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_tsc;
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 tsc_rate_min, tsc_rate_max;
39 unsigned long good_tsc_sum = 0;
40 unsigned long good_tsc_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 (jiffies <= (start_jiffies + 1)) {
71 read_current_timer(&start);
73 read_current_timer(&post_start);
78 (start_jiffies + 1 + DELAY_CALIBRATION_TICKS)) {
80 read_current_timer(&end);
82 read_current_timer(&post_end);
84 tsc_rate_max = (post_end - pre_start) / DELAY_CALIBRATION_TICKS;
85 tsc_rate_min = (pre_end - post_start) / DELAY_CALIBRATION_TICKS;
88 * If the upper limit and lower limit of the tsc_rate is
89 * >= 12.5% apart, redo calibration.
91 if (pre_start != 0 && pre_end != 0 &&
92 (tsc_rate_max - tsc_rate_min) < (tsc_rate_max >> 3)) {
94 good_tsc_sum += tsc_rate_max;
99 return (good_tsc_sum/good_tsc_count);
101 printk(KERN_WARNING "calibrate_delay_direct() failed to get a good "
102 "estimate for loops_per_jiffy.\nProbably due to long platform interrupts. Consider using \"lpj=\" boot option.\n");
106 static unsigned long __cpuinit calibrate_delay_direct(void) {return 0;}
110 * This is the number of bits of precision for the loops_per_jiffy. Each
111 * bit takes on average 1.5/HZ seconds. This (like the original) is a little
113 * For the boot cpu we can skip the delay calibration and assign it a value
114 * calculated based on the tsc frequency.
115 * For the rest of the CPUs we cannot assume that the tsc frequency is same as
116 * the cpu frequency, hence do the calibration for those.
120 void __cpuinit calibrate_delay(void)
122 unsigned long ticks, loopbit;
123 int lps_precision = LPS_PREC;
126 loops_per_jiffy = preset_lpj;
128 "Calibrating delay loop (skipped) preset value.. ");
129 } else if ((smp_processor_id() == 0) && lpj_tsc) {
130 loops_per_jiffy = lpj_tsc;
132 "Calibrating delay loop (skipped), "
133 "using tsc calculated value.. ");
134 } else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) {
136 "Calibrating delay using timer specific routine.. ");
138 loops_per_jiffy = (1<<12);
140 printk(KERN_INFO "Calibrating delay loop... ");
141 while ((loops_per_jiffy <<= 1) != 0) {
142 /* wait for "start of" clock tick */
144 while (ticks == jiffies)
148 __delay(loops_per_jiffy);
149 ticks = jiffies - ticks;
155 * Do a binary approximation to get loops_per_jiffy set to
156 * equal one clock (up to lps_precision bits)
158 loops_per_jiffy >>= 1;
159 loopbit = loops_per_jiffy;
160 while (lps_precision-- && (loopbit >>= 1)) {
161 loops_per_jiffy |= loopbit;
163 while (ticks == jiffies)
166 __delay(loops_per_jiffy);
167 if (jiffies != ticks) /* longer than 1 tick */
168 loops_per_jiffy &= ~loopbit;
171 printk(KERN_INFO "%lu.%02lu BogoMIPS (lpj=%lu)\n",
172 loops_per_jiffy/(500000/HZ),
173 (loops_per_jiffy/(5000/HZ)) % 100, loops_per_jiffy);