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: 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>

2 files changed, 13 insertions, 19 deletions

diff --git a/include/linux/timex.h b/include/linux/timex.h
index 48c3376dce71..9fbdd12a52f1 100644
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -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)
diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index c6ae0c249891..44491de312a0 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -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;
         }