[apple/xnu.git] / osfmk / kern / clock.c

/*
 * Copyright (c) 2000-2008 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 * 
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */
/*
 * @OSF_COPYRIGHT@
 */
/*
 */

#include <mach/mach_types.h>

#include <kern/lock.h>
#include <kern/spl.h>
#include <kern/sched_prim.h>
#include <kern/thread.h>
#include <kern/clock.h>
#include <kern/host_notify.h>

#include <IOKit/IOPlatformExpert.h>

#include <machine/commpage.h>

#include <mach/mach_traps.h>
#include <mach/mach_time.h>

uint32_t        hz_tick_interval = 1;


decl_simple_lock_data(,clock_lock)

#define clock_lock()    \
        simple_lock(&clock_lock)

#define clock_unlock()  \
        simple_unlock(&clock_lock)

#define clock_lock_init()       \
        simple_lock_init(&clock_lock, 0)


/*
 *      Time of day (calendar) variables.
 *
 *      Algorithm:
 *
 *      TOD <- (seconds + epoch, fraction) <- CONV(current absolute time + offset)
 *
 *      where CONV converts absolute time units into seconds and a fraction.
 */
static struct clock_calend {
        uint64_t        epoch;
        uint64_t        offset;

        int32_t         adjdelta;       /* Nanosecond time delta for this adjustment period */
        uint64_t        adjstart;       /* Absolute time value for start of this adjustment period */
        uint32_t        adjoffset;      /* Absolute time offset for this adjustment period as absolute value */
} clock_calend;

#if     CONFIG_DTRACE

/*
 *      Unlocked calendar flipflop; this is used to track a clock_calend such
 *      that we can safely access a snapshot of a valid  clock_calend structure
 *      without needing to take any locks to do it.
 *
 *      The trick is to use a generation count and set the low bit when it is
 *      being updated/read; by doing this, we guarantee, through use of the
 *      hw_atomic functions, that the generation is incremented when the bit
 *      is cleared atomically (by using a 1 bit add).
 */
static struct unlocked_clock_calend {
        struct clock_calend     calend;         /* copy of calendar */
        uint32_t                gen;            /* generation count */
} flipflop[ 2];

static void clock_track_calend_nowait(void);

#endif

/*
 *      Calendar adjustment variables and values.
 */
#define calend_adjperiod        (NSEC_PER_SEC / 100)    /* adjustment period, ns */
#define calend_adjskew          (40 * NSEC_PER_USEC)    /* "standard" skew, ns / period */
#define calend_adjbig           (NSEC_PER_SEC)                  /* use 10x skew above adjbig ns */

static int64_t                          calend_adjtotal;                /* Nanosecond remaining total adjustment */
static uint64_t                         calend_adjdeadline;             /* Absolute time value for next adjustment period */
static uint32_t                         calend_adjinterval;             /* Absolute time interval of adjustment period */

static timer_call_data_t        calend_adjcall;
static uint32_t                         calend_adjactive;

static uint32_t         calend_set_adjustment(
                                                long                    *secs,
                                                int                             *microsecs);

static void                     calend_adjust_call(void);
static uint32_t         calend_adjust(void);

static thread_call_data_t       calend_wakecall;

extern  void    IOKitResetTime(void);

void _clock_delay_until_deadline(uint64_t               interval,
                                                                 uint64_t               deadline);

static uint64_t         clock_boottime;                         /* Seconds boottime epoch */

#define TIME_ADD(rsecs, secs, rfrac, frac, unit)        \
MACRO_BEGIN                                                                                     \
        if (((rfrac) += (frac)) >= (unit)) {                    \
                (rfrac) -= (unit);                                                      \
                (rsecs) += 1;                                                           \
        }                                                                                               \
        (rsecs) += (secs);                                                              \
MACRO_END

#define TIME_SUB(rsecs, secs, rfrac, frac, unit)        \
MACRO_BEGIN                                                                                     \
        if ((int)((rfrac) -= (frac)) < 0) {                             \
                (rfrac) += (unit);                                                      \
                (rsecs) -= 1;                                                           \
        }                                                                                               \
        (rsecs) -= (secs);                                                              \
MACRO_END

