Linux-2.6.12-rc2

[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / arch / ppc64 / kernel / time.c

/*
 * 
 * Common time routines among all ppc machines.
 *
 * Written by Cort Dougan (cort@cs.nmt.edu) to merge
 * Paul Mackerras' version and mine for PReP and Pmac.
 * MPC8xx/MBX changes by Dan Malek (dmalek@jlc.net).
 * Converted for 64-bit by Mike Corrigan (mikejc@us.ibm.com)
 *
 * First round of bugfixes by Gabriel Paubert (paubert@iram.es)
 * to make clock more stable (2.4.0-test5). The only thing
 * that this code assumes is that the timebases have been synchronized
 * by firmware on SMP and are never stopped (never do sleep
 * on SMP then, nap and doze are OK).
 * 
 * Speeded up do_gettimeofday by getting rid of references to
 * xtime (which required locks for consistency). (mikejc@us.ibm.com)
 *
 * TODO (not necessarily in this file):
 * - improve precision and reproducibility of timebase frequency
 * measurement at boot time. (for iSeries, we calibrate the timebase
 * against the Titan chip's clock.)
 * - for astronomical applications: add a new function to get
 * non ambiguous timestamps even around leap seconds. This needs
 * a new timestamp format and a good name.
 *
 * 1997-09-10  Updated NTP code according to technical memorandum Jan '96
 *             "A Kernel Model for Precision Timekeeping" by Dave Mills
 *
 *      This program is free software; you can redistribute it and/or
 *      modify it under the terms of the GNU General Public License
 *      as published by the Free Software Foundation; either version
 *      2 of the License, or (at your option) any later version.
 */

#include <linux/config.h>
#include <linux/errno.h>
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/kernel.h>
#include <linux/param.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/interrupt.h>
#include <linux/timex.h>
#include <linux/kernel_stat.h>
#include <linux/mc146818rtc.h>
#include <linux/time.h>
#include <linux/init.h>
#include <linux/profile.h>
#include <linux/cpu.h>
#include <linux/security.h>

#include <asm/segment.h>
#include <asm/io.h>
#include <asm/processor.h>
#include <asm/nvram.h>
#include <asm/cache.h>
#include <asm/machdep.h>
#ifdef CONFIG_PPC_ISERIES
#include <asm/iSeries/ItLpQueue.h>
#include <asm/iSeries/HvCallXm.h>
#endif
#include <asm/uaccess.h>
#include <asm/time.h>
#include <asm/ppcdebug.h>
#include <asm/prom.h>
#include <asm/sections.h>
#include <asm/systemcfg.h>

u64 jiffies_64 __cacheline_aligned_in_smp = INITIAL_JIFFIES;

EXPORT_SYMBOL(jiffies_64);

/* keep track of when we need to update the rtc */
time_t last_rtc_update;
extern int piranha_simulator;
#ifdef CONFIG_PPC_ISERIES
unsigned long iSeries_recal_titan = 0;
unsigned long iSeries_recal_tb = 0; 
static unsigned long first_settimeofday = 1;
#endif

#define XSEC_PER_SEC (1024*1024)

unsigned long tb_ticks_per_jiffy;
unsigned long tb_ticks_per_usec = 100; /* sane default */
EXPORT_SYMBOL(tb_ticks_per_usec);
unsigned long tb_ticks_per_sec;
unsigned long tb_to_xs;
unsigned      tb_to_us;
unsigned long processor_freq;
DEFINE_SPINLOCK(rtc_lock);

unsigned long tb_to_ns_scale;
unsigned long tb_to_ns_shift;

struct gettimeofday_struct do_gtod;

extern unsigned long wall_jiffies;
extern unsigned long lpevent_count;
extern int smp_tb_synchronized;

extern struct timezone sys_tz;

void ppc_adjtimex(void);

static unsigned adjusting_time = 0;

