1 // SPDX-License-Identifier: GPL-2.0
2 /* calibrate.c: default delay calibration
3 *
4 * Excised from init/main.c
5 * Copyright (C) 1991, 1992 Linus Torvalds
6 */
7
8 #include <linux/jiffies.h>
9 #include <linux/delay.h>
10 #include <linux/init.h>
11 #include <linux/timex.h>
12 #include <linux/smp.h>
13 #include <linux/percpu.h>
14
15 unsigned long lpj_fine;
16 unsigned long preset_lpj;
17 static int __init lpj_setup(char *str)
18 {
19 preset_lpj = simple_strtoul(str,NULL,0);
20 return 1;
21 }
22
23 __setup("lpj=", lpj_setup);
24
25 #ifdef ARCH_HAS_READ_CURRENT_TIMER
26
27 /* This routine uses the read_current_timer() routine and gets the
28 * loops per jiffy directly, instead of guessing it using delay().
29 * Also, this code tries to handle non-maskable asynchronous events
30 * (like SMIs)
31 */
32 #define DELAY_CALIBRATION_TICKS ((HZ < 100) ? 1 : (HZ/100))
33 #define MAX_DIRECT_CALIBRATION_RETRIES 5
34
35 static unsigned long calibrate_delay_direct(void)
36 {
37 unsigned long pre_start, start, post_start;
38 unsigned long pre_end, end, post_end;
39 unsigned long start_jiffies;
40 unsigned long timer_rate_min, timer_rate_max;
41 unsigned long good_timer_sum = 0;
42 unsigned long good_timer_count = 0;
43 unsigned long measured_times[MAX_DIRECT_CALIBRATION_RETRIES];
44 int max = -1; /* index of measured_times with max/min values or not set */
45 int min = -1;
46 int i;
47
48 if (read_current_timer(&pre_start) < 0 )
49 return 0;
50
51 /*
52 * A simple loop like
53 * while ( jiffies < start_jiffies+1)
54 * start = read_current_timer();
55 * will not do. As we don't really know whether jiffy switch
56 * happened first or timer_value was read first. And some asynchronous
57 * event can happen between these two events introducing errors in lpj.
58 *
59 * So, we do
60 * 1. pre_start <- When we are sure that jiffy switch hasn't happened
61 * 2. check jiffy switch
62 * 3. start <- timer value before or after jiffy switch
63 * 4. post_start <- When we are sure that jiffy switch has happened
64 *
65 * Note, we don't know anything about order of 2 and 3.
66 * Now, by looking at post_start and pre_start difference, we can
67 * check whether any asynchronous event happened or not
68 */
69
70 for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
71 pre_start = 0;
72 read_current_timer(&start);
73 start_jiffies = jiffies;
74 while (time_before_eq(jiffies, start_jiffies + 1)) {
75 pre_start = start;
76 read_current_timer(&start);
77 }
78 read_current_timer(&post_start);
79
80 pre_end = 0;
81 end = post_start;
82 while (time_before_eq(jiffies, start_jiffies + 1 +
83 DELAY_CALIBRATION_TICKS)) {
84 pre_end = end;
85 read_current_timer(&end);
86 }
87 read_current_timer(&post_end);
88
89 timer_rate_max = (post_end - pre_start) /
90 DELAY_CALIBRATION_TICKS;
91 timer_rate_min = (pre_end - post_start) /
92 DELAY_CALIBRATION_TICKS;
93
94 /*
95 * If the upper limit and lower limit of the timer_rate is
96 * >= 12.5% apart, redo calibration.
97 */
98 if (start >= post_end)
99 printk(KERN_NOTICE "calibrate_delay_direct() ignoring "
100 "timer_rate as we had a TSC wrap around"
101 " start=%lu >=post_end=%lu\n",
102 start, post_end);
103 if (start < post_end && pre_start != 0 && pre_end != 0 &&
104 (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
105 good_timer_count++;
106 good_timer_sum += timer_rate_max;
107 measured_times[i] = timer_rate_max;
108 if (max < 0 || timer_rate_max > measured_times[max])
109 max = i;
110 if (min < 0 || timer_rate_max < measured_times[min])
111 min = i;
112 } else
113 measured_times[i] = 0;
114
115 }
116
117 /*
118 * Find the maximum & minimum - if they differ too much throw out the
119 * one with the largest difference from the mean and try again...
120 */
121 while (good_timer_count > 1) {
122 unsigned long estimate;
123 unsigned long maxdiff;
124
125 /* compute the estimate */
126 estimate = (good_timer_sum/good_timer_count);
127 maxdiff = estimate >> 3;
128
129 /* if range is within 12% let's take it */
130 if ((measured_times[max] - measured_times[min]) < maxdiff)
131 return estimate;
132
133 /* ok - drop the worse value and try again... */
134 good_timer_sum = 0;
135 good_timer_count = 0;
136 if ((measured_times[max] - estimate) <
137 (estimate - measured_times[min])) {
138 printk(KERN_NOTICE "calibrate_delay_direct() dropping "
139 "min bogoMips estimate %d = %lu\n",
140 min, measured_times[min]);
141 measured_times[min] = 0;
142 min = max;
143 } else {
144 printk(KERN_NOTICE "calibrate_delay_direct() dropping "
145 "max bogoMips estimate %d = %lu\n",
146 max, measured_times[max]);
147 measured_times[max] = 0;
148 max = min;
149 }
150
151 for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
152 if (measured_times[i] == 0)
153 continue;
154 good_timer_count++;
155 good_timer_sum += measured_times[i];
156 if (measured_times[i] < measured_times[min])
157 min = i;
158 if (measured_times[i] > measured_times[max])
159 max = i;
160 }
161
162 }
163
164 printk(KERN_NOTICE "calibrate_delay_direct() failed to get a good "
165 "estimate for loops_per_jiffy.\nProbably due to long platform "
166 "interrupts. Consider using \"lpj=\" boot option.\n");
167 return 0;
168 }
169 #else
170 static unsigned long calibrate_delay_direct(void)
171 {
172 return 0;
173 }
174 #endif
175
176 /*
177 * This is the number of bits of precision for the loops_per_jiffy. Each
178 * time we refine our estimate after the first takes 1.5/HZ seconds, so try
179 * to start with a good estimate.
