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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 #include <linux/percpu.h>
13 
14 unsigned long lpj_fine;
15 unsigned long preset_lpj;
lpj_setup(char * str)16 static int __init lpj_setup(char *str)
17 {
18 	preset_lpj = simple_strtoul(str,NULL,0);
19 	return 1;
20 }
21 
22 __setup("lpj=", lpj_setup);
23 
24 #ifdef ARCH_HAS_READ_CURRENT_TIMER
25 
26 /* This routine uses the read_current_timer() routine and gets the
27  * loops per jiffy directly, instead of guessing it using delay().
28  * Also, this code tries to handle non-maskable asynchronous events
29  * (like SMIs)
30  */
31 #define DELAY_CALIBRATION_TICKS			((HZ < 100) ? 1 : (HZ/100))
32 #define MAX_DIRECT_CALIBRATION_RETRIES		5
33 
calibrate_delay_direct(void)34 static unsigned long __cpuinit calibrate_delay_direct(void)
35 {
36 	unsigned long pre_start, start, post_start;
37 	unsigned long pre_end, end, post_end;
38 	unsigned long start_jiffies;
39 	unsigned long timer_rate_min, timer_rate_max;
40 	unsigned long good_timer_sum = 0;
41 	unsigned long good_timer_count = 0;
42 	unsigned long measured_times[MAX_DIRECT_CALIBRATION_RETRIES];
43 	int max = -1; /* index of measured_times with max/min values or not set */
44 	int min = -1;
45 	int i;
46 
47 	if (read_current_timer(&pre_start) < 0 )
48 		return 0;
49 
50 	/*
51 	 * A simple loop like
52 	 *	while ( jiffies < start_jiffies+1)
53 	 *		start = read_current_timer();
54 	 * will not do. As we don't really know whether jiffy switch
55 	 * happened first or timer_value was read first. And some asynchronous
56 	 * event can happen between these two events introducing errors in lpj.
57 	 *
58 	 * So, we do
59 	 * 1. pre_start <- When we are sure that jiffy switch hasn't happened
60 	 * 2. check jiffy switch
61 	 * 3. start <- timer value before or after jiffy switch
62 	 * 4. post_start <- When we are sure that jiffy switch has happened
63 	 *
64 	 * Note, we don't know anything about order of 2 and 3.
65 	 * Now, by looking at post_start and pre_start difference, we can
66 	 * check whether any asynchronous event happened or not
67 	 */
68 
69 	for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
70 		pre_start = 0;
71 		read_current_timer(&start);
72 		start_jiffies = jiffies;
73 		while (time_before_eq(jiffies, start_jiffies + 1)) {
74 			pre_start = start;
75 			read_current_timer(&start);
76 		}
77 		read_current_timer(&post_start);
78 
79 		pre_end = 0;
80 		end = post_start;
81 		while (time_before_eq(jiffies, start_jiffies + 1 +
82 					       DELAY_CALIBRATION_TICKS)) {
83 			pre_end = end;
84 			read_current_timer(&end);
85 		}
86 		read_current_timer(&post_end);
87 
88 		timer_rate_max = (post_end - pre_start) /
89 					DELAY_CALIBRATION_TICKS;
90 		timer_rate_min = (pre_end - post_start) /
91 					DELAY_CALIBRATION_TICKS;
92 
93 		/*
94 		 * If the upper limit and lower limit of the timer_rate is
95 		 * >= 12.5% apart, redo calibration.
96 		 */
97 		if (start >= post_end)
98 			printk(KERN_NOTICE "calibrate_delay_direct() ignoring "
99 					"timer_rate as we had a TSC wrap around"
100 					" start=%lu >=post_end=%lu\n",
101 				start, post_end);
102 		if (start < post_end && pre_start != 0 && pre_end != 0 &&
103 		    (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
104 			good_timer_count++;
105 			good_timer_sum += timer_rate_max;
106 			measured_times[i] = timer_rate_max;
107 			if (max < 0 || timer_rate_max > measured_times[max])
108 				max = i;
109 			if (min < 0 || timer_rate_max < measured_times[min])
110 				min = i;
111 		} else
112 			measured_times[i] = 0;
113 
114 	}
115 
116 	/*
117 	 * Find the maximum & minimum - if they differ too much throw out the
118 	 * one with the largest difference from the mean and try again...
119 	 */
120 	while (good_timer_count > 1) {
121 		unsigned long estimate;
122 		unsigned long maxdiff;
123 
124 		/* compute the estimate */
125 		estimate = (good_timer_sum/good_timer_count);
126 		maxdiff = estimate >> 3;
127 
128 		/* if range is within 12% let's take it */
129 		if ((measured_times[max] - measured_times[min]) < maxdiff)
130 			return estimate;
131 
132 		/* ok - drop the worse value and try again... */
133 		good_timer_sum = 0;
134 		good_timer_count = 0;
135 		if ((measured_times[max] - estimate) <
136 				(estimate - measured_times[min])) {
137 			printk(KERN_NOTICE "calibrate_delay_direct() dropping "
138 					"min bogoMips estimate %d = %lu\n",
139 				min, measured_times[min]);
140 			measured_times[min] = 0;
141 			min = max;
142 		} else {
143 			printk(KERN_NOTICE "calibrate_delay_direct() dropping "
144 					"max bogoMips estimate %d = %lu\n",
145 				max, measured_times[max]);
146 			measured_times[max] = 0;
147 			max = min;
148 		}
149 
150 		for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
151 			if (measured_times[i] == 0)
152 				continue;
153 			good_timer_count++;
154 			good_timer_sum += measured_times[i];
155 			if (measured_times[i] < measured_times[min])
156 				min = i;
157 			if (measured_times[i] > measured_times[max])
158 				max = i;
159 		}
160 
161 	}
162 
163 	printk(KERN_NOTICE "calibrate_delay_direct() failed to get a good "
164 	       "estimate for loops_per_jiffy.\nProbably due to long platform "
165 		"interrupts. Consider using \"lpj=\" boot option.\n");
166 	return 0;
167 }
168 #else
calibrate_delay_direct(void)169 static unsigned long __cpuinit calibrate_delay_direct(void) {return 0;}
170 #endif
171 
172 /*
173  * This is the number of bits of precision for the loops_per_jiffy.  Each
174  * time we refine our estimate after the first takes 1.5/HZ seconds, so try
175  * to start with a good estimate.
