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SubjectTime warps, current status.

So far i have found following problems that cause time warps:

- timer interrupt updating jiffies, but not xtime
- latencies bigger than LATCH/100
- some hardware is not providing monotonous timer ticks, small "jumps"
exist. Some motherboards do this, some not. No idea why this happens.
here is a log:

jiffie,counter
J:8623 C:8725 |t
J:8623 C:8676 |i
J:8623 C:8627 |m
J:8623 C:8508 |e
J:8623 C:8525 <------- here the timer counter jumps >upwards<
J:8623 C:8465 |
J:8623 C:5834 |
J:8623 C:5767 V

the attached time.c is from 1.99.4, and fixes all these problems, but
other problems can exist too. A program that detects time warps (posted
earlier on this list) is attached too.

-- mingo
/*
* linux/arch/i386/kernel/time.c
*
* Copyright (C) 1991, 1992, 1995 Linus Torvalds
*
* This file contains the PC-specific time handling details:
* reading the RTC at bootup, etc..
* 1994-07-02 Alan Modra
* fixed set_rtc_mmss, fixed time.year for >= 2000, new mktime
* 1995-03-26 Markus Kuhn
* fixed 500 ms bug at call to set_rtc_mmss, fixed DS12887
* precision CMOS clock update
*/
#include <linux/errno.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 <asm/segment.h>
#include <asm/io.h>
#include <asm/irq.h>

#include <linux/mc146818rtc.h>
#include <linux/timex.h>
#include <linux/config.h>

extern int setup_x86_irq(int, struct irqaction *);

#ifndef CONFIG_APM /* cycle counter may be unreliable */
/* Cycle counter value at the previous timer interrupt.. */
static unsigned long long last_timer_cc = 0;
static unsigned long long init_timer_cc = 0;

static unsigned long do_fast_gettimeoffset(void)
{
unsigned long time_low, time_high;
unsigned long offset_low, offset_high;
unsigned long quotient,remainder,missing_time=0;

/* Last jiffie when do_fast_gettimeoffset() was called.. */
static unsigned long last_jiffies=0;

/* Cached "clocks per usec" value.. */
static unsigned long quotient_c=0, remainder_c=0;

/* The "clocks per usec" value is calculated once each jiffie */
if( last_jiffies != jiffies ) {

/****
* test for hanging bottom handler (this means xtime is not
* updated yet)
*/
if( test_bit(TIMER_BH, &bh_active) )
missing_time = 997670/HZ;

last_jiffies = jiffies;

/* Get last timer tick in absolute kernel time */
__asm__("subl %2,%0\n\t"
"sbbl %3,%1"
:"=r" (time_low), "=r" (time_high)
:"m" (*(0+(long *)&init_timer_cc)),
"m" (*(1+(long *)&init_timer_cc)),
"0" (*(0+(long *)&last_timer_cc)),
"1" (*(1+(long *)&last_timer_cc)));

/*
* Divide the 64-bit time with the 32-bit jiffy counter,
* getting the quotient in clocks.
*
* Giving quotient = "average internal clocks per usec"
*/
__asm__("divl %2"
:"=a" (quotient), "=d" (remainder)
:"r" (last_jiffies),
"0" (time_low), "1" (time_high));

__asm__("divl %2"
:"=a" (quotient_c), "=d" (remainder_c)
:"r" (quotient),
"0" (0), "1" (997670/HZ));

}

/* Read the time counter */
__asm__(".byte 0x0f,0x31"
:"=a" (time_low), "=d" (time_high));

/* .. relative to previous jiffy (32 bits is enough) */
time_low -= (unsigned long) last_timer_cc;