static __inline__ void timer_check_rtc(void)
{
        /*
         * update the rtc when needed, this should be performed on the
         * right fraction of a second. Half or full second ?
         * Full second works on mk48t59 clocks, others need testing.
         * Note that this update is basically only used through 
         * the adjtimex system calls. Setting the HW clock in
         * any other way is a /dev/rtc and userland business.
         * This is still wrong by -0.5/+1.5 jiffies because of the
         * timer interrupt resolution and possible delay, but here we 
         * hit a quantization limit which can only be solved by higher
         * resolution timers and decoupling time management from timer
         * interrupts. This is also wrong on the clocks
         * which require being written at the half second boundary.
         * We should have an rtc call that only sets the minutes and
         * seconds like on Intel to avoid problems with non UTC clocks.
         */
        if ( (time_status & STA_UNSYNC) == 0 &&
             xtime.tv_sec - last_rtc_update >= 659 &&
             abs((xtime.tv_nsec/1000) - (1000000-1000000/HZ)) < 500000/HZ &&
             jiffies - wall_jiffies == 1) {
            struct rtc_time tm;
            to_tm(xtime.tv_sec+1, &tm);
            tm.tm_year -= 1900;
            tm.tm_mon -= 1;
            if (ppc_md.set_rtc_time(&tm) == 0)
                last_rtc_update = xtime.tv_sec+1;
            else
                /* Try again one minute later */
                last_rtc_update += 60;
        }
}

/*
 * This version of gettimeofday has microsecond resolution.
 */
static inline void __do_gettimeofday(struct timeval *tv, unsigned long tb_val)
{
        unsigned long sec, usec, tb_ticks;
        unsigned long xsec, tb_xsec;
        struct gettimeofday_vars * temp_varp;
        unsigned long temp_tb_to_xs, temp_stamp_xsec;

        /*
         * These calculations are faster (gets rid of divides)
         * if done in units of 1/2^20 rather than microseconds.
         * The conversion to microseconds at the end is done
         * without a divide (and in fact, without a multiply)
         */
        temp_varp = do_gtod.varp;
        tb_ticks = tb_val - temp_varp->tb_orig_stamp;
        temp_tb_to_xs = temp_varp->tb_to_xs;
        temp_stamp_xsec = temp_varp->stamp_xsec;
        tb_xsec = mulhdu( tb_ticks, temp_tb_to_xs );
        xsec = temp_stamp_xsec + tb_xsec;
        sec = xsec / XSEC_PER_SEC;
        xsec -= sec * XSEC_PER_SEC;
        usec = (xsec * USEC_PER_SEC)/XSEC_PER_SEC;

        tv->tv_sec = sec;
        tv->tv_usec = usec;
}

void do_gettimeofday(struct timeval *tv)
{
        __do_gettimeofday(tv, get_tb());
}

EXPORT_SYMBOL(do_gettimeofday);

/* Synchronize xtime with do_gettimeofday */ 

static inline void timer_sync_xtime(unsigned long cur_tb)
{
        struct timeval my_tv;

        __do_gettimeofday(&my_tv, cur_tb);

        if (xtime.tv_sec <= my_tv.tv_sec) {
                xtime.tv_sec = my_tv.tv_sec;
                xtime.tv_nsec = my_tv.tv_usec * 1000;
        }
}

/*
 * When the timebase - tb_orig_stamp gets too big, we do a manipulation
 * between tb_orig_stamp and stamp_xsec. The goal here is to keep the
 * difference tb - tb_orig_stamp small enough to always fit inside a
 * 32 bits number. This is a requirement of our fast 32 bits userland
 * implementation in the vdso. If we "miss" a call to this function
 * (interrupt latency, CPU locked in a spinlock, ...) and we end up
 * with a too big difference, then the vdso will fallback to calling
 * the syscall
 */
static __inline__ void timer_recalc_offset(unsigned long cur_tb)
{
        struct gettimeofday_vars * temp_varp;
        unsigned temp_idx;
        unsigned long offset, new_stamp_xsec, new_tb_orig_stamp;

        if (((cur_tb - do_gtod.varp->tb_orig_stamp) & 0x80000000u) == 0)
                return;

        temp_idx = (do_gtod.var_idx == 0);
        temp_varp = &do_gtod.vars[temp_idx];

