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 (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
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