/*
* Time offset = (997670/HZ * time_low) / quotient.
*/

__asm__("mul %2"
:"=a" (offset_low), "=d" (offset_high)
:"r" (quotient_c),
"0" (time_low), "1" (0));

/*
* Due to rounding errors (and jiffies inconsistencies),
* we need to check the result so that we'll get a timer
* that is monotonous.
*/
if (offset_high >= 997670/HZ)
offset_high = 997670/HZ-1;

return offset_high + missing_time;
}
#endif

/* This function must be called with interrupts disabled
* It was inspired by Steve McCanne's microtime-i386 for BSD. -- jrs
*
* However, the pc-audio speaker driver changes the divisor so that
* it gets interrupted rather more often - it loads 64 into the
* counter rather than 11932! This has an adverse impact on
* do_gettimeoffset() -- it stops working! What is also not
* good is that the interval that our timer function gets called
* is no longer 10.0002 ms, but 9.9767 ms. To get around this
* would require using a different timing source. Maybe someone
* could use the RTC - I know that this can interrupt at frequencies
* ranging from 8192Hz to 2Hz. If I had the energy, I'd somehow fix
* it so that at startup, the timer code in sched.c would select
* using either the RTC or the 8253 timer. The decision would be
* based on whether there was any other device around that needed
* to trample on the 8253. I'd set up the RTC to interrupt at 1024 Hz,
* and then do some jiggery to have a version of do_timer that
* advanced the clock by 1/1024 s. Every time that reached over 1/100
* of a second, then do all the old code. If the time was kept correct
* then do_gettimeoffset could just return 0 - there is no low order
* divider that can be accessed.
*
* Ideally, you would be able to use the RTC for the speaker driver,
* but it appears that the speaker driver really needs interrupt more
* often than every 120 us or so.
*
* Anyway, this needs more thought.... pjsg (1993-08-28)
*
* If you are really that interested, you should be reading
* comp.protocols.time.ntp!
*/

#define TICK_SIZE tick

static unsigned long do_slow_gettimeoffset(void)
{
int count;
static int count_p = 0;
unsigned long offset = 0;
static unsigned long jiffies_p = 0;

/*******
* cache volatile jiffies temporaly, we have IRQs turned off.
* watch out when going SMP tho ...
*/
unsigned long jiffies_t;

/* timer count may underflow right here */
outb_p(0x00, 0x43); /* latch the count ASAP */
count = inb_p(0x40); /* read the latched count */
count |= inb(0x40) << 8;

jiffies_t = jiffies;

/******
* avoid timer inconsistencies ...
*/
if( count > count_p ) {
if( jiffies_t == jiffies_p ) {
if( count > LATCH-LATCH/100 )
offset = TICK_SIZE;
else
/* argh, the timer is bugging */
count = count_p;
} else {
if( test_bit(TIMER_BH, &bh_active) ) {
/******
* we have detected a counter underflow.
*/
offset = TICK_SIZE;
count_p = count;
} else {
count_p = count;
jiffies_p = jiffies_t;
}
}
} else {
count_p = count;
jiffies_p = jiffies_t;
}

count = ((LATCH-1) - count) * TICK_SIZE;
count = (count + LATCH/2) / LATCH;
return offset + count;
}

static unsigned long (*do_gettimeoffset)(void) = do_slow_gettimeoffset;

/*
* This version of gettimeofday has near microsecond resolution.
*/
void do_gettimeofday(struct timeval *tv)
{
unsigned long flags;

save_flags(flags);
cli();
*tv = xtime;
tv->tv_usec += do_gettimeoffset();
if (tv->tv_usec >= 1000000) {
tv->tv_usec -= 1000000;
tv->tv_sec++;
}
restore_flags(flags);
}

void do_settimeofday(struct timeval *tv)
{
cli();
/* This is revolting. We need to set the xtime.tv_usec
* correctly. However, the value in this location is
* is value at the last tick.
* Discover what correction gettimeofday
* would have done, and then undo it!
*/
tv->tv_usec -= do_gettimeoffset();

if (tv->tv_usec < 0) {
tv->tv_usec += 1000000;
tv->tv_sec--;
}

xtime = *tv;
time_state = TIME_BAD;
time_maxerror = MAXPHASE;
time_esterror = MAXPHASE;
sti();
}