        new_tb_orig_stamp = cur_tb;
        offset = new_tb_orig_stamp - do_gtod.varp->tb_orig_stamp;
        new_stamp_xsec = do_gtod.varp->stamp_xsec + mulhdu(offset, do_gtod.varp->tb_to_xs);

        temp_varp->tb_to_xs = do_gtod.varp->tb_to_xs;
        temp_varp->tb_orig_stamp = new_tb_orig_stamp;
        temp_varp->stamp_xsec = new_stamp_xsec;
        mb();
        do_gtod.varp = temp_varp;
        do_gtod.var_idx = temp_idx;

        ++(systemcfg->tb_update_count);
        wmb();
        systemcfg->tb_orig_stamp = new_tb_orig_stamp;
        systemcfg->stamp_xsec = new_stamp_xsec;
        wmb();
        ++(systemcfg->tb_update_count);
}

#ifdef CONFIG_SMP
unsigned long profile_pc(struct pt_regs *regs)
{
        unsigned long pc = instruction_pointer(regs);

        if (in_lock_functions(pc))
                return regs->link;

        return pc;
}
EXPORT_SYMBOL(profile_pc);
#endif

#ifdef CONFIG_PPC_ISERIES

/* 
 * This function recalibrates the timebase based on the 49-bit time-of-day
 * value in the Titan chip.  The Titan is much more accurate than the value
 * returned by the service processor for the timebase frequency.  
 */

static void iSeries_tb_recal(void)
{
        struct div_result divres;
        unsigned long titan, tb;
        tb = get_tb();
        titan = HvCallXm_loadTod();
        if ( iSeries_recal_titan ) {
                unsigned long tb_ticks = tb - iSeries_recal_tb;
                unsigned long titan_usec = (titan - iSeries_recal_titan) >> 12;
                unsigned long new_tb_ticks_per_sec   = (tb_ticks * USEC_PER_SEC)/titan_usec;
                unsigned long new_tb_ticks_per_jiffy = (new_tb_ticks_per_sec+(HZ/2))/HZ;
                long tick_diff = new_tb_ticks_per_jiffy - tb_ticks_per_jiffy;
                char sign = '+';                
                /* make sure tb_ticks_per_sec and tb_ticks_per_jiffy are consistent */
                new_tb_ticks_per_sec = new_tb_ticks_per_jiffy * HZ;

                if ( tick_diff < 0 ) {
                        tick_diff = -tick_diff;
                        sign = '-';
                }
                if ( tick_diff ) {
                        if ( tick_diff < tb_ticks_per_jiffy/25 ) {
                                printk( "Titan recalibrate: new tb_ticks_per_jiffy = %lu (%c%ld)\n",
                                                new_tb_ticks_per_jiffy, sign, tick_diff );
                                tb_ticks_per_jiffy = new_tb_ticks_per_jiffy;
                                tb_ticks_per_sec   = new_tb_ticks_per_sec;
                                div128_by_32( XSEC_PER_SEC, 0, tb_ticks_per_sec, &divres );
                                do_gtod.tb_ticks_per_sec = tb_ticks_per_sec;
                                tb_to_xs = divres.result_low;
                                do_gtod.varp->tb_to_xs = tb_to_xs;
                                systemcfg->tb_ticks_per_sec = tb_ticks_per_sec;
                                systemcfg->tb_to_xs = tb_to_xs;
                        }
                        else {
                                printk( "Titan recalibrate: FAILED (difference > 4 percent)\n"
                                        "                   new tb_ticks_per_jiffy = %lu\n"
                                        "                   old tb_ticks_per_jiffy = %lu\n",
                                        new_tb_ticks_per_jiffy, tb_ticks_per_jiffy );
                        }
                }
        }
        iSeries_recal_titan = titan;
        iSeries_recal_tb = tb;
}
#endif

