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Commit 9f14f669 authored by Roman Zippel's avatar Roman Zippel Committed by Linus Torvalds
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ntp: increase time_offset resolution 



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: default avatarRoman Zippel <zippel@linux-m68k.org>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 074b3b87

include/linux/timex.h

+2 −7
Original line number Diff line number Diff line
@@ -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)

kernel/time/ntp.c

+11 −12
Original line number Diff line number Diff line
@@ -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;

	}