xref: /openbmc/linux/init/calibrate.c (revision 4800cd83)
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 
13 unsigned long lpj_fine;
14 unsigned long preset_lpj;
15 static 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 
33 static 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
108 static 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 
122 void __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