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 #include <linux/smp.h>
  12
  13 unsigned long lpj_fine;
  14 unsigned long preset_lpj;
  15 static int __init lpj_setup(char *str)
  16 {
  17         preset_lpj = simple_strtoul(str,NULL,0);
  18         return 1;
  19 }
  20
  21 __setup("lpj=", lpj_setup);
  22
  23 #ifdef ARCH_HAS_READ_CURRENT_TIMER
  24
  25 /* This routine uses the read_current_timer() routine and gets the
  26  * loops per jiffy directly, instead of guessing it using delay().
  27  * Also, this code tries to handle non-maskable asynchronous events
  28  * (like SMIs)
  29  */
  30 #define DELAY_CALIBRATION_TICKS                 ((HZ < 100) ? 1 : (HZ/100))
  31 #define MAX_DIRECT_CALIBRATION_RETRIES          5
  32
  33 static unsigned long __cpuinit calibrate_delay_direct(void)
  34 {
  35         unsigned long pre_start, start, post_start;
  36         unsigned long pre_end, end, post_end;
  37         unsigned long start_jiffies;
  38         unsigned long timer_rate_min, timer_rate_max;
  39         unsigned long good_timer_sum = 0;
  40         unsigned long good_timer_count = 0;
  41         int i;
  42
  43         if (read_current_timer(&pre_start) < 0 )
  44                 return 0;
  45
  46         /*
  47          * A simple loop like
  48          *      while ( jiffies < start_jiffies+1)
  49          *              start = read_current_timer();
  50          * will not do. As we don't really know whether jiffy switch
  51          * happened first or timer_value was read first. And some asynchronous
  52          * event can happen between these two events introducing errors in lpj.
  53          *
  54          * So, we do
  55          * 1. pre_start <- When we are sure that jiffy switch hasn't happened
  56          * 2. check jiffy switch
  57          * 3. start <- timer value before or after jiffy switch
  58          * 4. post_start <- When we are sure that jiffy switch has happened
  59          *
  60          * Note, we don't know anything about order of 2 and 3.
  61          * Now, by looking at post_start and pre_start difference, we can
  62          * check whether any asynchronous event happened or not
  63          */
  64
  65         for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
  66                 pre_start = 0;
  67                 read_current_timer(&start);
  68                 start_jiffies = jiffies;
  69                 while (time_before_eq(jiffies, start_jiffies + 1)) {
  70                         pre_start = start;
  71                         read_current_timer(&start);
  72                 }
  73                 read_current_timer(&post_start);
  74
  75                 pre_end = 0;
  76                 end = post_start;
  77                 while (time_before_eq(jiffies, start_jiffies + 1 +
  78                                                DELAY_CALIBRATION_TICKS)) {
  79                         pre_end = end;
  80                         read_current_timer(&end);
  81                 }
  82                 read_current_timer(&post_end);
  83
  84                 timer_rate_max = (post_end - pre_start) /
  85                                         DELAY_CALIBRATION_TICKS;
  86                 timer_rate_min = (pre_end - post_start) /
  87                                         DELAY_CALIBRATION_TICKS;
  88
  89                 /*
  90                  * If the upper limit and lower limit of the timer_rate is
  91                  * >= 12.5% apart, redo calibration.
  92                  */
  93                 if (pre_start != 0 && pre_end != 0 &&
  94                     (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
  95                         good_timer_count++;
  96                         good_timer_sum += timer_rate_max;
  97                 }
  98         }
  99
 100         if (good_timer_count)
 101                 return (good_timer_sum/good_timer_count);
 102
 103         printk(KERN_WARNING "calibrate_delay_direct() failed to get a good "
 104                "estimate for loops_per_jiffy.\nProbably due to long platform interrupts. Consider using \"lpj=\" boot option.\n");
 105         return 0;
 106 }
 107 #else
 108 static unsigned long __cpuinit calibrate_delay_direct(void) {return 0;}
 109 #endif
 110
 111 /*
 112  * This is the number of bits of precision for the loops_per_jiffy.  Each
 113  * time we refine our estimate after the first takes 1.5/HZ seconds, so try
 114  * to start with a good estimate.
