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ntp: improve adjtimex frequency rounding 

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>
This commit is contained in:
Roman Zippel 2008-09-22 14:42:44 -07:00 committed by Thomas Gleixner
parent 5cd1c9c5cf
commit d40e944c25
2 changed files with 3 additions and 4 deletions
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2
include/linux/timex.h
View File

@ -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)

5
kernel/time/ntp.c
View File

@ -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;