/*
 *      clock_config:
 *
 *      Called once at boot to configure the clock subsystem.
 */
void
clock_config(void)
{
        clock_lock_init();

        timer_call_setup(&calend_adjcall, (timer_call_func_t)calend_adjust_call, NULL);
        thread_call_setup(&calend_wakecall, (thread_call_func_t)IOKitResetTime, NULL);

        clock_oldconfig();
}

/*
 *      clock_init:
 *
 *      Called on a processor each time started.
 */
void
clock_init(void)
{
        clock_oldinit();
}

/*
 *      clock_timebase_init:
 *
 *      Called by machine dependent code
 *      to initialize areas dependent on the
 *      timebase value.  May be called multiple
 *      times during start up.
 */
void
clock_timebase_init(void)
{
        uint64_t        abstime;

        nanoseconds_to_absolutetime(calend_adjperiod, &abstime);
        calend_adjinterval = (uint32_t)abstime;

        nanoseconds_to_absolutetime(NSEC_PER_SEC / 100, &abstime);
        hz_tick_interval = (uint32_t)abstime;

        sched_timebase_init();
}

/*
 *      mach_timebase_info_trap:
 *
 *      User trap returns timebase constant.
 */
kern_return_t
mach_timebase_info_trap(
        struct mach_timebase_info_trap_args *args)
{
        mach_vm_address_t                       out_info_addr = args->info;
        mach_timebase_info_data_t       info;

        clock_timebase_info(&info);

        copyout((void *)&info, out_info_addr, sizeof (info));

        return (KERN_SUCCESS);
}

/*
 *      Calendar routines.
 */

/*
 *      clock_get_calendar_microtime:
 *
 *      Returns the current calendar value,
 *      microseconds as the fraction.
 */
void
clock_get_calendar_microtime(
        clock_sec_t                     *secs,
        clock_usec_t            *microsecs)
{
        clock_get_calendar_absolute_and_microtime(secs, microsecs, NULL);
}

/*
 *      clock_get_calendar_absolute_and_microtime:
 *
 *      Returns the current calendar value,
 *      microseconds as the fraction. Also
 *      returns mach_absolute_time if abstime
 *      is not NULL.
 */
void
clock_get_calendar_absolute_and_microtime(
        clock_sec_t                     *secs,
        clock_usec_t            *microsecs,
        uint64_t                *abstime)
{
        uint64_t                now;
        spl_t                   s;

        s = splclock();
        clock_lock();

        now = mach_absolute_time();
        if (abstime)
                *abstime = now;

        if (clock_calend.adjdelta < 0) {
                uint32_t        t32;

                /* 
                 * Since offset is decremented during a negative adjustment,
                 * ensure that time increases monotonically without going
                 * temporarily backwards.
                 * If the delta has not yet passed, now is set to the start
                 * of the current adjustment period; otherwise, we're between
                 * the expiry of the delta and the next call to calend_adjust(),
                 * and we offset accordingly.
                 */
                if (now > clock_calend.adjstart) {
                        t32 = (uint32_t)(now - clock_calend.adjstart);

                        if (t32 > clock_calend.adjoffset)
                                now -= clock_calend.adjoffset;
                        else
                                now = clock_calend.adjstart;
                }
        }

        now += clock_calend.offset;

        absolutetime_to_microtime(now, secs, microsecs);

        *secs += (clock_sec_t)clock_calend.epoch;

        clock_unlock();
        splx(s);
}

/*
 *      clock_get_calendar_nanotime:
 *
 *      Returns the current calendar value,
 *      nanoseconds as the fraction.
 *
 *      Since we do not have an interface to
 *      set the calendar with resolution greater
 *      than a microsecond, we honor that here.
 */
void
clock_get_calendar_nanotime(
        clock_sec_t                     *secs,
        clock_nsec_t            *nanosecs)
{
        uint64_t                now;
        spl_t                   s;

        s = splclock();
        clock_lock();

        now = mach_absolute_time();

        if (clock_calend.adjdelta < 0) {
                uint32_t        t32;

                if (now > clock_calend.adjstart) {
                        t32 = (uint32_t)(now - clock_calend.adjstart);

                        if (t32 > clock_calend.adjoffset)
                                now -= clock_calend.adjoffset;
                        else
                                now = clock_calend.adjstart;
                }
        }

        now += clock_calend.offset;

        absolutetime_to_microtime(now, secs, nanosecs);

        *nanosecs *= NSEC_PER_USEC;

