The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_tc.c

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    1 /*-
    2  * ----------------------------------------------------------------------------
    3  * "THE BEER-WARE LICENSE" (Revision 42):
    4  * <phk@FreeBSD.ORG> wrote this file.  As long as you retain this notice you
    5  * can do whatever you want with this stuff. If we meet some day, and you think
    6  * this stuff is worth it, you can buy me a beer in return.   Poul-Henning Kamp
    7  * ----------------------------------------------------------------------------
    8  *
    9  * $FreeBSD: releng/5.1/sys/kern/kern_tc.c 112367 2003-03-18 08:45:25Z phk $
   10  */
   11 
   12 #include "opt_ntp.h"
   13 
   14 #include <sys/param.h>
   15 #include <sys/kernel.h>
   16 #include <sys/sysctl.h>
   17 #include <sys/systm.h>
   18 #include <sys/timepps.h>
   19 #include <sys/timetc.h>
   20 #include <sys/timex.h>
   21 
   22 /*
   23  * Implement a dummy timecounter which we can use until we get a real one
   24  * in the air.  This allows the console and other early stuff to use
   25  * time services.
   26  */
   27 
   28 static u_int
   29 dummy_get_timecount(struct timecounter *tc)
   30 {
   31         static u_int now;
   32 
   33         return (++now);
   34 }
   35 
   36 static struct timecounter dummy_timecounter = {
   37         dummy_get_timecount, 0, ~0u, 1000000, "dummy",
   38 };
   39 
   40 struct timehands {
   41         /* These fields must be initialized by the driver. */
   42         struct timecounter      *th_counter;
   43         int64_t                 th_adjustment;
   44         u_int64_t               th_scale;
   45         u_int                   th_offset_count;
   46         struct bintime          th_offset;
   47         struct timeval          th_microtime;
   48         struct timespec         th_nanotime;
   49         /* Fields not to be copied in tc_windup start with th_generation. */
   50         volatile u_int          th_generation;
   51         struct timehands        *th_next;
   52 };
   53 
   54 extern struct timehands th0;
   55 static struct timehands th9 = { NULL, 0, 0, 0, {0, 0}, {0, 0}, {0, 0}, 0, &th0};
   56 static struct timehands th8 = { NULL, 0, 0, 0, {0, 0}, {0, 0}, {0, 0}, 0, &th9};
   57 static struct timehands th7 = { NULL, 0, 0, 0, {0, 0}, {0, 0}, {0, 0}, 0, &th8};
   58 static struct timehands th6 = { NULL, 0, 0, 0, {0, 0}, {0, 0}, {0, 0}, 0, &th7};
   59 static struct timehands th5 = { NULL, 0, 0, 0, {0, 0}, {0, 0}, {0, 0}, 0, &th6};
   60 static struct timehands th4 = { NULL, 0, 0, 0, {0, 0}, {0, 0}, {0, 0}, 0, &th5};
   61 static struct timehands th3 = { NULL, 0, 0, 0, {0, 0}, {0, 0}, {0, 0}, 0, &th4};
   62 static struct timehands th2 = { NULL, 0, 0, 0, {0, 0}, {0, 0}, {0, 0}, 0, &th3};
   63 static struct timehands th1 = { NULL, 0, 0, 0, {0, 0}, {0, 0}, {0, 0}, 0, &th2};
   64 static struct timehands th0 = {
   65         &dummy_timecounter,
   66         0,
   67         (uint64_t)-1 / 1000000,
   68         0,
   69         {1, 0},
   70         {0, 0},
   71         {0, 0},
   72         1,
   73         &th1
   74 };
   75 
   76 static struct timehands *volatile timehands = &th0;
   77 struct timecounter *timecounter = &dummy_timecounter;
   78 static struct timecounter *timecounters = &dummy_timecounter;
   79 
   80 time_t time_second = 1;
   81 time_t time_uptime = 0;
   82 
   83 static struct bintime boottimebin;
   84 struct timeval boottime;
   85 SYSCTL_STRUCT(_kern, KERN_BOOTTIME, boottime, CTLFLAG_RD,
   86     &boottime, timeval, "System boottime");
   87 
   88 SYSCTL_NODE(_kern, OID_AUTO, timecounter, CTLFLAG_RW, 0, "");
   89 
   90 #define TC_STATS(foo) \
   91         static u_int foo; \
   92         SYSCTL_UINT(_kern_timecounter, OID_AUTO, foo, CTLFLAG_RD, &foo, 0, "");\
   93         struct __hack
   94 
   95 TC_STATS(nbinuptime);    TC_STATS(nnanouptime);    TC_STATS(nmicrouptime);
   96 TC_STATS(nbintime);      TC_STATS(nnanotime);      TC_STATS(nmicrotime);
   97 TC_STATS(ngetbinuptime); TC_STATS(ngetnanouptime); TC_STATS(ngetmicrouptime);
   98 TC_STATS(ngetbintime);   TC_STATS(ngetnanotime);   TC_STATS(ngetmicrotime);
   99 TC_STATS(nsetclock);
  100 
  101 #undef TC_STATS
  102 
  103 static void tc_windup(void);
  104 
  105 /*
  106  * Return the difference between the timehands' counter value now and what
  107  * was when we copied it to the timehands' offset_count.