/*
 * For iSeries shared processors, we have to let the hypervisor
 * set the hardware decrementer.  We set a virtual decrementer
 * in the lppaca and call the hypervisor if the virtual
 * decrementer is less than the current value in the hardware
 * decrementer. (almost always the new decrementer value will
 * be greater than the current hardware decementer so the hypervisor
 * call will not be needed)
 */

unsigned long tb_last_stamp __cacheline_aligned_in_smp;

/*
 * timer_interrupt - gets called when the decrementer overflows,
 * with interrupts disabled.
 */
int timer_interrupt(struct pt_regs * regs)
{
        int next_dec;
        unsigned long cur_tb;
        struct paca_struct *lpaca = get_paca();
        unsigned long cpu = smp_processor_id();

        irq_enter();

#ifndef CONFIG_PPC_ISERIES
        profile_tick(CPU_PROFILING, regs);
#endif

        lpaca->lppaca.int_dword.fields.decr_int = 0;

        while (lpaca->next_jiffy_update_tb <= (cur_tb = get_tb())) {
                /*
                 * We cannot disable the decrementer, so in the period
                 * between this cpu's being marked offline in cpu_online_map
                 * and calling stop-self, it is taking timer interrupts.
                 * Avoid calling into the scheduler rebalancing code if this
                 * is the case.
                 */
                if (!cpu_is_offline(cpu))
                        update_process_times(user_mode(regs));
                /*
                 * No need to check whether cpu is offline here; boot_cpuid
                 * should have been fixed up by now.
                 */
                if (cpu == boot_cpuid) {
                        write_seqlock(&xtime_lock);
                        tb_last_stamp = lpaca->next_jiffy_update_tb;
                        timer_recalc_offset(lpaca->next_jiffy_update_tb);
                        do_timer(regs);
                        timer_sync_xtime(lpaca->next_jiffy_update_tb);
                        timer_check_rtc();
                        write_sequnlock(&xtime_lock);
                        if ( adjusting_time && (time_adjust == 0) )
                                ppc_adjtimex();
                }
                lpaca->next_jiffy_update_tb += tb_ticks_per_jiffy;
        }
        
        next_dec = lpaca->next_jiffy_update_tb - cur_tb;
        if (next_dec > lpaca->default_decr)
                next_dec = lpaca->default_decr;
        set_dec(next_dec);

#ifdef CONFIG_PPC_ISERIES
        {
                struct ItLpQueue *lpq = lpaca->lpqueue_ptr;
                if (lpq && ItLpQueue_isLpIntPending(lpq))
                        lpevent_count += ItLpQueue_process(lpq, regs);
        }
#endif

/* collect purr register values often, for accurate calculations */
#if defined(CONFIG_PPC_PSERIES)
        if (cur_cpu_spec->firmware_features & FW_FEATURE_SPLPAR) {
                struct cpu_usage *cu = &__get_cpu_var(cpu_usage_array);
                cu->current_tb = mfspr(SPRN_PURR);
        }
#endif

        irq_exit();

        return 1;
}

/*
 * Scheduler clock - returns current time in nanosec units.
 *
 * Note: mulhdu(a, b) (multiply high double unsigned) returns
 * the high 64 bits of a * b, i.e. (a * b) >> 64, where a and b
 * are 64-bit unsigned numbers.
 */
unsigned long long sched_clock(void)
{
        return mulhdu(get_tb(), tb_to_ns_scale) << tb_to_ns_shift;
}

int do_settimeofday(struct timespec *tv)
{
        time_t wtm_sec, new_sec = tv->tv_sec;
        long wtm_nsec, new_nsec = tv->tv_nsec;
        unsigned long flags;
        unsigned long delta_xsec;
        long int tb_delta;
        unsigned long new_xsec;

        if ((unsigned long)tv->tv_nsec >= NSEC_PER_SEC)
                return -EINVAL;