        *secs += (clock_sec_t)clock_calend.epoch;

        clock_unlock();
        splx(s);
}

/*
 *      clock_gettimeofday:
 *
 *      Kernel interface for commpage implementation of
 *      gettimeofday() syscall.
 *
 *      Returns the current calendar value, and updates the
 *      commpage info as appropriate.  Because most calls to
 *      gettimeofday() are handled in user mode by the commpage,
 *      this routine should be used infrequently.
 */
void
clock_gettimeofday(
        clock_sec_t             *secs,
        clock_usec_t    *microsecs)
{
        uint64_t                now;
        spl_t                   s;

        s = splclock();
        clock_lock();

        now = mach_absolute_time();

        if (clock_calend.adjdelta >= 0) {
                clock_gettimeofday_set_commpage(now, clock_calend.epoch, clock_calend.offset, secs, microsecs);
        }
        else {
                uint32_t        t32;

                if (now > clock_calend.adjstart) {
                        t32 = (uint32_t)(now - clock_calend.adjstart);

                        if (t32 > clock_calend.adjoffset)
                                now -= clock_calend.adjoffset;
                        else
                                now = clock_calend.adjstart;
                }

                now += clock_calend.offset;

                absolutetime_to_microtime(now, secs, microsecs);

                *secs += (clock_sec_t)clock_calend.epoch;
        }

        clock_unlock();
        splx(s);
}

/*
 *      clock_set_calendar_microtime:
 *
 *      Sets the current calendar value by
 *      recalculating the epoch and offset
 *      from the system clock.
 *
 *      Also adjusts the boottime to keep the
 *      value consistent, writes the new
 *      calendar value to the platform clock,
 *      and sends calendar change notifications.
 */
void
clock_set_calendar_microtime(
        clock_sec_t                     secs,
        clock_usec_t            microsecs)
{
        clock_sec_t                     sys;
        clock_usec_t            microsys;
        clock_sec_t                     newsecs;
        spl_t                           s;

        newsecs = (microsecs < 500*USEC_PER_SEC)? secs: secs + 1;

        s = splclock();
        clock_lock();

        commpage_disable_timestamp();

        /*
         *      Calculate the new calendar epoch based on
         *      the new value and the system clock.
         */
        clock_get_system_microtime(&sys, &microsys);
        TIME_SUB(secs, sys, microsecs, microsys, USEC_PER_SEC);

        /*
         *      Adjust the boottime based on the delta.
         */
        clock_boottime += secs - clock_calend.epoch;

        /*
         *      Set the new calendar epoch.
         */
        clock_calend.epoch = secs;

        nanoseconds_to_absolutetime((uint64_t)microsecs * NSEC_PER_USEC, &clock_calend.offset);

        /*
         *      Cancel any adjustment in progress.
         */
        calend_adjtotal = clock_calend.adjdelta = 0;

        clock_unlock();

        /*
         *      Set the new value for the platform clock.
         */
        PESetGMTTimeOfDay(newsecs);

        splx(s);

        /*
         *      Send host notifications.
         */
        host_notify_calendar_change();
        
#if CONFIG_DTRACE
        clock_track_calend_nowait();
#endif
}

/*
 *      clock_initialize_calendar:
 *
 *      Set the calendar and related clocks
 *      from the platform clock at boot or
 *      wake event.
 *
 *      Also sends host notifications.
 */
void
clock_initialize_calendar(void)
{
        clock_sec_t                     sys, secs = PEGetGMTTimeOfDay();
        clock_usec_t            microsys, microsecs = 0;
        spl_t                           s;

        s = splclock();
        clock_lock();

        commpage_disable_timestamp();

        if ((long)secs >= (long)clock_boottime) {
                /*
                 *      Initialize the boot time based on the platform clock.
                 */
                if (clock_boottime == 0)
                        clock_boottime = secs;

                /*
                 *      Calculate the new calendar epoch based on
                 *      the platform clock and the system clock.
                 */
                clock_get_system_microtime(&sys, &microsys);
                TIME_SUB(secs, sys, microsecs, microsys, USEC_PER_SEC);

                /*
                 *      Set the new calendar epoch.
                 */
                clock_calend.epoch = secs;

                nanoseconds_to_absolutetime((uint64_t)microsecs * NSEC_PER_USEC, &clock_calend.offset);

                /*
                 *       Cancel any adjustment in progress.
                 */
                calend_adjtotal = clock_calend.adjdelta = 0;
        }

        clock_unlock();
        splx(s);

