1 /* calibrate.c: default delay calibration
3 * Excised from init/main.c
4 * Copyright (C) 1991, 1992 Linus Torvalds
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>
13 unsigned long lpj_fine;
14 unsigned long preset_lpj;
15 static int __init lpj_setup(char *str)
17 preset_lpj = simple_strtoul(str,NULL,0);
21 __setup("lpj=", lpj_setup);
23 #ifdef ARCH_HAS_READ_CURRENT_TIMER
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
30 #define DELAY_CALIBRATION_TICKS ((HZ < 100) ? 1 : (HZ/100))
31 #define MAX_DIRECT_CALIBRATION_RETRIES 5
33 static unsigned long __cpuinit calibrate_delay_direct(void)
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;
43 if (read_current_timer(&pre_start) < 0 )
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.
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
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
65 for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
67 read_current_timer(&start);
68 start_jiffies = jiffies;
69 while (time_before_eq(jiffies, start_jiffies + 1)) {
71 read_current_timer(&start);
73 read_current_timer(&post_start);
77 while (time_before_eq(jiffies, start_jiffies + 1 +
78 DELAY_CALIBRATION_TICKS)) {
80 read_current_timer(&end);
82 read_current_timer(&post_end);
84 timer_rate_max = (post_end - pre_start) /
85 DELAY_CALIBRATION_TICKS;
86 timer_rate_min = (pre_end - post_start) /
87 DELAY_CALIBRATION_TICKS;
90 * If the upper limit and lower limit of the timer_rate is
91 * >= 12.5% apart, redo calibration.
93 if (pre_start != 0 && pre_end != 0 &&
94 (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
96 good_timer_sum += timer_rate_max;
100 if (good_timer_count)
101 return (good_timer_sum/good_timer_count);
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");
108 static unsigned long __cpuinit calibrate_delay_direct(void) {return 0;}
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
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.
122 static unsigned long __cpuinit calibrate_delay_converge(void)
124 unsigned long lpj, ticks, loopbit;
125 int lps_precision = LPS_PREC;
128 while ((lpj <<= 1) != 0) {
129 /* wait for "start of" clock tick */
131 while (ticks == jiffies)
136 ticks = jiffies - ticks;
142 * Do a binary approximation to get lpj set to
143 * equal one clock (up to lps_precision bits)
147 while (lps_precision-- && (loopbit >>= 1)) {
150 while (ticks == jiffies)
154 if (jiffies != ticks) /* longer than 1 tick */
161 void __cpuinit calibrate_delay(void)
166 loops_per_jiffy = preset_lpj;
168 pr_info("Calibrating delay loop (skipped) "
170 } else if ((!printed) && lpj_fine) {
171 loops_per_jiffy = lpj_fine;
172 pr_info("Calibrating delay loop (skipped), "
173 "value calculated using timer frequency.. ");
174 } else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) {
176 pr_info("Calibrating delay using timer "
177 "specific routine.. ");
180 pr_info("Calibrating delay loop... ");
181 loops_per_jiffy = calibrate_delay_converge();
184 pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
185 loops_per_jiffy/(500000/HZ),
186 (loops_per_jiffy/(5000/HZ)) % 100, loops_per_jiffy);