/*
* In order to set the CMOS clock precisely, set_rtc_mmss has to be
* called 500 ms after the second nowtime has started, because when
* nowtime is written into the registers of the CMOS clock, it will
* jump to the next second precisely 500 ms later. Check the Motorola
* MC146818A or Dallas DS12887 data sheet for details.
*/
static int set_rtc_mmss(unsigned long nowtime)
{
int retval = 0;
int real_seconds, real_minutes, cmos_minutes;
unsigned char save_control, save_freq_select;

save_control = CMOS_READ(RTC_CONTROL); /* tell the clock it's being set */
CMOS_WRITE((save_control|RTC_SET), RTC_CONTROL);

save_freq_select = CMOS_READ(RTC_FREQ_SELECT); /* stop and reset prescaler */
CMOS_WRITE((save_freq_select|RTC_DIV_RESET2), RTC_FREQ_SELECT);

cmos_minutes = CMOS_READ(RTC_MINUTES);
if (!(save_control & RTC_DM_BINARY) || RTC_ALWAYS_BCD)
BCD_TO_BIN(cmos_minutes);

/*
* since we're only adjusting minutes and seconds,
* don't interfere with hour overflow. This avoids
* messing with unknown time zones but requires your
* RTC not to be off by more than 15 minutes
*/
real_seconds = nowtime % 60;
real_minutes = nowtime / 60;
if (((abs(real_minutes - cmos_minutes) + 15)/30) & 1)
real_minutes += 30; /* correct for half hour time zone */
real_minutes %= 60;

if (abs(real_minutes - cmos_minutes) < 30) {
if (!(save_control & RTC_DM_BINARY) || RTC_ALWAYS_BCD) {
BIN_TO_BCD(real_seconds);
BIN_TO_BCD(real_minutes);
}
CMOS_WRITE(real_seconds,RTC_SECONDS);
CMOS_WRITE(real_minutes,RTC_MINUTES);
} else
retval = -1;

/* The following flags have to be released exactly in this order,
* otherwise the DS12887 (popular MC146818A clone with integrated
* battery and quartz) will not reset the oscillator and will not
* update precisely 500 ms later. You won't find this mentioned in
* the Dallas Semiconductor data sheets, but who believes data
* sheets anyway ... -- Markus Kuhn
*/
CMOS_WRITE(save_control, RTC_CONTROL);
CMOS_WRITE(save_freq_select, RTC_FREQ_SELECT);

return retval;
}

/* last time the cmos clock got updated */
static long last_rtc_update = 0;

/*
* timer_interrupt() needs to keep up the real-time clock,
* as well as call the "do_timer()" routine every clocktick
*/
static inline void timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
{
do_timer(regs);

/*
* If we have an externally synchronized Linux clock, then update
* CMOS clock accordingly every ~11 minutes. Set_rtc_mmss() has to be
* called as close as possible to 500 ms before the new second starts.
*/
if (time_state != TIME_BAD && xtime.tv_sec > last_rtc_update + 660 &&
xtime.tv_usec > 500000 - (tick >> 1) &&
xtime.tv_usec < 500000 + (tick >> 1))
if (set_rtc_mmss(xtime.tv_sec) == 0)
last_rtc_update = xtime.tv_sec;
else
last_rtc_update = xtime.tv_sec - 600; /* do it again in 60 s */
/* As we return to user mode fire off the other CPU schedulers.. this is
basically because we don't yet share IRQ's around. This message is
rigged to be safe on the 386 - basically it's a hack, so don't look
closely for now.. */
/*smp_message_pass(MSG_ALL_BUT_SELF, MSG_RESCHEDULE, 0L, 0); */

}

#ifndef CONFIG_APM /* cycle counter may be unreliable */
/*
* This is the same as the above, except we _also_ save the current
* cycle counter value at the time of the timer interrupt, so that
* we later on can estimate the time of day more exactly.
*/
static void pentium_timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
{
/* read Pentium cycle counter */
__asm__(".byte 0x0f,0x31"
:"=a" (((unsigned long *) &last_timer_cc)[0]),
"=d" (((unsigned long *) &last_timer_cc)[1]));
timer_interrupt(irq, NULL, regs);
}
#endif

