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Linux/kernel/time.c

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  1 /*
  2  *  linux/kernel/time.c
  3  *
  4  *  Copyright (C) 1991, 1992  Linus Torvalds
  5  *
  6  *  This file contains the interface functions for the various
  7  *  time related system calls: time, stime, gettimeofday, settimeofday,
  8  *                             adjtime
  9  */
 10 /*
 11  * Modification history kernel/time.c
 12  *
 13  * 1993-09-02    Philip Gladstone
 14  *      Created file with time related functions from sched/core.c and adjtimex()
 15  * 1993-10-08    Torsten Duwe
 16  *      adjtime interface update and CMOS clock write code
 17  * 1995-08-13    Torsten Duwe
 18  *      kernel PLL updated to 1994-12-13 specs (rfc-1589)
 19  * 1999-01-16    Ulrich Windl
 20  *      Introduced error checking for many cases in adjtimex().
 21  *      Updated NTP code according to technical memorandum Jan '96
 22  *      "A Kernel Model for Precision Timekeeping" by Dave Mills
 23  *      Allow time_constant larger than MAXTC(6) for NTP v4 (MAXTC == 10)
 24  *      (Even though the technical memorandum forbids it)
 25  * 2004-07-14    Christoph Lameter
 26  *      Added getnstimeofday to allow the posix timer functions to return
 27  *      with nanosecond accuracy
 28  */
 29 
 30 #include <linux/export.h>
 31 #include <linux/timex.h>
 32 #include <linux/capability.h>
 33 #include <linux/timekeeper_internal.h>
 34 #include <linux/errno.h>
 35 #include <linux/syscalls.h>
 36 #include <linux/security.h>
 37 #include <linux/fs.h>
 38 #include <linux/math64.h>
 39 #include <linux/ptrace.h>
 40 
 41 #include <asm/uaccess.h>
 42 #include <asm/unistd.h>
 43 
 44 #include "timeconst.h"
 45 
 46 /*
 47  * The timezone where the local system is located.  Used as a default by some
 48  * programs who obtain this value by using gettimeofday.
 49  */
 50 struct timezone sys_tz;
 51 
 52 EXPORT_SYMBOL(sys_tz);
 53 
 54 #ifdef __ARCH_WANT_SYS_TIME
 55 
 56 /*
 57  * sys_time() can be implemented in user-level using
 58  * sys_gettimeofday().  Is this for backwards compatibility?  If so,
 59  * why not move it into the appropriate arch directory (for those
 60  * architectures that need it).
 61  */
 62 SYSCALL_DEFINE1(time, time_t __user *, tloc)
 63 {
 64         time_t i = get_seconds();
 65 
 66         if (tloc) {
 67                 if (put_user(i,tloc))
 68                         return -EFAULT;
 69         }
 70         force_successful_syscall_return();
 71         return i;
 72 }
 73 
 74 /*
 75  * sys_stime() can be implemented in user-level using
 76  * sys_settimeofday().  Is this for backwards compatibility?  If so,
 77  * why not move it into the appropriate arch directory (for those
 78  * architectures that need it).
 79  */
 80 
 81 SYSCALL_DEFINE1(stime, time_t __user *, tptr)
 82 {
 83         struct timespec tv;
 84         int err;
 85 
 86         if (get_user(tv.tv_sec, tptr))
 87                 return -EFAULT;
 88 
 89         tv.tv_nsec = 0;
 90 
 91         err = security_settime(&tv, NULL);
 92         if (err)
 93                 return err;
 94 
 95         do_settimeofday(&tv);
 96         return 0;
 97 }
 98 
 99 #endif /* __ARCH_WANT_SYS_TIME */
100 
101 SYSCALL_DEFINE2(gettimeofday, struct timeval __user *, tv,
102                 struct timezone __user *, tz)
103 {
104         if (likely(tv != NULL)) {
105                 struct timeval ktv;
106                 do_gettimeofday(&ktv);
107                 if (copy_to_user(tv, &ktv, sizeof(ktv)))
108                         return -EFAULT;
109         }
110         if (unlikely(tz != NULL)) {
111                 if (copy_to_user(tz, &sys_tz, sizeof(sys_tz)))
112                         return -EFAULT;
113         }
114         return 0;
115 }
116 
117 /*
118  * Indicates if there is an offset between the system clock and the hardware
119  * clock/persistent clock/rtc.
120  */
121 int persistent_clock_is_local;
122 
123 /*
124  * Adjust the time obtained from the CMOS to be UTC time instead of
125  * local time.
