Root/init/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
13unsigned long lpj_fine;
14unsigned long preset_lpj;
15static 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
33static 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
108static 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
122void __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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