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ntp: increase time_offset resolution

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time_offset is already a 64bit value but its resolution barely used, so this
makes better use of it by replacing SHIFT_UPDATE with TICK_LENGTH_SHIFT.

Side note: the SHIFT_HZ in SHIFT_UPDATE was incorrect for CONFIG_NO_HZ and the
primary reason for changing time_offset to 64bit to avoid the overflow.

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Roman Zippel 2008-05-01 04:34:36 -07:00 committed by Linus Torvalds
parent 074b3b8794
commit 9f14f669d1
2 changed files with 13 additions and 19 deletions
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9
include/linux/timex.h
View File

@ -76,27 +76,22 @@
#define MAXTC 10 /* maximum time constant (shift) */
/*
* The SHIFT_UPDATE define establishes the decimal point of the
* time_offset variable which represents the current offset with
* respect to standard time.
*
* SHIFT_USEC defines the scaling (shift) of the time_freq and
* time_tolerance variables, which represent the current frequency
* offset and maximum frequency tolerance.
*/
#define SHIFT_UPDATE (SHIFT_HZ + 1) /* time offset scale (shift) */
#define SHIFT_USEC 16 /* frequency offset scale (shift) */
#define PPM_SCALE (NSEC_PER_USEC << (TICK_LENGTH_SHIFT - SHIFT_USEC))
#define PPM_SCALE_INV_SHIFT 20
#define PPM_SCALE_INV ((1ll << (PPM_SCALE_INV_SHIFT + TICK_LENGTH_SHIFT)) / \
PPM_SCALE + 1)
#define MAXPHASE 512000L /* max phase error (us) */
#define MAXPHASE 500000000l /* max phase error (ns) */
#define MAXFREQ 500000 /* max frequency error (ns/s) */
#define MAXFREQ_SCALED ((s64)MAXFREQ << TICK_LENGTH_SHIFT)
#define MINSEC 256 /* min interval between updates (s) */
#define MAXSEC 2048 /* max interval between updates (s) */
#define NTP_PHASE_LIMIT (MAXPHASE << 5) /* beyond max. dispersion */
#define NTP_PHASE_LIMIT ((MAXPHASE / NSEC_PER_USEC) << 5) /* beyond max. dispersion */
/*
* syscall interface - used (mainly by NTP daemon)

23
kernel/time/ntp.c
View File

@ -65,16 +65,15 @@ static void ntp_update_offset(long offset)
if (!(time_status & STA_PLL))
return;
time_offset = offset;
if (!(time_status & STA_NANO))
time_offset *= NSEC_PER_USEC;
offset *= NSEC_PER_USEC;
/*
* Scale the phase adjustment and
* clamp to the operating range.
*/
time_offset = min(time_offset, (s64)MAXPHASE * NSEC_PER_USEC);
time_offset = max(time_offset, (s64)-MAXPHASE * NSEC_PER_USEC);
offset = min(offset, MAXPHASE);
offset = max(offset, -MAXPHASE);
/*
* Select how the frequency is to be controlled
@ -85,19 +84,19 @@ static void ntp_update_offset(long offset)
mtemp = xtime.tv_sec - time_reftime;
time_reftime = xtime.tv_sec;
freq_adj = time_offset * mtemp;
freq_adj = (s64)offset * mtemp;
freq_adj <<= TICK_LENGTH_SHIFT - 2 * (SHIFT_PLL + 2 + time_constant);
time_status &= ~STA_MODE;
if (mtemp >= MINSEC && (time_status & STA_FLL || mtemp > MAXSEC)) {
freq_adj += div_s64(time_offset << (TICK_LENGTH_SHIFT - SHIFT_FLL),
freq_adj += div_s64((s64)offset << (TICK_LENGTH_SHIFT - SHIFT_FLL),
mtemp);
time_status |= STA_MODE;
}
freq_adj += time_freq;
freq_adj = min(freq_adj, MAXFREQ_SCALED);
time_freq = max(freq_adj, -MAXFREQ_SCALED);
time_offset = div_s64(time_offset, NTP_INTERVAL_FREQ);
time_offset <<= SHIFT_UPDATE;
time_offset = div_s64((s64)offset << TICK_LENGTH_SHIFT, NTP_INTERVAL_FREQ);
}
/**
@ -128,7 +127,7 @@ void ntp_clear(void)
*/
void second_overflow(void)
{
long time_adj;
s64 time_adj;
/* Bump the maxerror field */
time_maxerror += MAXFREQ / NSEC_PER_USEC;
@ -184,7 +183,7 @@ void second_overflow(void)
tick_length = tick_length_base;
time_adj = shift_right(time_offset, SHIFT_PLL + time_constant);
time_offset -= time_adj;
tick_length += (s64)time_adj << (TICK_LENGTH_SHIFT - SHIFT_UPDATE);
tick_length += time_adj;
if (unlikely(time_adjust)) {
if (time_adjust > MAX_TICKADJ) {
@ -363,8 +362,8 @@ int do_adjtimex(struct timex *txc)
(txc->modes == ADJ_OFFSET_SS_READ))
txc->offset = save_adjust;
else {
txc->offset = ((long)shift_right(time_offset, SHIFT_UPDATE)) *
NTP_INTERVAL_FREQ;
txc->offset = shift_right(time_offset * NTP_INTERVAL_FREQ,
TICK_LENGTH_SHIFT);
if (!(time_status & STA_NANO))
txc->offset /= NSEC_PER_USEC;
}