126  *
127  * This is ugly, but preferable to the alternatives.  Otherwise we
128  * would either need to write a program to do it in /etc/rc (and risk
129  * confusion if the program gets run more than once; it would also be
130  * hard to make the program warp the clock precisely n hours)  or
131  * compile in the timezone information into the kernel.  Bad, bad....
132  *
133  *                                              - TYT, 1992-01-01
134  *
135  * The best thing to do is to keep the CMOS clock in universal time (UTC)
136  * as real UNIX machines always do it. This avoids all headaches about
137  * daylight saving times and warping kernel clocks.
138  */
139 static inline void warp_clock(void)
140 {
141         if (sys_tz.tz_minuteswest != 0) {
142                 struct timespec adjust;
143 
144                 persistent_clock_is_local = 1;
145                 adjust.tv_sec = sys_tz.tz_minuteswest * 60;
146                 adjust.tv_nsec = 0;
147                 timekeeping_inject_offset(&adjust);
148         }
149 }
150 
151 /*
152  * In case for some reason the CMOS clock has not already been running
153  * in UTC, but in some local time: The first time we set the timezone,
154  * we will warp the clock so that it is ticking UTC time instead of
155  * local time. Presumably, if someone is setting the timezone then we
156  * are running in an environment where the programs understand about
157  * timezones. This should be done at boot time in the /etc/rc script,
158  * as soon as possible, so that the clock can be set right. Otherwise,
159  * various programs will get confused when the clock gets warped.
160  */
161 
162 int do_sys_settimeofday(const struct timespec *tv, const struct timezone *tz)
163 {
164         static int firsttime = 1;
165         int error = 0;
166 
167         if (tv && !timespec_valid(tv))
168                 return -EINVAL;
169 
170         error = security_settime(tv, tz);
171         if (error)
172                 return error;
173 
174         if (tz) {
175                 sys_tz = *tz;
176                 update_vsyscall_tz();
177                 if (firsttime) {
178                         firsttime = 0;
179                         if (!tv)
180                                 warp_clock();
181                 }
182         }
183         if (tv)
184                 return do_settimeofday(tv);
185         return 0;
186 }
187 
188 SYSCALL_DEFINE2(settimeofday, struct timeval __user *, tv,
189                 struct timezone __user *, tz)
190 {
191         struct timeval user_tv;
192         struct timespec new_ts;
193         struct timezone new_tz;
194 
195         if (tv) {
196                 if (copy_from_user(&user_tv, tv, sizeof(*tv)))
197                         return -EFAULT;
198                 new_ts.tv_sec = user_tv.tv_sec;
199                 new_ts.tv_nsec = user_tv.tv_usec * NSEC_PER_USEC;
200         }
201         if (tz) {
202                 if (copy_from_user(&new_tz, tz, sizeof(*tz)))
203                         return -EFAULT;
204         }
205 
206         return do_sys_settimeofday(tv ? &new_ts : NULL, tz ? &new_tz : NULL);
207 }
208 
209 SYSCALL_DEFINE1(adjtimex, struct timex __user *, txc_p)
210 {
211         struct timex txc;               /* Local copy of parameter */
212         int ret;
213 
214         /* Copy the user data space into the kernel copy
215          * structure. But bear in mind that the structures
216          * may change
217          */
218         if(copy_from_user(&txc, txc_p, sizeof(struct timex)))
219                 return -EFAULT;
220         ret = do_adjtimex(&txc);
221         return copy_to_user(txc_p, &txc, sizeof(struct timex)) ? -EFAULT : ret;
222 }
223 
224 /**
225  * current_fs_time - Return FS time
226  * @sb: Superblock.
227  *
228  * Return the current time truncated to the time granularity supported by
229  * the fs.
230  */
231 struct timespec current_fs_time(struct super_block *sb)
232 {
233         struct timespec now = current_kernel_time();
234         return timespec_trunc(now, sb->s_time_gran);
235 }
236 EXPORT_SYMBOL(current_fs_time);
237 
238 /*
239  * Convert jiffies to milliseconds and back.
