projects/linux·blob v2.6.13 · refs

173 lines · 4.7 KiB

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/sched.h>
8#include <linux/delay.h>
9#include <linux/init.h>
10
11#include <asm/timex.h>
12
13static unsigned long preset_lpj;
14static int __init lpj_setup(char *str)
15{
16 preset_lpj = simple_strtoul(str,NULL,0);
17 return 1;
18}
19
20__setup("lpj=", lpj_setup);
21
22#ifdef ARCH_HAS_READ_CURRENT_TIMER
23
24/* This routine uses the read_current_timer() routine and gets the
25 * loops per jiffy directly, instead of guessing it using delay().
26 * Also, this code tries to handle non-maskable asynchronous events
27 * (like SMIs)
28 */
29#define DELAY_CALIBRATION_TICKS ((HZ < 100) ? 1 : (HZ/100))
30#define MAX_DIRECT_CALIBRATION_RETRIES 5
31
32static unsigned long __devinit calibrate_delay_direct(void)
33{
34 unsigned long pre_start, start, post_start;
35 unsigned long pre_end, end, post_end;
36 unsigned long start_jiffies;
37 unsigned long tsc_rate_min, tsc_rate_max;
38 unsigned long good_tsc_sum = 0;
39 unsigned long good_tsc_count = 0;
40 int i;
41
42 if (read_current_timer(&pre_start) < 0 )
43 return 0;
44
45 /*
46 * A simple loop like
47 * while ( jiffies < start_jiffies+1)
48 * start = read_current_timer();
49 * will not do. As we don't really know whether jiffy switch
50 * happened first or timer_value was read first. And some asynchronous
51 * event can happen between these two events introducing errors in lpj.
52 *
53 * So, we do
54 * 1. pre_start <- When we are sure that jiffy switch hasn't happened
55 * 2. check jiffy switch
56 * 3. start <- timer value before or after jiffy switch
57 * 4. post_start <- When we are sure that jiffy switch has happened
58 *
59 * Note, we don't know anything about order of 2 and 3.
60 * Now, by looking at post_start and pre_start difference, we can
61 * check whether any asynchronous event happened or not
62 */
63
64 for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
65 pre_start = 0;
66 read_current_timer(&start);
67 start_jiffies = jiffies;
68 while (jiffies <= (start_jiffies + 1)) {
69 pre_start = start;
70 read_current_timer(&start);
71 }
72 read_current_timer(&post_start);
73
74 pre_end = 0;
75 end = post_start;
76 while (jiffies <=
77 (start_jiffies + 1 + DELAY_CALIBRATION_TICKS)) {
78 pre_end = end;
79 read_current_timer(&end);
80 }
81 read_current_timer(&post_end);
82
83 tsc_rate_max = (post_end - pre_start) / DELAY_CALIBRATION_TICKS;
84 tsc_rate_min = (pre_end - post_start) / DELAY_CALIBRATION_TICKS;
85
86 /*
87 * If the upper limit and lower limit of the tsc_rate is
88 * >= 12.5% apart, redo calibration.
89 */
90 if (pre_start != 0 && pre_end != 0 &&
91 (tsc_rate_max - tsc_rate_min) < (tsc_rate_max >> 3)) {
92 good_tsc_count++;
93 good_tsc_sum += tsc_rate_max;
94 }
95 }
96
97 if (good_tsc_count)
98 return (good_tsc_sum/good_tsc_count);
99
100 printk(KERN_WARNING "calibrate_delay_direct() failed to get a good "
101 "estimate for loops_per_jiffy.\nProbably due to long platform interrupts. Consider using \"lpj=\" boot option.\n");
102 return 0;
103}
104#else
105static unsigned long __devinit calibrate_delay_direct(void) {return 0;}
106#endif
107
108/*
109 * This is the number of bits of precision for the loops_per_jiffy. Each
110 * bit takes on average 1.5/HZ seconds. This (like the original) is a little
111 * better than 1%
112 */
113#define LPS_PREC 8
114
115void __devinit calibrate_delay(void)
116{
117 unsigned long ticks, loopbit;
118 int lps_precision = LPS_PREC;
119
120 if (preset_lpj) {
121 loops_per_jiffy = preset_lpj;
122 printk("Calibrating delay loop (skipped)... "
123 "%lu.%02lu BogoMIPS preset\n",
124 loops_per_jiffy/(500000/HZ),
125 (loops_per_jiffy/(5000/HZ)) % 100);
126 } else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) {
127 printk("Calibrating delay using timer specific routine.. ");
128 printk("%lu.%02lu BogoMIPS (lpj=%lu)\n",
129 loops_per_jiffy/(500000/HZ),
130 (loops_per_jiffy/(5000/HZ)) % 100,
131 loops_per_jiffy);
132 } else {
133 loops_per_jiffy = (1<<12);
134
135 printk(KERN_DEBUG "Calibrating delay loop... ");
136 while ((loops_per_jiffy <<= 1) != 0) {
137 /* wait for "start of" clock tick */
138 ticks = jiffies;
139 while (ticks == jiffies)
140 /* nothing */;
141 /* Go .. */
142 ticks = jiffies;
143 __delay(loops_per_jiffy);
144 ticks = jiffies - ticks;
145 if (ticks)
146 break;
147 }
148
149 /*
150 * Do a binary approximation to get loops_per_jiffy set to
151 * equal one clock (up to lps_precision bits)
152 */
153 loops_per_jiffy >>= 1;
154 loopbit = loops_per_jiffy;
155 while (lps_precision-- && (loopbit >>= 1)) {
156 loops_per_jiffy |= loopbit;
157 ticks = jiffies;
158 while (ticks == jiffies)
159 /* nothing */;
160 ticks = jiffies;
161 __delay(loops_per_jiffy);
162 if (jiffies != ticks) /* longer than 1 tick */
163 loops_per_jiffy &= ~loopbit;
164 }
165
166 /* Round the value and print it */
167 printk("%lu.%02lu BogoMIPS (lpj=%lu)\n",
168 loops_per_jiffy/(500000/HZ),
169 (loops_per_jiffy/(5000/HZ)) % 100,
170 loops_per_jiffy);
171 }
172
173}