LXR linux/init/calibrate.c

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