240  *
241  * Avoid unnecessary multiplications/divisions in the
242  * two most common HZ cases:
243  */
244 unsigned int jiffies_to_msecs(const unsigned long j)
245 {
246 #if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
247         return (MSEC_PER_SEC / HZ) * j;
248 #elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
249         return (j + (HZ / MSEC_PER_SEC) - 1)/(HZ / MSEC_PER_SEC);
250 #else
251 # if BITS_PER_LONG == 32
252         return (HZ_TO_MSEC_MUL32 * j) >> HZ_TO_MSEC_SHR32;
253 # else
254         return (j * HZ_TO_MSEC_NUM) / HZ_TO_MSEC_DEN;
255 # endif
256 #endif
257 }
258 EXPORT_SYMBOL(jiffies_to_msecs);
259 
260 unsigned int jiffies_to_usecs(const unsigned long j)
261 {
262 #if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)
263         return (USEC_PER_SEC / HZ) * j;
264 #elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)
265         return (j + (HZ / USEC_PER_SEC) - 1)/(HZ / USEC_PER_SEC);
266 #else
267 # if BITS_PER_LONG == 32
268         return (HZ_TO_USEC_MUL32 * j) >> HZ_TO_USEC_SHR32;
269 # else
270         return (j * HZ_TO_USEC_NUM) / HZ_TO_USEC_DEN;
271 # endif
272 #endif
273 }
274 EXPORT_SYMBOL(jiffies_to_usecs);
275 
276 /**
277  * timespec_trunc - Truncate timespec to a granularity
278  * @t: Timespec
279  * @gran: Granularity in ns.
280  *
281  * Truncate a timespec to a granularity. gran must be smaller than a second.
282  * Always rounds down.
283  *
284  * This function should be only used for timestamps returned by
285  * current_kernel_time() or CURRENT_TIME, not with do_gettimeofday() because
286  * it doesn't handle the better resolution of the latter.
287  */
288 struct timespec timespec_trunc(struct timespec t, unsigned gran)
289 {
290         /*
291          * Division is pretty slow so avoid it for common cases.
292          * Currently current_kernel_time() never returns better than
293          * jiffies resolution. Exploit that.
294          */
295         if (gran <= jiffies_to_usecs(1) * 1000) {
296                 /* nothing */
297         } else if (gran == 1000000000) {
298                 t.tv_nsec = 0;
299         } else {
300                 t.tv_nsec -= t.tv_nsec % gran;
301         }
302         return t;
303 }
304 EXPORT_SYMBOL(timespec_trunc);
305 
306 /* Converts Gregorian date to seconds since 1970-01-01 00:00:00.
307  * Assumes input in normal date format, i.e. 1980-12-31 23:59:59
308  * => year=1980, mon=12, day=31, hour=23, min=59, sec=59.
309  *
310  * [For the Julian calendar (which was used in Russia before 1917,
311  * Britain & colonies before 1752, anywhere else before 1582,
312  * and is still in use by some communities) leave out the
313  * -year/100+year/400 terms, and add 10.]
314  *
315  * This algorithm was first published by Gauss (I think).
316  *
317  * WARNING: this function will overflow on 2106-02-07 06:28:16 on
318  * machines where long is 32-bit! (However, as time_t is signed, we
319  * will already get problems at other places on 2038-01-19 03:14:08)
320  */
321 unsigned long
322 mktime(const unsigned int year0, const unsigned int mon0,
323        const unsigned int day, const unsigned int hour,
324        const unsigned int min, const unsigned int sec)
325 {
326         unsigned int mon = mon0, year = year0;
327 
328         /* 1..12 -> 11,12,1..10 */
329         if (0 >= (int) (mon -= 2)) {
330                 mon += 12;      /* Puts Feb last since it has leap day */
331                 year -= 1;
332         }
333 
334         return ((((unsigned long)
335                   (year/4 - year/100 + year/400 + 367*mon/12 + day) +
336                   year*365 - 719499
337             )*24 + hour /* now have hours */
338           )*60 + min /* now have minutes */
339         )*60 + sec; /* finally seconds */
340 }
341 
342 EXPORT_SYMBOL(mktime);
343 
344 /**
345  * set_normalized_timespec - set timespec sec and nsec parts and normalize
346  *
347  * @ts:         pointer to timespec variable to be set
348  * @sec:        seconds to set
349  * @nsec:       nanoseconds to set
350  *
351  * Set seconds and nanoseconds field of a timespec variable and
352  * normalize to the timespec storage format
353  *
354  * Note: The tv_nsec part is always in the range of
355  *      0 <= tv_nsec < NSEC_PER_SEC
356  * For negative values only the tv_sec field is negative !