  108  */
  109 static __inline u_int
  110 tc_delta(struct timehands *th)
  111 {
  112         struct timecounter *tc;
  113 
  114         tc = th->th_counter;
  115         return ((tc->tc_get_timecount(tc) - th->th_offset_count) &
  116             tc->tc_counter_mask);
  117 }
  118 
  119 /*
  120  * Functions for reading the time.  We have to loop until we are sure that
  121  * the timehands that we operated on was not updated under our feet.  See
  122  * the comment in <sys/time.h> for a description of these 12 functions.
  123  */
  124 
  125 void
  126 binuptime(struct bintime *bt)
  127 {
  128         struct timehands *th;
  129         u_int gen;
  130 
  131         nbinuptime++;
  132         do {
  133                 th = timehands;
  134                 gen = th->th_generation;
  135                 *bt = th->th_offset;
  136                 bintime_addx(bt, th->th_scale * tc_delta(th));
  137         } while (gen == 0 || gen != th->th_generation);
  138 }
  139 
  140 void
  141 nanouptime(struct timespec *tsp)
  142 {
  143         struct bintime bt;
  144 
  145         nnanouptime++;
  146         binuptime(&bt);
  147         bintime2timespec(&bt, tsp);
  148 }
  149 
  150 void
  151 microuptime(struct timeval *tvp)
  152 {
  153         struct bintime bt;
  154 
  155         nmicrouptime++;
  156         binuptime(&bt);
  157         bintime2timeval(&bt, tvp);
  158 }
  159 
  160 void
  161 bintime(struct bintime *bt)
  162 {
  163 
  164         nbintime++;
  165         binuptime(bt);
  166         bintime_add(bt, &boottimebin);
  167 }
  168 
  169 void
  170 nanotime(struct timespec *tsp)
  171 {
  172         struct bintime bt;
  173 
  174         nnanotime++;
  175         bintime(&bt);
  176         bintime2timespec(&bt, tsp);
  177 }
  178 
  179 void
  180 microtime(struct timeval *tvp)
  181 {
  182         struct bintime bt;
  183 
  184         nmicrotime++;
  185         bintime(&bt);
  186         bintime2timeval(&bt, tvp);
  187 }
  188 
  189 void
  190 getbinuptime(struct bintime *bt)
  191 {
  192         struct timehands *th;
  193         u_int gen;
  194 
  195         ngetbinuptime++;
  196         do {
  197                 th = timehands;
  198                 gen = th->th_generation;
  199                 *bt = th->th_offset;
  200         } while (gen == 0 || gen != th->th_generation);
  201 }
  202 
  203 void
  204 getnanouptime(struct timespec *tsp)
  205 {
  206         struct timehands *th;
  207         u_int gen;
  208 
  209         ngetnanouptime++;
  210         do {
  211                 th = timehands;
  212                 gen = th->th_generation;
  213                 bintime2timespec(&th->th_offset, tsp);
  214         } while (gen == 0 || gen != th->th_generation);
  215 }
  216 
  217 void
  218 getmicrouptime(struct timeval *tvp)
  219 {
  220         struct timehands *th;
  221         u_int gen;
  222 
  223         ngetmicrouptime++;
  224         do {
  225                 th = timehands;
  226                 gen = th->th_generation;
  227                 bintime2timeval(&th->th_offset, tvp);
  228         } while (gen == 0 || gen != th->th_generation);
  229 }
  230 
  231 void
  232 getbintime(struct bintime *bt)
  233 {
  234         struct timehands *th;
  235         u_int gen;
  236 
  237         ngetbintime++;
  238         do {
  239                 th = timehands;
  240                 gen = th->th_generation;
  241                 *bt = th->th_offset;
  242         } while (gen == 0 || gen != th->th_generation);
  243         bintime_add(bt, &boottimebin);
  244 }
  245 
  246 void
  247 getnanotime(struct timespec *tsp)
  248 {
  249         struct timehands *th;
  250         u_int gen;
  251 
  252         ngetnanotime++;
  253         do {
  254                 th = timehands;
  255                 gen = th->th_generation;
  256                 *tsp = th->th_nanotime;
  257         } while (gen == 0 || gen != th->th_generation);
  258 }
  259 
  260 void
  261 getmicrotime(struct timeval *tvp)
  262 {
  263         struct timehands *th;
  264         u_int gen;
  265 
  266         ngetmicrotime++;
  267         do {
  268                 th = timehands;
  269                 gen = th->th_generation;
  270                 *tvp = th->th_microtime;
  271         } while (gen == 0 || gen != th->th_generation);
  272 }
  273 
  274 /*
  275  * Initialize a new timecounter.