        write_seqlock_irqsave(&xtime_lock, flags);
        /* Updating the RTC is not the job of this code. If the time is
         * stepped under NTP, the RTC will be update after STA_UNSYNC
         * is cleared. Tool like clock/hwclock either copy the RTC
         * to the system time, in which case there is no point in writing
         * to the RTC again, or write to the RTC but then they don't call
         * settimeofday to perform this operation.
         */
#ifdef CONFIG_PPC_ISERIES
        if ( first_settimeofday ) {
                iSeries_tb_recal();
                first_settimeofday = 0;
        }
#endif
        tb_delta = tb_ticks_since(tb_last_stamp);
        tb_delta += (jiffies - wall_jiffies) * tb_ticks_per_jiffy;

        new_nsec -= tb_delta / tb_ticks_per_usec / 1000;

        wtm_sec  = wall_to_monotonic.tv_sec + (xtime.tv_sec - new_sec);
        wtm_nsec = wall_to_monotonic.tv_nsec + (xtime.tv_nsec - new_nsec);

        set_normalized_timespec(&xtime, new_sec, new_nsec);
        set_normalized_timespec(&wall_to_monotonic, wtm_sec, wtm_nsec);

        /* In case of a large backwards jump in time with NTP, we want the 
         * clock to be updated as soon as the PLL is again in lock.
         */
        last_rtc_update = new_sec - 658;

        time_adjust = 0;                /* stop active adjtime() */
        time_status |= STA_UNSYNC;
        time_maxerror = NTP_PHASE_LIMIT;
        time_esterror = NTP_PHASE_LIMIT;

        delta_xsec = mulhdu( (tb_last_stamp-do_gtod.varp->tb_orig_stamp),
                             do_gtod.varp->tb_to_xs );

        new_xsec = (new_nsec * XSEC_PER_SEC) / NSEC_PER_SEC;
        new_xsec += new_sec * XSEC_PER_SEC;
        if ( new_xsec > delta_xsec ) {
                do_gtod.varp->stamp_xsec = new_xsec - delta_xsec;
                systemcfg->stamp_xsec = new_xsec - delta_xsec;
        }
        else {
                /* This is only for the case where the user is setting the time
                 * way back to a time such that the boot time would have been
                 * before 1970 ... eg. we booted ten days ago, and we are setting
                 * the time to Jan 5, 1970 */
                do_gtod.varp->stamp_xsec = new_xsec;
                do_gtod.varp->tb_orig_stamp = tb_last_stamp;
                systemcfg->stamp_xsec = new_xsec;
                systemcfg->tb_orig_stamp = tb_last_stamp;
        }

        systemcfg->tz_minuteswest = sys_tz.tz_minuteswest;
        systemcfg->tz_dsttime = sys_tz.tz_dsttime;

        write_sequnlock_irqrestore(&xtime_lock, flags);
        clock_was_set();
        return 0;
}

EXPORT_SYMBOL(do_settimeofday);

void __init time_init(void)
{
        /* This function is only called on the boot processor */
        unsigned long flags;
        struct rtc_time tm;
        struct div_result res;
        unsigned long scale, shift;

        ppc_md.calibrate_decr();

        /*
         * Compute scale factor for sched_clock.
         * The calibrate_decr() function has set tb_ticks_per_sec,
         * which is the timebase frequency.
         * We compute 1e9 * 2^64 / tb_ticks_per_sec and interpret
         * the 128-bit result as a 64.64 fixed-point number.
         * We then shift that number right until it is less than 1.0,
         * giving us the scale factor and shift count to use in
         * sched_clock().
         */
        div128_by_32(1000000000, 0, tb_ticks_per_sec, &res);
        scale = res.result_low;
        for (shift = 0; res.result_high != 0; ++shift) {
                scale = (scale >> 1) | (res.result_high << 63);
                res.result_high >>= 1;
        }
        tb_to_ns_scale = scale;
        tb_to_ns_shift = shift;

