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
  13 static unsigned long preset_lpj;
  14 static 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
  32 static 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
 105 static 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
 115 void __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 }