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