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ntp: Clamp PLL update interval

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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>
Acked-by: john stultz <johnstul@us.ibm.com>
LKML-Reference: <1283870626-9472-1-git-send-email-mlichvar@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Miroslav Lichvar 2010-09-07 16:43:46 +02:00 committed by Thomas Gleixner
parent d56557af19
commit 8af3c153ba
1 changed files with 11 additions and 3 deletions
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14
kernel/time/ntp.c
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@ -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);