        /*
         *      Send host notifications.
         */
        host_notify_calendar_change();
        
#if CONFIG_DTRACE
        clock_track_calend_nowait();
#endif
}

/*
 *      clock_get_boottime_nanotime:
 *
 *      Return the boottime, used by sysctl.
 */
void
clock_get_boottime_nanotime(
        clock_sec_t                     *secs,
        clock_nsec_t            *nanosecs)
{
        spl_t   s;

        s = splclock();
        clock_lock();

        *secs = (clock_sec_t)clock_boottime;
        *nanosecs = 0;

        clock_unlock();
        splx(s);
}

/*
 *      clock_adjtime:
 *
 *      Interface to adjtime() syscall.
 *
 *      Calculates adjustment variables and
 *      initiates adjustment.
 */
void
clock_adjtime(
        long            *secs,
        int                     *microsecs)
{
        uint32_t        interval;
        spl_t           s;

        s = splclock();
        clock_lock();

        interval = calend_set_adjustment(secs, microsecs);
        if (interval != 0) {
                calend_adjdeadline = mach_absolute_time() + interval;
                if (!timer_call_enter(&calend_adjcall, calend_adjdeadline, TIMER_CALL_SYS_CRITICAL))
                        calend_adjactive++;
        }
        else
        if (timer_call_cancel(&calend_adjcall))
                calend_adjactive--;

        clock_unlock();
        splx(s);
}

static uint32_t
calend_set_adjustment(
        long                    *secs,
        int                             *microsecs)
{
        uint64_t                now, t64;
        int64_t                 total, ototal;
        uint32_t                interval = 0;

        /* 
         * Compute the total adjustment time in nanoseconds.
         */
        total = ((int64_t)*secs * (int64_t)NSEC_PER_SEC) + (*microsecs * (int64_t)NSEC_PER_USEC);

        /* 
         * Disable commpage gettimeofday().
         */
        commpage_disable_timestamp();

        /* 
         * Get current absolute time.
         */
        now = mach_absolute_time();

        /* 
         * Save the old adjustment total for later return.
         */
        ototal = calend_adjtotal;

        /*
         * Is a new correction specified?
         */
        if (total != 0) {
                /*
                 * Set delta to the standard, small, adjustment skew.
                 */
                int32_t         delta = calend_adjskew;

                if (total > 0) {
                        /*
                         * Positive adjustment. If greater than the preset 'big' 
                         * threshold, slew at a faster rate, capping if necessary.
                         */
                        if (total > (int64_t) calend_adjbig)
                                delta *= 10;
                        if (delta > total)
                                delta = (int32_t)total;

                        /* 
                         * Convert the delta back from ns to absolute time and store in adjoffset.
                         */
                        nanoseconds_to_absolutetime((uint64_t)delta, &t64);
                        clock_calend.adjoffset = (uint32_t)t64;
                }
                else {
                        /*
                         * Negative adjustment; therefore, negate the delta. If 
                         * greater than the preset 'big' threshold, slew at a faster 
                         * rate, capping if necessary.
                         */
                        if (total < (int64_t) -calend_adjbig)
                                delta *= 10;
                        delta = -delta;
                        if (delta < total)
                                delta = (int32_t)total;

                        /* 
                         * Save the current absolute time. Subsequent time operations occuring
                         * during this negative correction can make use of this value to ensure 
                         * that time increases monotonically.
                         */
                        clock_calend.adjstart = now;

                        /* 
                         * Convert the delta back from ns to absolute time and store in adjoffset.
                         */
                        nanoseconds_to_absolutetime((uint64_t)-delta, &t64);
                        clock_calend.adjoffset = (uint32_t)t64;
                }

                /* 
                 * Store the total adjustment time in ns. 
                 */
                calend_adjtotal = total;
                
                /* 
                 * Store the delta for this adjustment period in ns. 
                 */
                clock_calend.adjdelta = delta;

                /* 
                 * Set the interval in absolute time for later return. 
                 */
                interval = calend_adjinterval;
        }
        else {
                /* 
                 * No change; clear any prior adjustment.
                 */
                calend_adjtotal = clock_calend.adjdelta = 0;
        }

        /* 
         * If an prior correction was in progress, return the
         * remaining uncorrected time from it. 
         */
        if (ototal != 0) {
                *secs = (long)(ototal / (long)NSEC_PER_SEC);
                *microsecs = (int)((ototal % (int)NSEC_PER_SEC) / (int)NSEC_PER_USEC);
        }
        else
                *secs = *microsecs = 0;