  276  * We should really try to rank the timecounters and intelligently determine
  277  * if the new timecounter is better than the current one.  This is subject
  278  * to further study.  For now always use the new timecounter.
  279  */
  280 void
  281 tc_init(struct timecounter *tc)
  282 {
  283         unsigned u;
  284 
  285         printf("Timecounter \"%s\"  frequency %ju Hz",
  286             tc->tc_name, (intmax_t)tc->tc_frequency);
  287 
  288         u = tc->tc_frequency / tc->tc_counter_mask;
  289         if (u > hz) {
  290                 printf(" -- Insufficient hz, needs at least %u\n", u);
  291                 return;
  292         }
  293         tc->tc_next = timecounters;
  294         timecounters = tc;
  295         printf("\n");
  296         (void)tc->tc_get_timecount(tc);
  297         (void)tc->tc_get_timecount(tc);
  298         timecounter = tc;
  299 }
  300 
  301 /* Report the frequency of the current timecounter. */
  302 u_int64_t
  303 tc_getfrequency(void)
  304 {
  305 
  306         return (timehands->th_counter->tc_frequency);
  307 }
  308 
  309 /*
  310  * Step our concept of GMT.  This is done by modifying our estimate of
  311  * when we booted.  XXX: needs futher work.
  312  */
  313 void
  314 tc_setclock(struct timespec *ts)
  315 {
  316         struct timespec ts2;
  317 
  318         nsetclock++;
  319         nanouptime(&ts2);
  320         boottime.tv_sec = ts->tv_sec - ts2.tv_sec;
  321         /* XXX boottime should probably be a timespec. */
  322         boottime.tv_usec = (ts->tv_nsec - ts2.tv_nsec) / 1000;
  323         if (boottime.tv_usec < 0) {
  324                 boottime.tv_usec += 1000000;
  325                 boottime.tv_sec--;
  326         }
  327         timeval2bintime(&boottime, &boottimebin);
  328 
  329         /* XXX fiddle all the little crinkly bits around the fiords... */
  330         tc_windup();
  331 }
  332 
  333 /*
  334  * Initialize the next struct timehands in the ring and make
  335  * it the active timehands.  Along the way we might switch to a different
  336  * timecounter and/or do seconds processing in NTP.  Slightly magic.
  337  */
  338 static void
  339 tc_windup(void)
  340 {
  341         struct bintime bt;
  342         struct timehands *th, *tho;
  343         u_int64_t scale;
  344         u_int delta, ncount, ogen;
  345         int i;
  346 
  347         /*
  348          * Make the next timehands a copy of the current one, but do not
  349          * overwrite the generation or next pointer.  While we update
  350          * the contents, the generation must be zero.
  351          */
  352         tho = timehands;
  353         th = tho->th_next;
  354         ogen = th->th_generation;
  355         th->th_generation = 0;
  356         bcopy(tho, th, offsetof(struct timehands, th_generation));
  357 
  358         /*
  359          * Capture a timecounter delta on the current timecounter and if
  360          * changing timecounters, a counter value from the new timecounter.
  361          * Update the offset fields accordingly.
  362          */
  363         delta = tc_delta(th);
  364         if (th->th_counter != timecounter)
  365                 ncount = timecounter->tc_get_timecount(timecounter);
  366         else
  367                 ncount = 0;
  368         th->th_offset_count += delta;
  369         th->th_offset_count &= th->th_counter->tc_counter_mask;
  370         bintime_addx(&th->th_offset, th->th_scale * delta);
  371 
  372         /*
  373          * Hardware latching timecounters may not generate interrupts on
  374          * PPS events, so instead we poll them.  There is a finite risk that
  375          * the hardware might capture a count which is later than the one we
  376          * got above, and therefore possibly in the next NTP second which might
  377          * have a different rate than the current NTP second.  It doesn't
  378          * matter in practice.
