cpu_loops_per_jiffy, this_cpu);
 283                if (!printed)
 284                        pr_info("Calibrating delay loop (skipped) "
 285                                "already calibrated this CPU");
 286        } else if (preset_lpj) {
 287                lpj = preset_lpj;
 288                if (!printed)
 289                        pr_info("Calibrating delay loop (skipped) "
 290                                "preset value.. ");
 291        } else if ((!printed) && lpj_fine) {
 292                lpj = lpj_fine;
 293                pr_info("Calibrating delay loop (skipped), "
 294                        "value calculated using timer frequency.. ");
 295        } else if ((lpj = calibrate_delay_is_known())) {
 296                ;
 297        } else if ((lpj = calibrate_delay_direct()) != 0) {
 298                if (!printed)
 299                        pr_info("Calibrating delay using timer "
 300                                "specific routine.. ");
 301        } else {
 302                if (!printed)
 303                        pr_info("Calibrating delay loop... ");
 304                lpj = calibrate_delay_converge();
 305        }
 306        per_cpu(cpu_loops_per_jiffy, this_cpu) = lpj;
 307        if (!printed)
 308                pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
 309                        lpj/(500000/HZ),
 310                        (lpj/(5000/HZ)) % 100, lpj);
 311
 312        loops_per_jiffy = lpj;
 313        printed = true;
 314
 315        calibration_delay_done();
 316}
 317
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