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Linux/init/calibrate.c

  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;
 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 
 34 static unsigned long 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
169 static unsigned long calibrate_delay_direct(void)
170 {
171         return 0;
172 }
173 #endif
174 
175 /*
176  * This is the number of bits of precision for the loops_per_jiffy.  Each
177  * time we refine our estimate after the first takes 1.5/HZ seconds, so try
178  * to start with a good estimate.
179  * For the boot cpu we can skip the delay calibration and assign it a value
180  * calculated based on the timer frequency.
181  * For the rest of the CPUs we cannot assume that the timer frequency is same as
182  * the cpu frequency, hence do the calibration for those.
183  */
184 #define LPS_PREC 8
185 
186 static unsigned long calibrate_delay_converge(void)
187 {
188         /* First stage - slowly accelerate to find initial bounds */
189         unsigned long lpj, lpj_base, ticks, loopadd, loopadd_base, chop_limit;
190         int trials = 0, band = 0, trial_in_band = 0;
191 
192         lpj = (1<<12);
193 
194         /* wait for "start of" clock tick */
195         ticks = jiffies;
196         while (ticks == jiffies)
197                 ; /* nothing */
198         /* Go .. */
199         ticks = jiffies;
200         do {
201                 if (++trial_in_band == (1<<band)) {
202                         ++band;
203                         trial_in_band = 0;
204                 }
205                 __delay(lpj * band);
206                 trials += band;
207         } while (ticks == jiffies);
208         /*
209          * We overshot, so retreat to a clear underestimate. Then estimate
210          * the largest likely undershoot. This defines our chop bounds.
211          */
212         trials -= band;
213         loopadd_base = lpj * band;
214         lpj_base = lpj * trials;
215 
216 recalibrate:
217         lpj = lpj_base;
218         loopadd = loopadd_base;
219 
220         /*
221          * Do a binary approximation to get lpj set to
222          * equal one clock (up to LPS_PREC bits)
223          */
224         chop_limit = lpj >> LPS_PREC;
225         while (loopadd > chop_limit) {
226                 lpj += loopadd;
227                 ticks = jiffies;
228                 while (ticks == jiffies)
229                         ; /* nothing */
230                 ticks = jiffies;
231                 __delay(lpj);
232                 if (jiffies != ticks)   /* longer than 1 tick */
233                         lpj -= loopadd;
234                 loopadd >>= 1;
235         }
236         /*
237          * If we incremented every single time possible, presume we've
238          * massively underestimated initially, and retry with a higher
239          * start, and larger range. (Only seen on x86_64, due to SMIs)
240          */
241         if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
242                 lpj_base = lpj;
243                 loopadd_base <<= 2;
244                 goto recalibrate;
245         }
246 
247         return lpj;
248 }
249 
250 static DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };
251 
252 /*
253  * Check if cpu calibration delay is already known. For example,
254  * some processors with multi-core sockets may have all cores
255  * with the same calibration delay.
256  *
257  * Architectures should override this function if a faster calibration
258  * method is available.
259  */
260 unsigned long __attribute__((weak)) calibrate_delay_is_known(void)
261 {
262         return 0;
263 }
264 
265 /*
266  * Indicate the cpu delay calibration is done. This can be used by
267  * architectures to stop accepting delay timer registrations after this point.
268  */
269 
270 void __attribute__((weak)) calibration_delay_done(void)
271 {
272 }
273 
274 void calibrate_delay(void)
275 {
276         unsigned long lpj;
277         static bool printed;
278         int this_cpu = smp_processor_id();
279 
280         if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
281                 lpj = per_cpu(cpu_loops_per_jiffy, this_cpu);
282                 if (!printed)
283                         pr_info("Calibrating delay loop (skipped) "
284                                 "already calibrated this CPU");
285         } else if (preset_lpj) {
286                 lpj = preset_lpj;
287                 if (!printed)
288                         pr_info("Calibrating delay loop (skipped) "
289                                 "preset value.. ");
290         } else if ((!printed) && lpj_fine) {
291                 lpj = lpj_fine;
292                 pr_info("Calibrating delay loop (skipped), "
293                         "value calculated using timer frequency.. ");
294         } else if ((lpj = calibrate_delay_is_known())) {
295                 ;
296         } else if ((lpj = calibrate_delay_direct()) != 0) {
297                 if (!printed)
298                         pr_info("Calibrating delay using timer "
299                                 "specific routine.. ");
300         } else {
301                 if (!printed)
302                         pr_info("Calibrating delay loop... ");
303                 lpj = calibrate_delay_converge();
304         }
305         per_cpu(cpu_loops_per_jiffy, this_cpu) = lpj;
306         if (!printed)
307                 pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
308                         lpj/(500000/HZ),
309                         (lpj/(5000/HZ)) % 100, lpj);
310 
311         loops_per_jiffy = lpj;
312         printed = true;
313 
314         calibration_delay_done();
315 }
316 

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