  379          */
  380         if (tho->th_counter->tc_poll_pps)
  381                 tho->th_counter->tc_poll_pps(tho->th_counter);
  382 
  383         /*
  384          * Deal with NTP second processing.  The for loop normally only
  385          * iterates once, but in extreme situations it might keep NTP sane
  386          * if timeouts are not run for several seconds.
  387          */
  388         for (i = th->th_offset.sec - tho->th_offset.sec; i > 0; i--)
  389                 ntp_update_second(&th->th_adjustment, &th->th_offset.sec);
  390 
  391         /* Now is a good time to change timecounters. */
  392         if (th->th_counter != timecounter) {
  393                 th->th_counter = timecounter;
  394                 th->th_offset_count = ncount;
  395         }
  396 
  397         /*-
  398          * Recalculate the scaling factor.  We want the number of 1/2^64
  399          * fractions of a second per period of the hardware counter, taking
  400          * into account the th_adjustment factor which the NTP PLL/adjtime(2)
  401          * processing provides us with.
  402          *
  403          * The th_adjustment is nanoseconds per second with 32 bit binary
  404          * fraction and want 64 bit binary fraction of second:
  405          *
  406          *       x = a * 2^32 / 10^9 = a * 4.294967296
  407          *
  408          * The range of th_adjustment is +/- 5000PPM so inside a 64bit int
  409          * we can only multiply by about 850 without overflowing, but that
  410          * leaves suitably precise fractions for multiply before divide.
  411          *
  412          * Divide before multiply with a fraction of 2199/512 results in a
  413          * systematic undercompensation of 10PPM of th_adjustment.  On a
  414          * 5000PPM adjustment this is a 0.05PPM error.  This is acceptable.
  415          *
  416          * We happily sacrifice the lowest of the 64 bits of our result
  417          * to the goddess of code clarity.
  418          *
  419          */
  420         scale = (u_int64_t)1 << 63;
  421         scale += (th->th_adjustment / 1024) * 2199;
  422         scale /= th->th_counter->tc_frequency;
  423         th->th_scale = scale * 2;
  424 
  425         /* Update the GMT timestamps used for the get*() functions. */
  426         bt = th->th_offset;
  427         bintime_add(&bt, &boottimebin);
  428         bintime2timeval(&bt, &th->th_microtime);
  429         bintime2timespec(&bt, &th->th_nanotime);
  430 
  431         /*
  432          * Now that the struct timehands is again consistent, set the new
  433          * generation number, making sure to not make it zero.
  434          */
  435         if (++ogen == 0)
  436                 ogen = 1;
  437         th->th_generation = ogen;
  438 
  439         /* Go live with the new struct timehands. */
  440         time_second = th->th_microtime.tv_sec;
  441         time_uptime = th->th_offset.sec;
  442         timehands = th;
  443 }
  444 
  445 /* Report or change the active timecounter hardware. */
  446 static int
  447 sysctl_kern_timecounter_hardware(SYSCTL_HANDLER_ARGS)
  448 {
  449         char newname[32];
  450         struct timecounter *newtc, *tc;
  451         int error;
  452 
  453         tc = timecounter;
  454         strlcpy(newname, tc->tc_name, sizeof(newname));
  455 
  456         error = sysctl_handle_string(oidp, &newname[0], sizeof(newname), req);
  457         if (error != 0 || req->newptr == NULL ||
  458             strcmp(newname, tc->tc_name) == 0)
  459                 return (error);
  460         for (newtc = timecounters; newtc != NULL; newtc = newtc->tc_next) {
  461                 if (strcmp(newname, newtc->tc_name) != 0)
  462                         continue;
  463 
  464                 /* Warm up new timecounter. */
  465                 (void)newtc->tc_get_timecount(newtc);
  466                 (void)newtc->tc_get_timecount(newtc);
  467 
  468                 timecounter = newtc;
  469                 return (0);
  470         }
  471         return (EINVAL);
  472 }
  473 
  474 SYSCTL_PROC(_kern_timecounter, OID_AUTO, hardware, CTLTYPE_STRING | CTLFLAG_RW,
  475     0, 0, sysctl_kern_timecounter_hardware, "A", "");
  476 
  477 /*
  478  * RFC 2783 PPS-API implementation.