#ifdef CONFIG_PPC_ISERIES
        if (!piranha_simulator)
#endif
                ppc_md.get_boot_time(&tm);

        write_seqlock_irqsave(&xtime_lock, flags);
        xtime.tv_sec = mktime(tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
                              tm.tm_hour, tm.tm_min, tm.tm_sec);
        tb_last_stamp = get_tb();
        do_gtod.varp = &do_gtod.vars[0];
        do_gtod.var_idx = 0;
        do_gtod.varp->tb_orig_stamp = tb_last_stamp;
        do_gtod.varp->stamp_xsec = xtime.tv_sec * XSEC_PER_SEC;
        do_gtod.tb_ticks_per_sec = tb_ticks_per_sec;
        do_gtod.varp->tb_to_xs = tb_to_xs;
        do_gtod.tb_to_us = tb_to_us;
        systemcfg->tb_orig_stamp = tb_last_stamp;
        systemcfg->tb_update_count = 0;
        systemcfg->tb_ticks_per_sec = tb_ticks_per_sec;
        systemcfg->stamp_xsec = xtime.tv_sec * XSEC_PER_SEC;
        systemcfg->tb_to_xs = tb_to_xs;

        time_freq = 0;

        xtime.tv_nsec = 0;
        last_rtc_update = xtime.tv_sec;
        set_normalized_timespec(&wall_to_monotonic,
                                -xtime.tv_sec, -xtime.tv_nsec);
        write_sequnlock_irqrestore(&xtime_lock, flags);

        /* Not exact, but the timer interrupt takes care of this */
        set_dec(tb_ticks_per_jiffy);
}

/* 
 * After adjtimex is called, adjust the conversion of tb ticks
 * to microseconds to keep do_gettimeofday synchronized 
 * with ntpd.
 *
 * Use the time_adjust, time_freq and time_offset computed by adjtimex to 
 * adjust the frequency.
 */

/* #define DEBUG_PPC_ADJTIMEX 1 */

void ppc_adjtimex(void)
{
        unsigned long den, new_tb_ticks_per_sec, tb_ticks, old_xsec, new_tb_to_xs, new_xsec, new_stamp_xsec;
        unsigned long tb_ticks_per_sec_delta;
        long delta_freq, ltemp;
        struct div_result divres; 
        unsigned long flags;
        struct gettimeofday_vars * temp_varp;
        unsigned temp_idx;
        long singleshot_ppm = 0;

        /* Compute parts per million frequency adjustment to accomplish the time adjustment
           implied by time_offset to be applied over the elapsed time indicated by time_constant.
           Use SHIFT_USEC to get it into the same units as time_freq. */
        if ( time_offset < 0 ) {
                ltemp = -time_offset;
                ltemp <<= SHIFT_USEC - SHIFT_UPDATE;
                ltemp >>= SHIFT_KG + time_constant;
                ltemp = -ltemp;
        }
        else {
                ltemp = time_offset;
                ltemp <<= SHIFT_USEC - SHIFT_UPDATE;
                ltemp >>= SHIFT_KG + time_constant;
        }
        
        /* If there is a single shot time adjustment in progress */
        if ( time_adjust ) {
#ifdef DEBUG_PPC_ADJTIMEX
                printk("ppc_adjtimex: ");
                if ( adjusting_time == 0 )
                        printk("starting ");
                printk("single shot time_adjust = %ld\n", time_adjust);
#endif  
        
                adjusting_time = 1;
                
                /* Compute parts per million frequency adjustment to match time_adjust */
                singleshot_ppm = tickadj * HZ;  
                /*
                 * The adjustment should be tickadj*HZ to match the code in
                 * linux/kernel/timer.c, but experiments show that this is too
                 * large. 3/4 of tickadj*HZ seems about right
                 */
                singleshot_ppm -= singleshot_ppm / 4;
                /* Use SHIFT_USEC to get it into the same units as time_freq */ 
                singleshot_ppm <<= SHIFT_USEC;
                if ( time_adjust < 0 )
                        singleshot_ppm = -singleshot_ppm;
        }
        else {
#ifdef DEBUG_PPC_ADJTIMEX
                if ( adjusting_time )
                        printk("ppc_adjtimex: ending single shot time_adjust\n");
#endif
                adjusting_time = 0;
        }
        
        /* Add up all of the frequency adjustments */
        delta_freq = time_freq + ltemp + singleshot_ppm;
        
