linux/init/calibrate.c
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   1/* calibrate.c: default delay calibration
   2 *
   3 * Excised from init/main.c
   4 *  Copyright (C) 1991, 1992  Linus Torvalds
   5 */
   6
   7#include <linux/jiffies.h>
   8#include <linux/delay.h>
   9#include <linux/init.h>
  10#include <linux/timex.h>
  11#include <linux/smp.h>
  12
  13unsigned long lpj_fine;
  14unsigned long preset_lpj;
  15static int __init lpj_setup(char *str)
  16{
  17        preset_lpj = simple_strtoul(str,NULL,0);
  18        return 1;
  19}
  20
  21__setup("lpj=", lpj_setup);
  22
  23#ifdef ARCH_HAS_READ_CURRENT_TIMER
  24
  25/* This routine uses the read_current_timer() routine and gets the
  26 * loops per jiffy directly, instead of guessing it using delay().
  27 * Also, this code tries to handle non-maskable asynchronous events
  28 * (like SMIs)
  29 */
  30#define DELAY_CALIBRATION_TICKS                 ((HZ < 100) ? 1 : (HZ/100))
  31#define MAX_DIRECT_CALIBRATION_RETRIES          5
  32
  33static unsigned long __cpuinit calibrate_delay_direct(void)
  34{
  35        unsigned long pre_start, start, post_start;
  36        unsigned long pre_end, end, post_end;
  37        unsigned long start_jiffies;
  38        unsigned long timer_rate_min, timer_rate_max;
  39        unsigned long good_timer_sum = 0;
  40        unsigned long good_timer_count = 0;
  41        int i;
  42
  43        if (read_current_timer(&pre_start) < 0 )
  44                return 0;
  45
  46        /*
  47         * A simple loop like
  48         *      while ( jiffies < start_jiffies+1)
  49         *              start = read_current_timer();
  50         * will not do. As we don't really know whether jiffy switch
  51         * happened first or timer_value was read first. And some asynchronous
  52         * event can happen between these two events introducing errors in lpj.
  53         *
  54         * So, we do
  55         * 1. pre_start <- When we are sure that jiffy switch hasn't happened
  56         * 2. check jiffy switch
  57         * 3. start <- timer value before or after jiffy switch
  58         * 4. post_start <- When we are sure that jiffy switch has happened
  59         *
  60         * Note, we don't know anything about order of 2 and 3.
  61         * Now, by looking at post_start and pre_start difference, we can
  62         * check whether any asynchronous event happened or not
  63         */
  64
  65        for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
  66                pre_start = 0;
  67                read_current_timer(&start);
  68                start_jiffies = jiffies;
  69                while (jiffies <= (start_jiffies + 1)) {
  70                        pre_start = start;
  71                        read_current_timer(&start);
  72                }
  73                read_current_timer(&post_start);
  74
  75                pre_end = 0;
  76                end = post_start;
  77                while (jiffies <=
  78                       (start_jiffies + 1 + DELAY_CALIBRATION_TICKS)) {
  79                        pre_end = end;
  80                        read_current_timer(&end);
  81                }
  82                read_current_timer(&post_end);
  83
  84                timer_rate_max = (post_end - pre_start) /
  85                                        DELAY_CALIBRATION_TICKS;
  86                timer_rate_min = (pre_end - post_start) /
  87                                        DELAY_CALIBRATION_TICKS;
  88
  89                /*
  90                 * If the upper limit and lower limit of the timer_rate is
  91                 * >= 12.5% apart, redo calibration.
  92                 */
  93                if (pre_start != 0 && pre_end != 0 &&
  94                    (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
  95                        good_timer_count++;
  96                        good_timer_sum += timer_rate_max;
  97                }
  98        }
  99
 100        if (good_timer_count)
 101                return (good_timer_sum/good_timer_count);
 102
 103        printk(KERN_WARNING "calibrate_delay_direct() failed to get a good "
 104               "estimate for loops_per_jiffy.\nProbably due to long platform interrupts. Consider using \"lpj=\" boot option.\n");
 105        return 0;
 106}
 107#else
 108static unsigned long __cpuinit calibrate_delay_direct(void) {return 0;}
 109#endif
 110
 111/*
 112 * This is the number of bits of precision for the loops_per_jiffy.  Each
 113 * bit takes on average 1.5/HZ seconds.  This (like the original) is a little
 114 * better than 1%
 115 * For the boot cpu we can skip the delay calibration and assign it a value
 116 * calculated based on the timer frequency.
 117 * For the rest of the CPUs we cannot assume that the timer frequency is same as
 118 * the cpu frequency, hence do the calibration for those.
 119 */
 120#define LPS_PREC 8
 121
 122void __cpuinit calibrate_delay(void)
 123{
 124        unsigned long ticks, loopbit;
 125        int lps_precision = LPS_PREC;
 126
 127        if (preset_lpj) {
 128                loops_per_jiffy = preset_lpj;
 129                printk(KERN_INFO
 130                        "Calibrating delay loop (skipped) preset value.. ");
 131        } else if ((smp_processor_id() == 0) && lpj_fine) {
 132                loops_per_jiffy = lpj_fine;
 133                printk(KERN_INFO
 134                        "Calibrating delay loop (skipped), "
 135                        "value calculated using timer frequency.. ");
 136        } else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) {
 137                printk(KERN_INFO
 138                        "Calibrating delay using timer specific routine.. ");
 139        } else {
 140                loops_per_jiffy = (1<<12);
 141
 142                printk(KERN_INFO "Calibrating delay loop... ");
 143                while ((loops_per_jiffy <<= 1) != 0) {
 144                        /* wait for "start of" clock tick */
 145                        ticks = jiffies;
 146                        while (ticks == jiffies)
 147                                /* nothing */;
 148                        /* Go .. */
 149                        ticks = jiffies;
 150                        __delay(loops_per_jiffy);
 151                        ticks = jiffies - ticks;
 152                        if (ticks)
 153                                break;
 154                }
 155
 156                /*
 157                 * Do a binary approximation to get loops_per_jiffy set to
 158                 * equal one clock (up to lps_precision bits)
 159                 */
 160                loops_per_jiffy >>= 1;
 161                loopbit = loops_per_jiffy;
 162                while (lps_precision-- && (loopbit >>= 1)) {
 163                        loops_per_jiffy |= loopbit;
 164                        ticks = jiffies;
 165                        while (ticks == jiffies)
 166                                /* nothing */;
 167                        ticks = jiffies;
 168                        __delay(loops_per_jiffy);
 169                        if (jiffies != ticks)   /* longer than 1 tick */
 170                                loops_per_jiffy &= ~loopbit;
 171                }
 172        }
 173        printk(KERN_CONT "%lu.%02lu BogoMIPS (lpj=%lu)\n",
 174                        loops_per_jiffy/(500000/HZ),
 175                        (loops_per_jiffy/(5000/HZ)) % 100, loops_per_jiffy);
 176}
 177
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