  479  */
  480 
  481 int
  482 pps_ioctl(u_long cmd, caddr_t data, struct pps_state *pps)
  483 {
  484         pps_params_t *app;
  485         struct pps_fetch_args *fapi;
  486 #ifdef PPS_SYNC
  487         struct pps_kcbind_args *kapi;
  488 #endif
  489 
  490         switch (cmd) {
  491         case PPS_IOC_CREATE:
  492                 return (0);
  493         case PPS_IOC_DESTROY:
  494                 return (0);
  495         case PPS_IOC_SETPARAMS:
  496                 app = (pps_params_t *)data;
  497                 if (app->mode & ~pps->ppscap)
  498                         return (EINVAL);
  499                 pps->ppsparam = *app;
  500                 return (0);
  501         case PPS_IOC_GETPARAMS:
  502                 app = (pps_params_t *)data;
  503                 *app = pps->ppsparam;
  504                 app->api_version = PPS_API_VERS_1;
  505                 return (0);
  506         case PPS_IOC_GETCAP:
  507                 *(int*)data = pps->ppscap;
  508                 return (0);
  509         case PPS_IOC_FETCH:
  510                 fapi = (struct pps_fetch_args *)data;
  511                 if (fapi->tsformat && fapi->tsformat != PPS_TSFMT_TSPEC)
  512                         return (EINVAL);
  513                 if (fapi->timeout.tv_sec || fapi->timeout.tv_nsec)
  514                         return (EOPNOTSUPP);
  515                 pps->ppsinfo.current_mode = pps->ppsparam.mode;
  516                 fapi->pps_info_buf = pps->ppsinfo;
  517                 return (0);
  518         case PPS_IOC_KCBIND:
  519 #ifdef PPS_SYNC
  520                 kapi = (struct pps_kcbind_args *)data;
  521                 /* XXX Only root should be able to do this */
  522                 if (kapi->tsformat && kapi->tsformat != PPS_TSFMT_TSPEC)
  523                         return (EINVAL);
  524                 if (kapi->kernel_consumer != PPS_KC_HARDPPS)
  525                         return (EINVAL);
  526                 if (kapi->edge & ~pps->ppscap)
  527                         return (EINVAL);
  528                 pps->kcmode = kapi->edge;
  529                 return (0);
  530 #else
  531                 return (EOPNOTSUPP);
  532 #endif
  533         default:
  534                 return (ENOTTY);
  535         }
  536 }
  537 
  538 void
  539 pps_init(struct pps_state *pps)
  540 {
  541         pps->ppscap |= PPS_TSFMT_TSPEC;
  542         if (pps->ppscap & PPS_CAPTUREASSERT)
  543                 pps->ppscap |= PPS_OFFSETASSERT;
  544         if (pps->ppscap & PPS_CAPTURECLEAR)
  545                 pps->ppscap |= PPS_OFFSETCLEAR;
  546 }
  547 
  548 void
  549 pps_capture(struct pps_state *pps)
  550 {
  551         struct timehands *th;
  552 
  553         th = timehands;
  554         pps->capgen = th->th_generation;
  555         pps->capth = th;
  556         pps->capcount = th->th_counter->tc_get_timecount(th->th_counter);
  557         if (pps->capgen != th->th_generation)
  558                 pps->capgen = 0;
  559 }
  560 
  561 void
  562 pps_event(struct pps_state *pps, int event)
  563 {
  564         struct bintime bt;
  565         struct timespec ts, *tsp, *osp;
  566         u_int tcount, *pcount;
  567         int foff, fhard;
  568         pps_seq_t *pseq;
  569 
  570         /* If the timecounter was wound up underneath us, bail out. */
  571         if (pps->capgen == 0 || pps->capgen != pps->capth->th_generation)
  572                 return;
  573 
  574         /* Things would be easier with arrays. */
  575         if (event == PPS_CAPTUREASSERT) {
  576                 tsp = &pps->ppsinfo.assert_timestamp;
  577                 osp = &pps->ppsparam.assert_offset;
  578                 foff = pps->ppsparam.mode & PPS_OFFSETASSERT;
  579                 fhard = pps->kcmode & PPS_CAPTUREASSERT;
  580                 pcount = &pps->ppscount[0];
  581                 pseq = &pps->ppsinfo.assert_sequence;
  582         } else {
  583                 tsp = &pps->ppsinfo.clear_timestamp;
  584                 osp = &pps->ppsparam.clear_offset;
  585                 foff = pps->ppsparam.mode & PPS_OFFSETCLEAR;
  586                 fhard = pps->kcmode & PPS_CAPTURECLEAR;
  587                 pcount = &pps->ppscount[1];
  588                 pseq = &pps->ppsinfo.clear_sequence;
  589         }
  590 
  591         /*
  592          * If the timecounter changed, we cannot compare the count values, so
  593          * we have to drop the rest of the PPS-stuff until the next event.