180 * For the boot cpu we can skip the delay calibration and assign it a value
181 * calculated based on the timer frequency.
182 * For the rest of the CPUs we cannot assume that the timer frequency is same as
183 * the cpu frequency, hence do the calibration for those.
184 */
185 #define LPS_PREC 8
186
187 static unsigned long calibrate_delay_converge(void)
188 {
189 /* First stage - slowly accelerate to find initial bounds */
190 unsigned long lpj, lpj_base, ticks, loopadd, loopadd_base, chop_limit;
191 int trials = 0, band = 0, trial_in_band = 0;
192
193 lpj = (1<<12);
194
195 /* wait for "start of" clock tick */
196 ticks = jiffies;
197 while (ticks == jiffies)
198 ; /* nothing */
199 /* Go .. */
200 ticks = jiffies;
201 do {
202 if (++trial_in_band == (1<<band)) {
203 ++band;
204 trial_in_band = 0;
205 }
206 __delay(lpj * band);
207 trials += band;
208 } while (ticks == jiffies);
209 /*
210 * We overshot, so retreat to a clear underestimate. Then estimate
211 * the largest likely undershoot. This defines our chop bounds.
212 */
213 trials -= band;
214 loopadd_base = lpj * band;
215 lpj_base = lpj * trials;
216
217 recalibrate:
218 lpj = lpj_base;
219 loopadd = loopadd_base;
220
221 /*
222 * Do a binary approximation to get lpj set to
223 * equal one clock (up to LPS_PREC bits)
224 */
225 chop_limit = lpj >> LPS_PREC;
226 while (loopadd > chop_limit) {
227 lpj += loopadd;
228 ticks = jiffies;
229 while (ticks == jiffies)
230 ; /* nothing */
231 ticks = jiffies;
232 __delay(lpj);
233 if (jiffies != ticks) /* longer than 1 tick */
234 lpj -= loopadd;
235 loopadd >>= 1;
236 }
237 /*
238 * If we incremented every single time possible, presume we've
239 * massively underestimated initially, and retry with a higher
240 * start, and larger range. (Only seen on x86_64, due to SMIs)
241 */
242 if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
243 lpj_base = lpj;
244 loopadd_base <<= 2;
245 goto recalibrate;
246 }
247
248 return lpj;
249 }
250
251 static DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };
252
253 /*
254 * Check if cpu calibration delay is already known. For example,
255 * some processors with multi-core sockets may have all cores
256 * with the same calibration delay.
257 *
258 * Architectures should override this function if a faster calibration
259 * method is available.
260 */
261 unsigned long __attribute__((weak)) calibrate_delay_is_known(void)
262 {
263 return 0;
264 }
265
266 /*
267 * Indicate the cpu delay calibration is done. This can be used by
268 * architectures to stop accepting delay timer registrations after this point.
269 */
270
271 void __attribute__((weak)) calibration_delay_done(void)
272 {
273 }
274
275 void calibrate_delay(void)
276 {
277 unsigned long lpj;
278 static bool printed;
279 int this_cpu = smp_processor_id();
280
281 if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
282 lpj = per_cpu(cpu_loops_per_jiffy, this_cpu);
283 if (!printed)
284 pr_info("Calibrating delay loop (skipped) "
285 "already calibrated this CPU");
286 } else if (preset_lpj) {
287 lpj = preset_lpj;
288 if (!printed)
289 pr_info("Calibrating delay loop (skipped) "
290 "preset value.. ");
291 } else if ((!printed) && lpj_fine) {
292 lpj = lpj_fine;
293 pr_info("Calibrating delay loop (skipped), "
294 "value calculated using timer frequency.. ");
295 } else if ((lpj = calibrate_delay_is_known())) {
296 ;
297 } else if ((lpj = calibrate_delay_direct()) != 0) {
298 if (!printed)
299 pr_info("Calibrating delay using timer "
300 "specific routine.. ");
301 } else {
302 if (!printed)
303 pr_info("Calibrating delay loop... ");
304 lpj = calibrate_delay_converge();
305 }
306 per_cpu(cpu_loops_per_jiffy, this_cpu) = lpj;
307 if (!printed)
308 pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
309 lpj/(500000/HZ),
310 (lpj/(5000/HZ)) % 100, lpj);
311
312 loops_per_jiffy = lpj;
313 printed = true;
314
315 calibration_delay_done();
316 }
317
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