ntp: improve adjtimex frequency rounding (d40e944c) · Commits · 戴 / test

Change PPM_SCALE_INV_SHIFT so that it doesn't throw away any input bits (19 is the amount of the factor 2 in PPM_SCALE), the output frequency can then be calculated back to its input value, as the inverse divide produce a slightly larger value, which is then correctly rounded by the final shift. Reported-by:

Martin Ziegler <ziegler@uni-freiburg.de> Signed-off-by:

Roman Zippel <zippel@linux-m68k.org> Cc: John Stultz <johnstul@us.ibm.com> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Thomas Gleixner <tglx@linutronix.de>

include/linux/timex.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -82,7 +82,7 @@
		*/
		#define SHIFT_USEC 16 /* frequency offset scale (shift) */
		#define PPM_SCALE (NSEC_PER_USEC << (NTP_SCALE_SHIFT - SHIFT_USEC))
		#define PPM_SCALE_INV_SHIFT 20
		#define PPM_SCALE_INV_SHIFT 19
		#define PPM_SCALE_INV ((1ll << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \
		PPM_SCALE + 1)

kernel/time/ntp.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -406,9 +406,8 @@ adj_done:
		if (time_status & (STA_UNSYNC\|STA_CLOCKERR))
		result = TIME_ERROR;

		txc->freq = shift_right((s32)(time_freq >> PPM_SCALE_INV_SHIFT) *
		(s64)PPM_SCALE_INV,
		NTP_SCALE_SHIFT);
		txc->freq = shift_right((time_freq >> PPM_SCALE_INV_SHIFT) *
		(s64)PPM_SCALE_INV, NTP_SCALE_SHIFT);
		txc->maxerror = time_maxerror;
		txc->esterror = time_esterror;
		txc->status = time_status;

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