#if CONFIG_DTRACE
        clock_track_calend_nowait();
#endif
        
        return (interval);
}

static void
calend_adjust_call(void)
{
        uint32_t        interval;
        spl_t           s;

        s = splclock();
        clock_lock();

        if (--calend_adjactive == 0) {
                interval = calend_adjust();
                if (interval != 0) {
                        clock_deadline_for_periodic_event(interval, mach_absolute_time(), &calend_adjdeadline);

                        if (!timer_call_enter(&calend_adjcall, calend_adjdeadline, TIMER_CALL_SYS_CRITICAL))
                                calend_adjactive++;
                }
        }

        clock_unlock();
        splx(s);
}

static uint32_t
calend_adjust(void)
{
        uint64_t                now, t64;
        int32_t                 delta;
        uint32_t                interval = 0;

        commpage_disable_timestamp();

        now = mach_absolute_time();

        delta = clock_calend.adjdelta;

        if (delta > 0) {
                clock_calend.offset += clock_calend.adjoffset;

                calend_adjtotal -= delta;
                if (delta > calend_adjtotal) {
                        clock_calend.adjdelta = delta = (int32_t)calend_adjtotal;

                        nanoseconds_to_absolutetime((uint64_t)delta, &t64);
                        clock_calend.adjoffset = (uint32_t)t64;
                }
        }
        else
                if (delta < 0) {
                        clock_calend.offset -= clock_calend.adjoffset;

                        calend_adjtotal -= delta;
                        if (delta < calend_adjtotal) {
                                clock_calend.adjdelta = delta = (int32_t)calend_adjtotal;

                                nanoseconds_to_absolutetime((uint64_t)-delta, &t64);
                                clock_calend.adjoffset = (uint32_t)t64;
                        }

                        if (clock_calend.adjdelta != 0)
                                clock_calend.adjstart = now;
                }

        if (clock_calend.adjdelta != 0)
                interval = calend_adjinterval;

#if CONFIG_DTRACE
        clock_track_calend_nowait();
#endif

        return (interval);
}

/*
 *      clock_wakeup_calendar:
 *
 *      Interface to power management, used
 *      to initiate the reset of the calendar
 *      on wake from sleep event.
 */
void
clock_wakeup_calendar(void)
{
        thread_call_enter(&calend_wakecall);
}

/*
 *      Wait / delay routines.
 */
static void
mach_wait_until_continue(
        __unused void   *parameter,
        wait_result_t   wresult)
{
        thread_syscall_return((wresult == THREAD_INTERRUPTED)? KERN_ABORTED: KERN_SUCCESS);
        /*NOTREACHED*/
}

/*
 * mach_wait_until_trap: Suspend execution of calling thread until the specified time has passed
 *
 * Parameters:    args->deadline          Amount of time to wait
 *
 * Returns:        0                      Success
 *                !0                      Not success           
 *
 */
kern_return_t
mach_wait_until_trap(
        struct mach_wait_until_trap_args        *args)
{
        uint64_t                deadline = args->deadline;
        wait_result_t   wresult;

        wresult = assert_wait_deadline_with_leeway((event_t)mach_wait_until_trap, THREAD_ABORTSAFE,
                                                   TIMEOUT_URGENCY_USER_NORMAL, deadline, 0);
        if (wresult == THREAD_WAITING)
                wresult = thread_block(mach_wait_until_continue);

        return ((wresult == THREAD_INTERRUPTED)? KERN_ABORTED: KERN_SUCCESS);
}

void
clock_delay_until(
        uint64_t                deadline)
{
        uint64_t                now = mach_absolute_time();

        if (now >= deadline)
                return;

        _clock_delay_until_deadline(deadline - now, deadline);
}

/*
 * Preserve the original precise interval that the client
 * requested for comparison to the spin threshold.
 */
void
_clock_delay_until_deadline(
        uint64_t                interval,
        uint64_t                deadline)
{

        if (interval == 0)
                return;

        if (    ml_delay_should_spin(interval)  ||
                        get_preemption_level() != 0                             ||
                        ml_get_interrupts_enabled() == FALSE    ) {
                machine_delay_until(interval, deadline);
        } else {
                assert_wait_deadline((event_t)clock_delay_until, THREAD_UNINT, deadline);

                thread_block(THREAD_CONTINUE_NULL);
        }
}


void
delay_for_interval(
        uint32_t                interval,
        uint32_t                scale_factor)
{
        uint64_t                abstime;

        clock_interval_to_absolutetime_interval(interval, scale_factor, &abstime);

