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 (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 (jiffies <=
 78                        (start_jiffies + 1 + 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  * bit takes on average 1.5/HZ seconds.  This (like the original) is a little
114  * better than 1%
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 void __cpuinit calibrate_delay(void)
123 {
124         unsigned long ticks, loopbit;
125         int lps_precision = LPS_PREC;
126         static bool printed;
127 
128         if (preset_lpj) {
129                 loops_per_jiffy = preset_lpj;
130                 if (!printed)
131                         pr_info("Calibrating delay loop (skipped) "
132                                 "preset value.. ");
133         } else if ((!printed) && lpj_fine) {
134                 loops_per_jiffy = lpj_fine;
135                 pr_info("Calibrating delay loop (skipped), "
136                         "value calculated using timer frequency.. ");
137         } else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) {
138                 if (!printed)
139                         pr_info("Calibrating delay using timer "
140                                 "specific routine.. ");
141         } else {
142                 loops_per_jiffy = (1<<12);
143 
144                 if (!printed)
145                         pr_info("Calibrating delay loop... ");
146                 while ((loops_per_jiffy <<= 1) != 0) {
147                         /* wait for "start of" clock tick */
148                         ticks = jiffies;
149                         while (ticks == jiffies)
150                                 /* nothing */;
151                         /* Go .. */
152                         ticks = jiffies;
153                         __delay(loops_per_jiffy);
154                         ticks = jiffies - ticks;
155                         if (ticks)
156                                 break;
157                 }
158 
159                 /*
160                  * Do a binary approximation to get loops_per_jiffy set to
161                  * equal one clock (up to lps_precision bits)
162                  */
163                 loops_per_jiffy >>= 1;
164                 loopbit = loops_per_jiffy;
165                 while (lps_precision-- && (loopbit >>= 1)) {
166                         loops_per_jiffy |= loopbit;
167                         ticks = jiffies;
168                         while (ticks == jiffies)
169                                 /* nothing */;
170                         ticks = jiffies;
171                         __delay(loops_per_jiffy);
172                         if (jiffies != ticks)   /* longer than 1 tick */
173                                 loops_per_jiffy &= ~loopbit;
174                 }
175         }
176         if (!printed)
177                 pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
178                         loops_per_jiffy/(500000/HZ),
179                         (loops_per_jiffy/(5000/HZ)) % 100, loops_per_jiffy);
180 
181         printed = true;
182 }
183
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Linux/init/calibrate.c
