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Subject[PATCH] ntp: Clamp PLL update interval
Date
Clamp update interval to reduce PLL gain with low sampling rate (e.g.
intermittent network connection) to avoid instability.

The clamp roughly corresponds to the loop time constant, it's 8 * poll
interval for SHIFT_PLL 2 and 32 * poll interval for SHIFT_PLL 4. This
gives good results without affecting the gain in normal conditions where
ntpd skips only up to seven consecutive samples.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
---
kernel/time/ntp.c | 14 +++++++++++---
1 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index c631168..d232189 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -149,10 +149,18 @@ static void ntp_update_offset(long offset)
time_reftime = get_seconds();

offset64 = offset;
- freq_adj = (offset64 * secs) <<
- (NTP_SCALE_SHIFT - 2 * (SHIFT_PLL + 2 + time_constant));
+ freq_adj = ntp_update_offset_fll(offset64, secs);

- freq_adj += ntp_update_offset_fll(offset64, secs);
+ /*
+ * Clamp update interval to reduce PLL gain with low
+ * sampling rate (e.g. intermittent network connection)
+ * to avoid instability.
+ */
+ if (unlikely(secs > 1 << (SHIFT_PLL + 1 + time_constant)))
+ secs = 1 << (SHIFT_PLL + 1 + time_constant);
+
+ freq_adj += (offset64 * secs) <<
+ (NTP_SCALE_SHIFT - 2 * (SHIFT_PLL + 2 + time_constant));

freq_adj = min(freq_adj + time_freq, MAXFREQ_SCALED);

--
1.7.2


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