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SubjectRe: [PATCH] ntp: Clamp PLL update interval
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On Tue, 2010-09-07 at 16:43 +0200, Miroslav Lichvar wrote:
> 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>

Thanks Miroslav!

Acked-by: John Stultz <johnstul@us.ibm.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);
>




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