  594          */
  595         if (pps->ppstc != pps->capth->th_counter) {
  596                 pps->ppstc = pps->capth->th_counter;
  597                 *pcount = pps->capcount;
  598                 pps->ppscount[2] = pps->capcount;
  599                 return;
  600         }
  601 
  602         /* Return if nothing really happened. */
  603         if (*pcount == pps->capcount)
  604                 return;
  605 
  606         /* Convert the count to a timespec. */
  607         tcount = pps->capcount - pps->capth->th_offset_count;
  608         tcount &= pps->capth->th_counter->tc_counter_mask;
  609         bt = pps->capth->th_offset;
  610         bintime_addx(&bt, pps->capth->th_scale * tcount);
  611         bintime_add(&bt, &boottimebin);
  612         bintime2timespec(&bt, &ts);
  613 
  614         /* If the timecounter was wound up underneath us, bail out. */
  615         if (pps->capgen != pps->capth->th_generation)
  616                 return;
  617 
  618         *pcount = pps->capcount;
  619         (*pseq)++;
  620         *tsp = ts;
  621 
  622         if (foff) {
  623                 timespecadd(tsp, osp);
  624                 if (tsp->tv_nsec < 0) {
  625                         tsp->tv_nsec += 1000000000;
  626                         tsp->tv_sec -= 1;
  627                 }
  628         }
  629 #ifdef PPS_SYNC
  630         if (fhard) {
  631                 u_int64_t scale;
  632 
  633                 /*
  634                  * Feed the NTP PLL/FLL.
  635                  * The FLL wants to know how many (hardware) nanoseconds
  636                  * elapsed since the previous event.
  637                  */
  638                 tcount = pps->capcount - pps->ppscount[2];
  639                 pps->ppscount[2] = pps->capcount;
  640                 tcount &= pps->capth->th_counter->tc_counter_mask;
  641                 scale = (u_int64_t)1 << 63;
  642                 scale /= pps->capth->th_counter->tc_frequency;
  643                 scale *= 2;
  644                 bt.sec = 0;
  645                 bt.frac = 0;
  646                 bintime_addx(&bt, scale * tcount);
  647                 bintime2timespec(&bt, &ts);
  648                 hardpps(tsp, ts.tv_nsec + 1000000000 * ts.tv_sec);
  649         }
  650 #endif
  651 }
  652 
  653 /*
  654  * Timecounters need to be updated every so often to prevent the hardware
  655  * counter from overflowing.  Updating also recalculates the cached values
  656  * used by the get*() family of functions, so their precision depends on
  657  * the update frequency.
  658  */
  659 
  660 static int tc_tick;
  661 SYSCTL_INT(_kern_timecounter, OID_AUTO, tick, CTLFLAG_RD, &tc_tick, 0, "");
  662 
  663 void
  664 tc_ticktock(void)
  665 {
  666         static int count;
  667 
  668         if (++count < tc_tick)
  669                 return;
  670         count = 0;
  671         tc_windup();
  672 }
  673 
  674 static void
  675 inittimecounter(void *dummy)
  676 {
  677         u_int p;
  678 
  679         /*
  680          * Set the initial timeout to
  681          * max(1, <approx. number of hardclock ticks in a millisecond>).
  682          * People should probably not use the sysctl to set the timeout
  683          * to smaller than its inital value, since that value is the
  684          * smallest reasonable one.  If they want better timestamps they
  685          * should use the non-"get"* functions.
  686          */
  687         if (hz > 1000)
  688                 tc_tick = (hz + 500) / 1000;
  689         else
  690                 tc_tick = 1;
  691         p = (tc_tick * 1000000) / hz;
  692         printf("Timecounters tick every %d.%03u msec\n", p / 1000, p % 1000);
  693 
  694         /* warm up new timecounter (again) and get rolling. */
  695         (void)timecounter->tc_get_timecount(timecounter);
  696         (void)timecounter->tc_get_timecount(timecounter);
  697 }
  698 
  699 SYSINIT(timecounter, SI_SUB_CLOCKS, SI_ORDER_SECOND, inittimecounter, NULL)

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