        /* Compute a new value for tb_ticks_per_sec based on the frequency adjustment */
        den = 1000000 * (1 << (SHIFT_USEC - 8));
        if ( delta_freq < 0 ) {
                tb_ticks_per_sec_delta = ( tb_ticks_per_sec * ( (-delta_freq) >> (SHIFT_USEC - 8))) / den;
                new_tb_ticks_per_sec = tb_ticks_per_sec + tb_ticks_per_sec_delta;
        }
        else {
                tb_ticks_per_sec_delta = ( tb_ticks_per_sec * ( delta_freq >> (SHIFT_USEC - 8))) / den;
                new_tb_ticks_per_sec = tb_ticks_per_sec - tb_ticks_per_sec_delta;
        }
        
#ifdef DEBUG_PPC_ADJTIMEX
        printk("ppc_adjtimex: ltemp = %ld, time_freq = %ld, singleshot_ppm = %ld\n", ltemp, time_freq, singleshot_ppm);
        printk("ppc_adjtimex: tb_ticks_per_sec - base = %ld  new = %ld\n", tb_ticks_per_sec, new_tb_ticks_per_sec);
#endif
                                
        /* Compute a new value of tb_to_xs (used to convert tb to microseconds and a new value of 
           stamp_xsec which is the time (in 1/2^20 second units) corresponding to tb_orig_stamp.  This 
           new value of stamp_xsec compensates for the change in frequency (implied by the new tb_to_xs)
           which guarantees that the current time remains the same */ 
        write_seqlock_irqsave( &xtime_lock, flags );
        tb_ticks = get_tb() - do_gtod.varp->tb_orig_stamp;
        div128_by_32( 1024*1024, 0, new_tb_ticks_per_sec, &divres );
        new_tb_to_xs = divres.result_low;
        new_xsec = mulhdu( tb_ticks, new_tb_to_xs );

        old_xsec = mulhdu( tb_ticks, do_gtod.varp->tb_to_xs );
        new_stamp_xsec = do_gtod.varp->stamp_xsec + old_xsec - new_xsec;

        /* There are two copies of tb_to_xs and stamp_xsec so that no lock is needed to access and use these
           values in do_gettimeofday.  We alternate the copies and as long as a reasonable time elapses between
           changes, there will never be inconsistent values.  ntpd has a minimum of one minute between updates */

        temp_idx = (do_gtod.var_idx == 0);
        temp_varp = &do_gtod.vars[temp_idx];

        temp_varp->tb_to_xs = new_tb_to_xs;
        temp_varp->stamp_xsec = new_stamp_xsec;
        temp_varp->tb_orig_stamp = do_gtod.varp->tb_orig_stamp;
        mb();
        do_gtod.varp = temp_varp;
        do_gtod.var_idx = temp_idx;

        /*
         * tb_update_count is used to allow the problem state gettimeofday code
         * to assure itself that it sees a consistent view of the tb_to_xs and
         * stamp_xsec variables.  It reads the tb_update_count, then reads
         * tb_to_xs and stamp_xsec and then reads tb_update_count again.  If
         * the two values of tb_update_count match and are even then the
         * tb_to_xs and stamp_xsec values are consistent.  If not, then it
         * loops back and reads them again until this criteria is met.
         */
        ++(systemcfg->tb_update_count);
        wmb();
        systemcfg->tb_to_xs = new_tb_to_xs;
        systemcfg->stamp_xsec = new_stamp_xsec;
        wmb();
        ++(systemcfg->tb_update_count);

        write_sequnlock_irqrestore( &xtime_lock, flags );

}


#define TICK_SIZE tick
#define FEBRUARY        2
#define STARTOFTIME     1970
#define SECDAY          86400L
#define SECYR           (SECDAY * 365)
#define leapyear(year)          ((year) % 4 == 0)
#define days_in_year(a)         (leapyear(a) ? 366 : 365)
#define days_in_month(a)        (month_days[(a) - 1])

static int month_days[12] = {
        31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
};

/*
 * This only works for the Gregorian calendar - i.e. after 1752 (in the UK)
 */
void GregorianDay(struct rtc_time * tm)
{
        int leapsToDate;
        int lastYear;
        int day;
        int MonthOffset[] = { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };

        lastYear=tm->tm_year-1;

