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 * 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