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1// SPDX-License-Identifier: GPL-2.02/* calibrate.c: default delay calibration3 *4 * Excised from init/main.c5 * Copyright (C) 1991, 1992 Linus Torvalds6 */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_TIMER26 27/* This routine uses the read_current_timer() routine and gets the28 * loops per jiffy directly, instead of guessing it using delay().29 * Also, this code tries to handle non-maskable asynchronous events30 * (like SMIs)31 */32#define DELAY_CALIBRATION_TICKS ((HZ < 100) ? 1 : (HZ/100))33#define MAX_DIRECT_CALIBRATION_RETRIES 534 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 like53 * while ( jiffies < start_jiffies+1)54 * start = read_current_timer();55 * will not do. As we don't really know whether jiffy switch56 * happened first or timer_value was read first. And some asynchronous57 * event can happen between these two events introducing errors in lpj.58 *59 * So, we do60 * 1. pre_start <- When we are sure that jiffy switch hasn't happened61 * 2. check jiffy switch62 * 3. start <- timer value before or after jiffy switch63 * 4. post_start <- When we are sure that jiffy switch has happened64 *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 can67 * check whether any asynchronous event happened or not68 */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 is96 * >= 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 } else113 measured_times[i] = 0;114 115 }116 117 /*118 * Find the maximum & minimum - if they differ too much throw out the119 * 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#else170static unsigned long calibrate_delay_direct(void)171{172 return 0;173}174#endif175 176/*177 * This is the number of bits of precision for the loops_per_jiffy. Each178 * time we refine our estimate after the first takes 1.5/HZ seconds, so try179 * to start with a good estimate.180 * For the boot cpu we can skip the delay calibration and assign it a value181 * calculated based on the timer frequency.182 * For the rest of the CPUs we cannot assume that the timer frequency is same as183 * the cpu frequency, hence do the calibration for those.184 */185#define LPS_PREC 8186 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 estimate211 * 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 to223 * 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've239 * massively underestimated initially, and retry with a higher240 * 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 cores256 * with the same calibration delay.257 *258 * Architectures should override this function if a faster calibration259 * 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 by268 * 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