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         unsigned long measured_times[MAX_DIRECT_CALIBRATION_RETRIES];
  42         int max = -1; /* index of measured_times with max/min values or not set */
  43         int min = -1;
  44         int i;
  45
  46         if (read_current_timer(&pre_start) < 0 )
  47                 return 0;
  48
  49         /*
  50          * A simple loop like
  51          *      while ( jiffies < start_jiffies+1)
  52          *              start = read_current_timer();
  53          * will not do. As we don't really know whether jiffy switch
  54          * happened first or timer_value was read first. And some asynchronous
  55          * event can happen between these two events introducing errors in lpj.
  56          *
  57          * So, we do
  58          * 1. pre_start <- When we are sure that jiffy switch hasn't happened
  59          * 2. check jiffy switch
  60          * 3. start <- timer value before or after jiffy switch
  61          * 4. post_start <- When we are sure that jiffy switch has happened
  62          *
  63          * Note, we don't know anything about order of 2 and 3.
  64          * Now, by looking at post_start and pre_start difference, we can
  65          * check whether any asynchronous event happened or not
  66          */
  67
  68         for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
  69                 pre_start = 0;
  70                 read_current_timer(&start);
  71                 start_jiffies = jiffies;
  72                 while (time_before_eq(jiffies, start_jiffies + 1)) {
  73                         pre_start = start;
  74                         read_current_timer(&start);
  75                 }
  76                 read_current_timer(&post_start);
  77
  78                 pre_end = 0;
  79                 end = post_start;
  80                 while (time_before_eq(jiffies, start_jiffies + 1 +
  81                                                DELAY_CALIBRATION_TICKS)) {
  82                         pre_end = end;
  83                         read_current_timer(&end);
  84                 }
  85                 read_current_timer(&post_end);
  86
  87                 timer_rate_max = (post_end - pre_start) /
  88                                         DELAY_CALIBRATION_TICKS;
  89                 timer_rate_min = (pre_end - post_start) /
  90                                         DELAY_CALIBRATION_TICKS;
  91
  92                 /*
  93                  * If the upper limit and lower limit of the timer_rate is
  94                  * >= 12.5% apart, redo calibration.
  95                  */
  96                 if (start >= post_end)
  97                         printk(KERN_NOTICE "calibrate_delay_direct() ignoring "
  98                                         "timer_rate as we had a TSC wrap around"
  99                                         " start=%lu >=post_end=%lu\n",
 100                                 start, post_end);
 101                 if (start < post_end && pre_start != 0 && pre_end != 0 &&
 102                     (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
 103                         good_timer_count++;
 104                         good_timer_sum += timer_rate_max;
 105                         measured_times[i] = timer_rate_max;
 106                         if (max < 0 || timer_rate_max > measured_times[max])
 107                                 max = i;
 108                         if (min < 0 || timer_rate_max < measured_times[min])
 109                                 min = i;
 110                 } else
 111                         measured_times[i] = 0;
 112
 113         }
 114
 115         /*
 116          * Find the maximum & minimum - if they differ too much throw out the
 117          * one with the largest difference from the mean and try again...
 118          */
 119         while (good_timer_count > 1) {
 120                 unsigned long estimate;
 121                 unsigned long maxdiff;
 122
 123                 /* compute the estimate */
 124                 estimate = (good_timer_sum/good_timer_count);
 125                 maxdiff = estimate >> 3;
 126
 127                 /* if range is within 12% let's take it */
 128                 if ((measured_times[max] - measured_times[min]) < maxdiff)
 129                         return estimate;
 130
 131                 /* ok - drop the worse value and try again... */
 132                 good_timer_sum = 0;
 133                 good_timer_count = 0;
 134                 if ((measured_times[max] - estimate) <
 135                                 (estimate - measured_times[min])) {
 136                         printk(KERN_NOTICE "calibrate_delay_direct() dropping "
 137                                         "min bogoMips estimate %d = %lu\n",
 138                                 min, measured_times[min]);
 139                         measured_times[min] = 0;
 140                         min = max;
 141                 } else {
 142                         printk(KERN_NOTICE "calibrate_delay_direct() dropping "
 143                                         "max bogoMips estimate %d = %lu\n",
 144                                 max, measured_times[max]);
 145                         measured_times[max] = 0;
 146                         max = min;
 147                 }
 148
 149                 for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
 150                         if (measured_times[i] == 0)
 151                                 continue;
 152                         good_timer_count++;
 153                         good_timer_sum += measured_times[i];
 154                         if (measured_times[i] < measured_times[min])
 155                                 min = i;
 156                         if (measured_times[i] > measured_times[max])
 157                                 max = i;
 158                 }
 159
 160         }
 161
 162         printk(KERN_NOTICE "calibrate_delay_direct() failed to get a good "
 163                "estimate for loops_per_jiffy.\nProbably due to long platform "
 164                 "interrupts. Consider using \"lpj=\" boot option.\n");
 165         return 0;
 166 }
 167 #else
 168 static unsigned long __cpuinit calibrate_delay_direct(void) {return 0;}
 169 #endif
 170
 171 /*
 172  * This is the number of bits of precision for the loops_per_jiffy.  Each
 173  * time we refine our estimate after the first takes 1.5/HZ seconds, so try
 174  * to start with a good estimate.
