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 #include <linux/timex.h>
  11
  12 unsigned long preset_lpj;
  13 static 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
  31 static 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
 104 static 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
 114 void __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 }