Root/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
14unsigned long lpj_fine;
15unsigned long preset_lpj;
16static 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
34static 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
169static 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
186static 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
216recalibrate:
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
250static 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 */
260unsigned long __attribute__((weak)) calibrate_delay_is_known(void)
261{
262    return 0;
263}
264
265void calibrate_delay(void)
266{
267    unsigned long lpj;
268    static bool printed;
269    int this_cpu = smp_processor_id();
270
271    if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
272        lpj = per_cpu(cpu_loops_per_jiffy, this_cpu);
273        if (!printed)
274            pr_info("Calibrating delay loop (skipped) "
275                "already calibrated this CPU");
276    } else if (preset_lpj) {
277        lpj = preset_lpj;
278        if (!printed)
279            pr_info("Calibrating delay loop (skipped) "
280                "preset value.. ");
281    } else if ((!printed) && lpj_fine) {
282        lpj = lpj_fine;
283        pr_info("Calibrating delay loop (skipped), "
284            "value calculated using timer frequency.. ");
285    } else if ((lpj = calibrate_delay_is_known())) {
286        ;
287    } else if ((lpj = calibrate_delay_direct()) != 0) {
288        if (!printed)
289            pr_info("Calibrating delay using timer "
290                "specific routine.. ");
291    } else {
292        if (!printed)
293            pr_info("Calibrating delay loop... ");
294        lpj = calibrate_delay_converge();
295    }
296    per_cpu(cpu_loops_per_jiffy, this_cpu) = lpj;
297    if (!printed)
298        pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
299            lpj/(500000/HZ),
300            (lpj/(5000/HZ)) % 100, lpj);
301
302    loops_per_jiffy = lpj;
303    printed = true;
304}
305

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