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/sched.h>
   8 #include <linux/delay.h>
   9 #include <linux/init.h>
  10
  11 static unsigned long preset_lpj;
  12 static int __init lpj_setup(char *str)
  13 {
  14         preset_lpj = simple_strtoul(str,NULL,0);
  15         return 1;
  16 }
  17
  18 __setup("lpj=", lpj_setup);
  19
  20 /*
  21  * This is the number of bits of precision for the loops_per_jiffy.  Each
  22  * bit takes on average 1.5/HZ seconds.  This (like the original) is a little
  23  * better than 1%
  24  */
  25 #define LPS_PREC 8
  26
  27 void __devinit calibrate_delay(void)
  28 {
  29         unsigned long ticks, loopbit;
  30         int lps_precision = LPS_PREC;
  31
  32         if (preset_lpj) {
  33                 loops_per_jiffy = preset_lpj;
  34                 printk("Calibrating delay loop (skipped)... "
  35                         "%lu.%02lu BogoMIPS preset\n",
  36                         loops_per_jiffy/(500000/HZ),
  37                         (loops_per_jiffy/(5000/HZ)) % 100);
  38         } else {
  39                 loops_per_jiffy = (1<<12);
  40
  41                 printk(KERN_DEBUG "Calibrating delay loop... ");
  42                 while ((loops_per_jiffy <<= 1) != 0) {
  43                         /* wait for "start of" clock tick */
  44                         ticks = jiffies;
  45                         while (ticks == jiffies)
  46                                 /* nothing */;
  47                         /* Go .. */
  48                         ticks = jiffies;
  49                         __delay(loops_per_jiffy);
  50                         ticks = jiffies - ticks;
  51                         if (ticks)
  52                                 break;
  53                 }
  54
  55                 /*
  56                  * Do a binary approximation to get loops_per_jiffy set to
  57                  * equal one clock (up to lps_precision bits)
  58                  */
  59                 loops_per_jiffy >>= 1;
  60                 loopbit = loops_per_jiffy;
  61                 while (lps_precision-- && (loopbit >>= 1)) {
  62                         loops_per_jiffy |= loopbit;
  63                         ticks = jiffies;
  64                         while (ticks == jiffies)
  65                                 /* nothing */;
  66                         ticks = jiffies;
  67                         __delay(loops_per_jiffy);
  68                         if (jiffies != ticks)   /* longer than 1 tick */
  69                                 loops_per_jiffy &= ~loopbit;
  70                 }
  71
  72                 /* Round the value and print it */
  73                 printk("%lu.%02lu BogoMIPS (lpj=%lu)\n",
  74                         loops_per_jiffy/(500000/HZ),
  75                         (loops_per_jiffy/(5000/HZ)) % 100,
  76                         loops_per_jiffy);
  77         }
  78
  79 }