176  * For the boot cpu we can skip the delay calibration and assign it a value
177  * calculated based on the timer frequency.
178  * For the rest of the CPUs we cannot assume that the timer frequency is same as
179  * the cpu frequency, hence do the calibration for those.
180  */
181 #define LPS_PREC 8
182 
calibrate_delay_converge(void)183 static unsigned long __cpuinit calibrate_delay_converge(void)
184 {
185 	/* First stage - slowly accelerate to find initial bounds */
186 	unsigned long lpj, lpj_base, ticks, loopadd, loopadd_base, chop_limit;
187 	int trials = 0, band = 0, trial_in_band = 0;
188 
189 	lpj = (1<<12);
190 
191 	/* wait for "start of" clock tick */
192 	ticks = jiffies;
193 	while (ticks == jiffies)
194 		; /* nothing */
195 	/* Go .. */
196 	ticks = jiffies;
197 	do {
198 		if (++trial_in_band == (1<<band)) {
199 			++band;
200 			trial_in_band = 0;
201 		}
202 		__delay(lpj * band);
203 		trials += band;
204 	} while (ticks == jiffies);
205 	/*
206 	 * We overshot, so retreat to a clear underestimate. Then estimate
207 	 * the largest likely undershoot. This defines our chop bounds.
208 	 */
209 	trials -= band;
210 	loopadd_base = lpj * band;
211 	lpj_base = lpj * trials;
212 
213 recalibrate:
214 	lpj = lpj_base;
215 	loopadd = loopadd_base;
216 
217 	/*
218 	 * Do a binary approximation to get lpj set to
219 	 * equal one clock (up to LPS_PREC bits)
220 	 */
221 	chop_limit = lpj >> LPS_PREC;
222 	while (loopadd > chop_limit) {
223 		lpj += loopadd;
224 		ticks = jiffies;
225 		while (ticks == jiffies)
226 			; /* nothing */
227 		ticks = jiffies;
228 		__delay(lpj);
229 		if (jiffies != ticks)	/* longer than 1 tick */
230 			lpj -= loopadd;
231 		loopadd >>= 1;
232 	}
233 	/*
234 	 * If we incremented every single time possible, presume we've
235 	 * massively underestimated initially, and retry with a higher
236 	 * start, and larger range. (Only seen on x86_64, due to SMIs)
237 	 */
238 	if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
239 		lpj_base = lpj;
240 		loopadd_base <<= 2;
241 		goto recalibrate;
242 	}
243 
244 	return lpj;
245 }
246 
247 static DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };
248 
249 /*
250  * Check if cpu calibration delay is already known. For example,
251  * some processors with multi-core sockets may have all cores
252  * with the same calibration delay.
253  *
254  * Architectures should override this function if a faster calibration
255  * method is available.
256  */
calibrate_delay_is_known(void)257 unsigned long __attribute__((weak)) __cpuinit calibrate_delay_is_known(void)
258 {
259 	return 0;
260 }
261 
calibrate_delay(void)262 void __cpuinit calibrate_delay(void)
263 {
264 	unsigned long lpj;
265 	static bool printed;
266 	int this_cpu = smp_processor_id();
267 
268 	if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
269 		lpj = per_cpu(cpu_loops_per_jiffy, this_cpu);
270 		if (!printed)
271 			pr_info("Calibrating delay loop (skipped) "
272 				"already calibrated this CPU");
273 	} else if (preset_lpj) {
274 		lpj = preset_lpj;
275 		if (!printed)
276 			pr_info("Calibrating delay loop (skipped) "
277 				"preset value.. ");
278 	} else if ((!printed) && lpj_fine) {
279 		lpj = lpj_fine;
280 		pr_info("Calibrating delay loop (skipped), "
281 			"value calculated using timer frequency.. ");
282 	} else if ((lpj = calibrate_delay_is_known())) {
283 		;
284 	} else if ((lpj = calibrate_delay_direct()) != 0) {
285 		if (!printed)
286 			pr_info("Calibrating delay using timer "
287 				"specific routine.. ");
288 	} else {
289 		if (!printed)
290 			pr_info("Calibrating delay loop... ");
291 		lpj = calibrate_delay_converge();
292 	}
293 	per_cpu(cpu_loops_per_jiffy, this_cpu) = lpj;
294 	if (!printed)
295 		pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
296 			lpj/(500000/HZ),
297 			(lpj/(5000/HZ)) % 100, lpj);
298 
299 	loops_per_jiffy = lpj;
300 	printed = true;
301 }
302