 115  * For the boot cpu we can skip the delay calibration and assign it a value
 116  * calculated based on the timer frequency.
 117  * For the rest of the CPUs we cannot assume that the timer frequency is same as
 118  * the cpu frequency, hence do the calibration for those.
 119  */
 120 #define LPS_PREC 8
 121
 122 static unsigned long __cpuinit calibrate_delay_converge(void)
 123 {
 124         /* First stage - slowly accelerate to find initial bounds */
 125         unsigned long lpj, lpj_base, ticks, loopadd, loopadd_base, chop_limit;
 126         int trials = 0, band = 0, trial_in_band = 0;
 127
 128         lpj = (1<<12);
 129
 130         /* wait for "start of" clock tick */
 131         ticks = jiffies;
 132         while (ticks == jiffies)
 133                 ; /* nothing */
 134         /* Go .. */
 135         ticks = jiffies;
 136         do {
 137                 if (++trial_in_band == (1<<band)) {
 138                         ++band;
 139                         trial_in_band = 0;
 140                 }
 141                 __delay(lpj * band);
 142                 trials += band;
 143         } while (ticks == jiffies);
 144         /*
 145          * We overshot, so retreat to a clear underestimate. Then estimate
 146          * the largest likely undershoot. This defines our chop bounds.
 147          */
 148         trials -= band;
 149         loopadd_base = lpj * band;
 150         lpj_base = lpj * trials;
 151
 152 recalibrate:
 153         lpj = lpj_base;
 154         loopadd = loopadd_base;
 155
 156         /*
 157          * Do a binary approximation to get lpj set to
 158          * equal one clock (up to LPS_PREC bits)
 159          */
 160         chop_limit = lpj >> LPS_PREC;
 161         while (loopadd > chop_limit) {
 162                 lpj += loopadd;
 163                 ticks = jiffies;
 164                 while (ticks == jiffies)
 165                         ; /* nothing */
 166                 ticks = jiffies;
 167                 __delay(lpj);
 168                 if (jiffies != ticks)   /* longer than 1 tick */
 169                         lpj -= loopadd;
 170                 loopadd >>= 1;
 171         }
 172         /*
 173          * If we incremented every single time possible, presume we've
 174          * massively underestimated initially, and retry with a higher
 175          * start, and larger range. (Only seen on x86_64, due to SMIs)
 176          */
 177         if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
 178                 lpj_base = lpj;
 179                 loopadd_base <<= 2;
 180                 goto recalibrate;
 181         }
 182
 183         return lpj;
 184 }
 185
 186 void __cpuinit calibrate_delay(void)
 187 {
 188         unsigned long lpj;
 189         static bool printed;
 190
 191         if (preset_lpj) {
 192                 lpj = preset_lpj;
 193                 if (!printed)
 194                         pr_info("Calibrating delay loop (skipped) "
 195                                 "preset value.. ");
 196         } else if ((!printed) && lpj_fine) {
 197                 lpj = lpj_fine;
 198                 pr_info("Calibrating delay loop (skipped), "
 199                         "value calculated using timer frequency.. ");
 200         } else if ((lpj = calibrate_delay_direct()) != 0) {
 201                 if (!printed)
 202                         pr_info("Calibrating delay using timer "
 203                                 "specific routine.. ");
 204         } else {
 205                 if (!printed)
 206                         pr_info("Calibrating delay loop... ");
 207                 lpj = calibrate_delay_converge();
 208         }
 209         if (!printed)
 210                 pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
 211                         lpj/(500000/HZ),
 212                         (lpj/(5000/HZ)) % 100, lpj);
 213
 214         loops_per_jiffy = lpj;
 215         printed = true;
 216 }