357  */
358 void set_normalized_timespec(struct timespec *ts, time_t sec, s64 nsec)
359 {
360         while (nsec >= NSEC_PER_SEC) {
361                 /*
362                  * The following asm() prevents the compiler from
363                  * optimising this loop into a modulo operation. See
364                  * also __iter_div_u64_rem() in include/linux/time.h
365                  */
366                 asm("" : "+rm"(nsec));
367                 nsec -= NSEC_PER_SEC;
368                 ++sec;
369         }
370         while (nsec < 0) {
371                 asm("" : "+rm"(nsec));
372                 nsec += NSEC_PER_SEC;
373                 --sec;
374         }
375         ts->tv_sec = sec;
376         ts->tv_nsec = nsec;
377 }
378 EXPORT_SYMBOL(set_normalized_timespec);
379 
380 /**
381  * ns_to_timespec - Convert nanoseconds to timespec
382  * @nsec:       the nanoseconds value to be converted
383  *
384  * Returns the timespec representation of the nsec parameter.
385  */
386 struct timespec ns_to_timespec(const s64 nsec)
387 {
388         struct timespec ts;
389         s32 rem;
390 
391         if (!nsec)
392                 return (struct timespec) {0, 0};
393 
394         ts.tv_sec = div_s64_rem(nsec, NSEC_PER_SEC, &rem);
395         if (unlikely(rem < 0)) {
396                 ts.tv_sec--;
397                 rem += NSEC_PER_SEC;
398         }
399         ts.tv_nsec = rem;
400 
401         return ts;
402 }
403 EXPORT_SYMBOL(ns_to_timespec);
404 
405 /**
406  * ns_to_timeval - Convert nanoseconds to timeval
407  * @nsec:       the nanoseconds value to be converted
408  *
409  * Returns the timeval representation of the nsec parameter.
410  */
411 struct timeval ns_to_timeval(const s64 nsec)
412 {
413         struct timespec ts = ns_to_timespec(nsec);
414         struct timeval tv;
415 
416         tv.tv_sec = ts.tv_sec;
417         tv.tv_usec = (suseconds_t) ts.tv_nsec / 1000;
418 
419         return tv;
420 }
421 EXPORT_SYMBOL(ns_to_timeval);
422 
423 /*
424  * When we convert to jiffies then we interpret incoming values
425  * the following way:
426  *
427  * - negative values mean 'infinite timeout' (MAX_JIFFY_OFFSET)
428  *
429  * - 'too large' values [that would result in larger than
430  *   MAX_JIFFY_OFFSET values] mean 'infinite timeout' too.
431  *
432  * - all other values are converted to jiffies by either multiplying
433  *   the input value by a factor or dividing it with a factor
434  *
435  * We must also be careful about 32-bit overflows.
436  */
437 unsigned long msecs_to_jiffies(const unsigned int m)
438 {
439         /*
440          * Negative value, means infinite timeout:
441          */
442         if ((int)m < 0)
443                 return MAX_JIFFY_OFFSET;
444 
445 #if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
446         /*
447          * HZ is equal to or smaller than 1000, and 1000 is a nice
448          * round multiple of HZ, divide with the factor between them,
449          * but round upwards:
450          */
451         return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
452 #elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
453         /*
454          * HZ is larger than 1000, and HZ is a nice round multiple of
455          * 1000 - simply multiply with the factor between them.
456          *
457          * But first make sure the multiplication result cannot
458          * overflow:
459          */
460         if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
461                 return MAX_JIFFY_OFFSET;
462 
463         return m * (HZ / MSEC_PER_SEC);
464 #else
465         /*
466          * Generic case - multiply, round and divide. But first
467          * check that if we are doing a net multiplication, that
468          * we wouldn't overflow:
469          */
470         if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
471                 return MAX_JIFFY_OFFSET;
472 
473         return (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32)
474                 >> MSEC_TO_HZ_SHR32;
475 #endif
476 }
477 EXPORT_SYMBOL(msecs_to_jiffies);
478 
479 unsigned long usecs_to_jiffies(const unsigned int u)
480 {
481         if (u > jiffies_to_usecs(MAX_JIFFY_OFFSET))
482                 return MAX_JIFFY_OFFSET;
483 #if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)
484         return (u + (USEC_PER_SEC / HZ) - 1) / (USEC_PER_SEC / HZ);
485 #elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)
486         return u * (HZ / USEC_PER_SEC);
487 #else
488         return (USEC_TO_HZ_MUL32 * u + USEC_TO_HZ_ADJ32)
489                 >> USEC_TO_HZ_SHR32;
490 #endif
491 }
492 EXPORT_SYMBOL(usecs_to_jiffies);
493 
494 /*
495  * The TICK_NSEC - 1 rounds up the value to the next resolution.  Note
496  * that a remainder subtract here would not do the right thing as the
497  * resolution values don't fall on second boundries.  I.e. the line:
498  * nsec -= nsec % TICK_NSEC; is NOT a correct resolution rounding.