        /*
         * Number of leap corrections to apply up to end of last year
         */
        leapsToDate = lastYear/4 - lastYear/100 + lastYear/400;

        /*
         * This year is a leap year if it is divisible by 4 except when it is
         * divisible by 100 unless it is divisible by 400
         *
         * e.g. 1904 was a leap year, 1900 was not, 1996 is, and 2000 will be
         */
        if((tm->tm_year%4==0) &&
           ((tm->tm_year%100!=0) || (tm->tm_year%400==0)) &&
           (tm->tm_mon>2))
        {
                /*
                 * We are past Feb. 29 in a leap year
                 */
                day=1;
        }
        else
        {
                day=0;
        }

        day += lastYear*365 + leapsToDate + MonthOffset[tm->tm_mon-1] +
                   tm->tm_mday;

        tm->tm_wday=day%7;
}

void to_tm(int tim, struct rtc_time * tm)
{
        register int    i;
        register long   hms, day;

        day = tim / SECDAY;
        hms = tim % SECDAY;

        /* Hours, minutes, seconds are easy */
        tm->tm_hour = hms / 3600;
        tm->tm_min = (hms % 3600) / 60;
        tm->tm_sec = (hms % 3600) % 60;

        /* Number of years in days */
        for (i = STARTOFTIME; day >= days_in_year(i); i++)
                day -= days_in_year(i);
        tm->tm_year = i;

        /* Number of months in days left */
        if (leapyear(tm->tm_year))
                days_in_month(FEBRUARY) = 29;
        for (i = 1; day >= days_in_month(i); i++)
                day -= days_in_month(i);
        days_in_month(FEBRUARY) = 28;
        tm->tm_mon = i;

        /* Days are what is left over (+1) from all that. */
        tm->tm_mday = day + 1;

        /*
         * Determine the day of week
         */
        GregorianDay(tm);
}

/* Auxiliary function to compute scaling factors */
/* Actually the choice of a timebase running at 1/4 the of the bus
 * frequency giving resolution of a few tens of nanoseconds is quite nice.
 * It makes this computation very precise (27-28 bits typically) which
 * is optimistic considering the stability of most processor clock
 * oscillators and the precision with which the timebase frequency
 * is measured but does not harm.
 */
unsigned mulhwu_scale_factor(unsigned inscale, unsigned outscale) {
        unsigned mlt=0, tmp, err;
        /* No concern for performance, it's done once: use a stupid
         * but safe and compact method to find the multiplier.
         */
  
        for (tmp = 1U<<31; tmp != 0; tmp >>= 1) {
                if (mulhwu(inscale, mlt|tmp) < outscale) mlt|=tmp;
        }
  
        /* We might still be off by 1 for the best approximation.
         * A side effect of this is that if outscale is too large
         * the returned value will be zero.
         * Many corner cases have been checked and seem to work,
         * some might have been forgotten in the test however.
         */
  
        err = inscale*(mlt+1);
        if (err <= inscale/2) mlt++;
        return mlt;
  }

/*
 * Divide a 128-bit dividend by a 32-bit divisor, leaving a 128 bit
 * result.
 */

void div128_by_32( unsigned long dividend_high, unsigned long dividend_low,
                   unsigned divisor, struct div_result *dr )
{
        unsigned long a,b,c,d, w,x,y,z, ra,rb,rc;

        a = dividend_high >> 32;
        b = dividend_high & 0xffffffff;
        c = dividend_low >> 32;
        d = dividend_low & 0xffffffff;

        w = a/divisor;
        ra = (a - (w * divisor)) << 32;

        x = (ra + b)/divisor;
        rb = ((ra + b) - (x * divisor)) << 32;

        y = (rb + c)/divisor;
        rc = ((rb + b) - (y * divisor)) << 32;

        z = (rc + d)/divisor;

        dr->result_high = (w << 32) + x;
        dr->result_low  = (y << 32) + z;

}