        _clock_delay_until_deadline(abstime, mach_absolute_time() + abstime);
}

void
delay(
        int             usec)
{
        delay_for_interval((usec < 0)? -usec: usec, NSEC_PER_USEC);
}

/*
 *      Miscellaneous routines.
 */
void
clock_interval_to_deadline(
        uint32_t                        interval,
        uint32_t                        scale_factor,
        uint64_t                        *result)
{
        uint64_t        abstime;

        clock_interval_to_absolutetime_interval(interval, scale_factor, &abstime);

        *result = mach_absolute_time() + abstime;
}

void
clock_absolutetime_interval_to_deadline(
        uint64_t                        abstime,
        uint64_t                        *result)
{
        *result = mach_absolute_time() + abstime;
}

void
clock_get_uptime(
        uint64_t        *result)
{
        *result = mach_absolute_time();
}

void
clock_deadline_for_periodic_event(
        uint64_t                        interval,
        uint64_t                        abstime,
        uint64_t                        *deadline)
{
        assert(interval != 0);

        *deadline += interval;

        if (*deadline <= abstime) {
                *deadline = abstime + interval;
                abstime = mach_absolute_time();

                if (*deadline <= abstime)
                        *deadline = abstime + interval;
        }
}

#if     CONFIG_DTRACE

/*
 * clock_get_calendar_nanotime_nowait
 *
 * Description: Non-blocking version of clock_get_calendar_nanotime()
 *
 * Notes:       This function operates by separately tracking calendar time
 *              updates using a two element structure to copy the calendar
 *              state, which may be asynchronously modified.  It utilizes
 *              barrier instructions in the tracking process and in the local
 *              stable snapshot process in order to ensure that a consistent
 *              snapshot is used to perform the calculation.
 */
void
clock_get_calendar_nanotime_nowait(
        clock_sec_t                     *secs,
        clock_nsec_t            *nanosecs)
{
        int i = 0;
        uint64_t                now;
        struct unlocked_clock_calend stable;

        for (;;) {
                stable = flipflop[i];           /* take snapshot */

                /*
                 * Use a barrier instructions to ensure atomicity.  We AND
                 * off the "in progress" bit to get the current generation
                 * count.
                 */
                (void)hw_atomic_and(&stable.gen, ~(uint32_t)1);

                /*
                 * If an update _is_ in progress, the generation count will be
                 * off by one, if it _was_ in progress, it will be off by two,
                 * and if we caught it at a good time, it will be equal (and
                 * our snapshot is threfore stable).
                 */
                if (flipflop[i].gen == stable.gen)
                        break;

                /* Switch to the oher element of the flipflop, and try again. */
                i ^= 1;
        }

        now = mach_absolute_time();

        if (stable.calend.adjdelta < 0) {
                uint32_t        t32;

                if (now > stable.calend.adjstart) {
                        t32 = (uint32_t)(now - stable.calend.adjstart);

                        if (t32 > stable.calend.adjoffset)
                                now -= stable.calend.adjoffset;
                        else
                                now = stable.calend.adjstart;
                }
        }

        now += stable.calend.offset;

        absolutetime_to_microtime(now, secs, nanosecs);
        *nanosecs *= NSEC_PER_USEC;

        *secs += (clock_sec_t)stable.calend.epoch;
}

static void 
clock_track_calend_nowait(void)
{
        int i;

        for (i = 0; i < 2; i++) {
                struct clock_calend tmp = clock_calend;

                /*
                 * Set the low bit if the generation count; since we use a
                 * barrier instruction to do this, we are guaranteed that this
                 * will flag an update in progress to an async caller trying
                 * to examine the contents.
                 */
                (void)hw_atomic_or(&flipflop[i].gen, 1);

                flipflop[i].calend = tmp;

                /*
                 * Increment the generation count to clear the low bit to
                 * signal completion.  If a caller compares the generation
                 * count after taking a copy while in progress, the count
                 * will be off by two.
                 */
                (void)hw_atomic_add(&flipflop[i].gen, 1);
        }
}

#endif  /* CONFIG_DTRACE */