ntp: adjust SHIFT_PLL to improve NTP convergence

#include <asm/timex.h>

/*

 * SHIFT_KG and SHIFT_KF establish the damping of the PLL and are chosen

 * for a slightly underdamped convergence characteristic. SHIFT_KH

 * establishes the damping of the FLL and is chosen by wisdom and black

 * art.

 * SHIFT_PLL is used as a dampening factor to define how much we

 * adjust the frequency correction for a given offset in PLL mode.

 * It also used in dampening the offset correction, to define how

 * much of the current value in time_offset we correct for each

 * second. Changing this value changes the stiffness of the ntp

 * adjustment code. A lower value makes it more flexible, reducing

 * NTP convergence time. A higher value makes it stiffer, increasing

 * convergence time, but making the clock more stable.

 *

 * MAXTC establishes the maximum time constant of the PLL. With the

 * SHIFT_KG and SHIFT_KF values given and a time constant range from

 * zero to MAXTC, the PLL will converge in 15 minutes to 16 hours,

 * respectively.

 * In David Mills' nanokenrel reference implmentation SHIFT_PLL is 4.

 * However this seems to increase convergence time much too long.

 *

 * https://lists.ntp.org/pipermail/hackers/2008-January/003487.html

 *

 * In the above mailing list discussion, it seems the value of 4

 * was appropriate for other Unix systems with HZ=100, and that

 * SHIFT_PLL should be decreased as HZ increases. However, Linux's

 * clock steering implementation is HZ independent.

 *

 * Through experimentation, a SHIFT_PLL value of 2 was found to allow

 * for fast convergence (very similar to the NTPv3 code used prior to

 * v2.6.19), with good clock stability.

 *

 *

 * SHIFT_FLL is used as a dampening factor to define how much we

 * adjust the frequency correction for a given offset in FLL mode.

 * In David Mills' nanokenrel reference implmentation SHIFT_PLL is 2.

 *

 * MAXTC establishes the maximum time constant of the PLL.

 */

#define SHIFT_PLL	4	/* PLL frequency factor (shift) */

#define SHIFT_PLL	2	/* PLL frequency factor (shift) */

#define SHIFT_FLL	2	/* FLL frequency factor (shift) */

#define MAXTC		10	/* maximum time constant (shift) */

#define SHIFT_USEC 16		/* frequency offset scale (shift) */

#define PPM_SCALE ((s64)NSEC_PER_USEC << (NTP_SCALE_SHIFT - SHIFT_USEC))

#define PPM_SCALE_INV_SHIFT 19

#define PPM_SCALE_INV ((1ll << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \

#define PPM_SCALE_INV ((1LL << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \

		       PPM_SCALE + 1)

#define MAXPHASE 500000000l	/* max phase error (ns) */

#define MAXPHASE 500000000L	/* max phase error (ns) */

#define MAXFREQ 500000		/* max frequency error (ns/s) */

#define MAXFREQ_SCALED ((s64)MAXFREQ << NTP_SCALE_SHIFT)

#define MINSEC 256		/* min interval between updates (s) */