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>

include/linux/timex.h

@@ -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

@@ -406,9 +406,8 @@ int do_adjtimex(struct timex *txc)
 	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;