499  *
500  * Rather, we just shift the bits off the right.
501  *
502  * The >> (NSEC_JIFFIE_SC - SEC_JIFFIE_SC) converts the scaled nsec
503  * value to a scaled second value.
504  */
505 unsigned long
506 timespec_to_jiffies(const struct timespec *value)
507 {
508         unsigned long sec = value->tv_sec;
509         long nsec = value->tv_nsec + TICK_NSEC - 1;
510 
511         if (sec >= MAX_SEC_IN_JIFFIES){
512                 sec = MAX_SEC_IN_JIFFIES;
513                 nsec = 0;
514         }
515         return (((u64)sec * SEC_CONVERSION) +
516                 (((u64)nsec * NSEC_CONVERSION) >>
517                  (NSEC_JIFFIE_SC - SEC_JIFFIE_SC))) >> SEC_JIFFIE_SC;
518 
519 }
520 EXPORT_SYMBOL(timespec_to_jiffies);
521 
522 void
523 jiffies_to_timespec(const unsigned long jiffies, struct timespec *value)
524 {
525         /*
526          * Convert jiffies to nanoseconds and separate with
527          * one divide.
528          */
529         u32 rem;
530         value->tv_sec = div_u64_rem((u64)jiffies * TICK_NSEC,
531                                     NSEC_PER_SEC, &rem);
532         value->tv_nsec = rem;
533 }
534 EXPORT_SYMBOL(jiffies_to_timespec);
535 
536 /* Same for "timeval"
537  *
538  * Well, almost.  The problem here is that the real system resolution is
539  * in nanoseconds and the value being converted is in micro seconds.
540  * Also for some machines (those that use HZ = 1024, in-particular),
541  * there is a LARGE error in the tick size in microseconds.
542 
543  * The solution we use is to do the rounding AFTER we convert the
544  * microsecond part.  Thus the USEC_ROUND, the bits to be shifted off.
545  * Instruction wise, this should cost only an additional add with carry
546  * instruction above the way it was done above.
547  */
548 unsigned long
549 timeval_to_jiffies(const struct timeval *value)
550 {
551         unsigned long sec = value->tv_sec;
552         long usec = value->tv_usec;
553 
554         if (sec >= MAX_SEC_IN_JIFFIES){
555                 sec = MAX_SEC_IN_JIFFIES;
556                 usec = 0;
557         }
558         return (((u64)sec * SEC_CONVERSION) +
559                 (((u64)usec * USEC_CONVERSION + USEC_ROUND) >>
560                  (USEC_JIFFIE_SC - SEC_JIFFIE_SC))) >> SEC_JIFFIE_SC;
561 }
562 EXPORT_SYMBOL(timeval_to_jiffies);
563 
564 void jiffies_to_timeval(const unsigned long jiffies, struct timeval *value)
565 {
566         /*
567          * Convert jiffies to nanoseconds and separate with
568          * one divide.
569          */
570         u32 rem;
571 
572         value->tv_sec = div_u64_rem((u64)jiffies * TICK_NSEC,
573                                     NSEC_PER_SEC, &rem);
574         value->tv_usec = rem / NSEC_PER_USEC;
575 }
576 EXPORT_SYMBOL(jiffies_to_timeval);
577 
578 /*
579  * Convert jiffies/jiffies_64 to clock_t and back.
580  */
581 clock_t jiffies_to_clock_t(unsigned long x)
582 {
583 #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0
584 # if HZ < USER_HZ
585         return x * (USER_HZ / HZ);
586 # else
587         return x / (HZ / USER_HZ);
588 # endif
589 #else
590         return div_u64((u64)x * TICK_NSEC, NSEC_PER_SEC / USER_HZ);
591 #endif
592 }
593 EXPORT_SYMBOL(jiffies_to_clock_t);
594 
595 unsigned long clock_t_to_jiffies(unsigned long x)
596 {
597 #if (HZ % USER_HZ)==0
598         if (x >= ~0UL / (HZ / USER_HZ))
599                 return ~0UL;
600         return x * (HZ / USER_HZ);
601 #else
602         /* Don't worry about loss of precision here .. */
603         if (x >= ~0UL / HZ * USER_HZ)
604                 return ~0UL;
605 
606         /* .. but do try to contain it here */
607         return div_u64((u64)x * HZ, USER_HZ);
608 #endif
609 }
610 EXPORT_SYMBOL(clock_t_to_jiffies);
611 
612 u64 jiffies_64_to_clock_t(u64 x)
613 {
614 #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0
615 # if HZ < USER_HZ
616         x = div_u64(x * USER_HZ, HZ);
617 # elif HZ > USER_HZ
618         x = div_u64(x, HZ / USER_HZ);
619 # else
620         /* Nothing to do */
621 # endif
622 #else
623         /*
624          * There are better ways that don't overflow early,
625          * but even this doesn't overflow in hundreds of years
626          * in 64 bits, so..