/* Converts Gregorian date to seconds since 1970-01-01 00:00:00.
* Assumes input in normal date format, i.e. 1980-12-31 23:59:59
* => year=1980, mon=12, day=31, hour=23, min=59, sec=59.
*
* [For the Julian calendar (which was used in Russia before 1917,
* Britain & colonies before 1752, anywhere else before 1582,
* and is still in use by some communities) leave out the
* -year/100+year/400 terms, and add 10.]
*
* This algorithm was first published by Gauss (I think).
*
* WARNING: this function will overflow on 2106-02-07 06:28:16 on
* machines were long is 32-bit! (However, as time_t is signed, we
* will already get problems at other places on 2038-01-19 03:14:08)
*/
static inline unsigned long mktime(unsigned int year, unsigned int mon,
unsigned int day, unsigned int hour,
unsigned int min, unsigned int sec)
{
if (0 >= (int) (mon -= 2)) { /* 1..12 -> 11,12,1..10 */
mon += 12; /* Puts Feb last since it has leap day */
year -= 1;
}
return (((
(unsigned long)(year/4 - year/100 + year/400 + 367*mon/12 + day) +
year*365 - 719499
)*24 + hour /* now have hours */
)*60 + min /* now have minutes */
)*60 + sec; /* finally seconds */
}

unsigned long get_cmos_time(void)
{
unsigned int year, mon, day, hour, min, sec;
int i;

/* The Linux interpretation of the CMOS clock register contents:
* When the Update-In-Progress (UIP) flag goes from 1 to 0, the
* RTC registers show the second which has precisely just started.
* Let's hope other operating systems interpret the RTC the same way.
*/
/* read RTC exactly on falling edge of update flag */
for (i = 0 ; i < 1000000 ; i++) /* may take up to 1 second... */
if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP)
break;
for (i = 0 ; i < 1000000 ; i++) /* must try at least 2.228 ms */
if (!(CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP))
break;
do { /* Isn't this overkill ? UIP above should guarantee consistency */
sec = CMOS_READ(RTC_SECONDS);
min = CMOS_READ(RTC_MINUTES);
hour = CMOS_READ(RTC_HOURS);
day = CMOS_READ(RTC_DAY_OF_MONTH);
mon = CMOS_READ(RTC_MONTH);
year = CMOS_READ(RTC_YEAR);
} while (sec != CMOS_READ(RTC_SECONDS));
if (!(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY) || RTC_ALWAYS_BCD)
{
BCD_TO_BIN(sec);
BCD_TO_BIN(min);
BCD_TO_BIN(hour);
BCD_TO_BIN(day);
BCD_TO_BIN(mon);
BCD_TO_BIN(year);
}
if ((year += 1900) < 1970)
year += 100;
return mktime(year, mon, day, hour, min, sec);
}

static struct irqaction irq0 = { timer_interrupt, 0, 0, "timer", NULL, NULL};

void time_init(void)
{
xtime.tv_sec = get_cmos_time();
xtime.tv_usec = 0;

/* If we have the CPU hardware time counters, use them */
#ifndef CONFIG_APM
/* Don't use them if a suspend/resume could
corrupt the timer value. This problem
needs more debugging. */
if (x86_capability & 16) {
do_gettimeoffset = do_fast_gettimeoffset;
/* read Pentium cycle counter */
__asm__(".byte 0x0f,0x31"
:"=a" (((unsigned long *) &init_timer_cc)[0]),
"=d" (((unsigned long *) &init_timer_cc)[1]));
irq0.handler = pentium_timer_interrupt;
}
#endif
setup_x86_irq(0, &irq0);
}


#include <sys/time.h>
#include <unistd.h>
#include <stdio.h>

void main()
{
struct timeval t, t_old;
long i;

timerclear(&t);
for(i=0; i<100000000l; i++) {
gettimeofday(&t, NULL);
if(i>0 && t.tv_sec==t_old.tv_sec && t.tv_usec<t_old.tv_usec)
{
printf("old=%d new=%d\n", t_old.tv_usec, t.tv_usec);
fflush(stdout);
}
t_old = t;
}
}

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