 175  * For the boot cpu we can skip the delay calibration and assign it a value
 176  * calculated based on the timer frequency.
 177  * For the rest of the CPUs we cannot assume that the timer frequency is same as
 178  * the cpu frequency, hence do the calibration for those.
 179  */
 180 #define LPS_PREC 8
 181
 182 static unsigned long __cpuinit calibrate_delay_converge(void)
 183 {
 184         /* First stage - slowly accelerate to find initial bounds */
 185         unsigned long lpj, lpj_base, ticks, loopadd, loopadd_base, chop_limit;
 186         int trials = 0, band = 0, trial_in_band = 0;
 187
 188         lpj = (1<<12);
 189
 190         /* wait for "start of" clock tick */
 191         ticks = jiffies;
 192         while (ticks == jiffies)
 193                 ; /* nothing */
 194         /* Go .. */
 195         ticks = jiffies;
 196         do {
 197                 if (++trial_in_band == (1<<band)) {
 198                         ++band;
 199                         trial_in_band = 0;
 200                 }
 201                 __delay(lpj * band);
 202                 trials += band;
 203         } while (ticks == jiffies);
 204         /*
 205          * We overshot, so retreat to a clear underestimate. Then estimate
 206          * the largest likely undershoot. This defines our chop bounds.
 207          */
 208         trials -= band;
 209         loopadd_base = lpj * band;
 210         lpj_base = lpj * trials;
 211
 212 recalibrate:
 213         lpj = lpj_base;
 214         loopadd = loopadd_base;
 215
 216         /*
 217          * Do a binary approximation to get lpj set to
 218          * equal one clock (up to LPS_PREC bits)
 219          */
 220         chop_limit = lpj >> LPS_PREC;
 221         while (loopadd > chop_limit) {
 222                 lpj += loopadd;
 223                 ticks = jiffies;
 224                 while (ticks == jiffies)
 225                         ; /* nothing */
 226                 ticks = jiffies;
 227                 __delay(lpj);
 228                 if (jiffies != ticks)   /* longer than 1 tick */
 229                         lpj -= loopadd;
 230                 loopadd >>= 1;
 231         }
 232         /*
 233          * If we incremented every single time possible, presume we've
 234          * massively underestimated initially, and retry with a higher
 235          * start, and larger range. (Only seen on x86_64, due to SMIs)
 236          */
 237         if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
 238                 lpj_base = lpj;
 239                 loopadd_base <<= 2;
 240                 goto recalibrate;
 241         }
 242
 243         return lpj;
 244 }
 245
 246 void __cpuinit calibrate_delay(void)
 247 {
 248         unsigned long lpj;
 249         static bool printed;
 250
 251         if (preset_lpj) {
 252                 lpj = preset_lpj;
 253                 if (!printed)
 254                         pr_info("Calibrating delay loop (skipped) "
 255                                 "preset value.. ");
 256         } else if ((!printed) && lpj_fine) {
 257                 lpj = lpj_fine;
 258                 pr_info("Calibrating delay loop (skipped), "
 259                         "value calculated using timer frequency.. ");
 260         } else if ((lpj = calibrate_delay_direct()) != 0) {
 261                 if (!printed)
 262                         pr_info("Calibrating delay using timer "
 263                                 "specific routine.. ");
 264         } else {
 265                 if (!printed)
 266                         pr_info("Calibrating delay loop... ");
 267                 lpj = calibrate_delay_converge();
 268         }
 269         if (!printed)
 270                 pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
 271                         lpj/(500000/HZ),
 272                         (lpj/(5000/HZ)) % 100, lpj);
 273
 274         loops_per_jiffy = lpj;
 275         printed = true;
 276 }