627          */
628         x = div_u64(x * TICK_NSEC, (NSEC_PER_SEC / USER_HZ));
629 #endif
630         return x;
631 }
632 EXPORT_SYMBOL(jiffies_64_to_clock_t);
633 
634 u64 nsec_to_clock_t(u64 x)
635 {
636 #if (NSEC_PER_SEC % USER_HZ) == 0
637         return div_u64(x, NSEC_PER_SEC / USER_HZ);
638 #elif (USER_HZ % 512) == 0
639         return div_u64(x * USER_HZ / 512, NSEC_PER_SEC / 512);
640 #else
641         /*
642          * max relative error 5.7e-8 (1.8s per year) for USER_HZ <= 1024,
643          * overflow after 64.99 years.
644          * exact for HZ=60, 72, 90, 120, 144, 180, 300, 600, 900, ...
645          */
646         return div_u64(x * 9, (9ull * NSEC_PER_SEC + (USER_HZ / 2)) / USER_HZ);
647 #endif
648 }
649 
650 /**
651  * nsecs_to_jiffies64 - Convert nsecs in u64 to jiffies64
652  *
653  * @n:  nsecs in u64
654  *
655  * Unlike {m,u}secs_to_jiffies, type of input is not unsigned int but u64.
656  * And this doesn't return MAX_JIFFY_OFFSET since this function is designed
657  * for scheduler, not for use in device drivers to calculate timeout value.
658  *
659  * note:
660  *   NSEC_PER_SEC = 10^9 = (5^9 * 2^9) = (1953125 * 512)
661  *   ULLONG_MAX ns = 18446744073.709551615 secs = about 584 years
662  */
663 u64 nsecs_to_jiffies64(u64 n)
664 {
665 #if (NSEC_PER_SEC % HZ) == 0
666         /* Common case, HZ = 100, 128, 200, 250, 256, 500, 512, 1000 etc. */
667         return div_u64(n, NSEC_PER_SEC / HZ);
668 #elif (HZ % 512) == 0
669         /* overflow after 292 years if HZ = 1024 */
670         return div_u64(n * HZ / 512, NSEC_PER_SEC / 512);
671 #else
672         /*
673          * Generic case - optimized for cases where HZ is a multiple of 3.
674          * overflow after 64.99 years, exact for HZ = 60, 72, 90, 120 etc.
675          */
676         return div_u64(n * 9, (9ull * NSEC_PER_SEC + HZ / 2) / HZ);
677 #endif
678 }
679 
680 /**
681  * nsecs_to_jiffies - Convert nsecs in u64 to jiffies
682  *
683  * @n:  nsecs in u64
684  *
685  * Unlike {m,u}secs_to_jiffies, type of input is not unsigned int but u64.
686  * And this doesn't return MAX_JIFFY_OFFSET since this function is designed
687  * for scheduler, not for use in device drivers to calculate timeout value.
688  *
689  * note:
690  *   NSEC_PER_SEC = 10^9 = (5^9 * 2^9) = (1953125 * 512)
691  *   ULLONG_MAX ns = 18446744073.709551615 secs = about 584 years
692  */
693 unsigned long nsecs_to_jiffies(u64 n)
694 {
695         return (unsigned long)nsecs_to_jiffies64(n);
696 }
697 
698 /*
699  * Add two timespec values and do a safety check for overflow.
700  * It's assumed that both values are valid (>= 0)
701  */
702 struct timespec timespec_add_safe(const struct timespec lhs,
703                                   const struct timespec rhs)
704 {
705         struct timespec res;
706 
707         set_normalized_timespec(&res, lhs.tv_sec + rhs.tv_sec,
708                                 lhs.tv_nsec + rhs.tv_nsec);
709 
710         if (res.tv_sec < lhs.tv_sec || res.tv_sec < rhs.tv_sec)
711                 res.tv_sec = TIME_T_MAX;
712 
713         return res;
714 }
715 

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