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TOMOYO Linux Cross Reference
Linux/init/calibrate.c

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  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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