linux/include/linux/timex.h
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   1/*****************************************************************************
   2 *                                                                           *
   3 * Copyright (c) David L. Mills 1993                                         *
   4 *                                                                           *
   5 * Permission to use, copy, modify, and distribute this software and its     *
   6 * documentation for any purpose and without fee is hereby granted, provided *
   7 * that the above copyright notice appears in all copies and that both the   *
   8 * copyright notice and this permission notice appear in supporting          *
   9 * documentation, and that the name University of Delaware not be used in    *
  10 * advertising or publicity pertaining to distribution of the software       *
  11 * without specific, written prior permission.  The University of Delaware   *
  12 * makes no representations about the suitability this software for any      *
  13 * purpose.  It is provided "as is" without express or implied warranty.     *
  14 *                                                                           *
  15 *****************************************************************************/
  16
  17/*
  18 * Modification history timex.h
  19 *
  20 * 29 Dec 97    Russell King
  21 *      Moved CLOCK_TICK_RATE, CLOCK_TICK_FACTOR and FINETUNE to asm/timex.h
  22 *      for ARM machines
  23 *
  24 *  9 Jan 97    Adrian Sun
  25 *      Shifted LATCH define to allow access to alpha machines.
  26 *
  27 * 26 Sep 94    David L. Mills
  28 *      Added defines for hybrid phase/frequency-lock loop.
  29 *
  30 * 19 Mar 94    David L. Mills
  31 *      Moved defines from kernel routines to header file and added new
  32 *      defines for PPS phase-lock loop.
  33 *
  34 * 20 Feb 94    David L. Mills
  35 *      Revised status codes and structures for external clock and PPS
  36 *      signal discipline.
  37 *
  38 * 28 Nov 93    David L. Mills
  39 *      Adjusted parameters to improve stability and increase poll
  40 *      interval.
  41 *
  42 * 17 Sep 93    David L. Mills
  43 *      Created file $NTP/include/sys/timex.h
  44 * 07 Oct 93    Torsten Duwe
  45 *      Derived linux/timex.h
  46 * 1995-08-13    Torsten Duwe
  47 *      kernel PLL updated to 1994-12-13 specs (rfc-1589)
  48 * 1997-08-30    Ulrich Windl
  49 *      Added new constant NTP_PHASE_LIMIT
  50 * 2004-08-12    Christoph Lameter
  51 *      Reworked time interpolation logic
  52 */
  53#ifndef _LINUX_TIMEX_H
  54#define _LINUX_TIMEX_H
  55
  56#include <uapi/linux/timex.h>
  57
  58#define ADJ_ADJTIME             0x8000  /* switch between adjtime/adjtimex modes */
  59#define ADJ_OFFSET_SINGLESHOT   0x0001  /* old-fashioned adjtime */
  60#define ADJ_OFFSET_READONLY     0x2000  /* read-only adjtime */
  61#include <linux/compiler.h>
  62#include <linux/types.h>
  63#include <linux/param.h>
  64
  65#include <asm/timex.h>
  66
  67/*
  68 * SHIFT_PLL is used as a dampening factor to define how much we
  69 * adjust the frequency correction for a given offset in PLL mode.
  70 * It also used in dampening the offset correction, to define how
  71 * much of the current value in time_offset we correct for each
  72 * second. Changing this value changes the stiffness of the ntp
  73 * adjustment code. A lower value makes it more flexible, reducing
  74 * NTP convergence time. A higher value makes it stiffer, increasing
  75 * convergence time, but making the clock more stable.
  76 *
  77 * In David Mills' nanokernel reference implementation SHIFT_PLL is 4.
  78 * However this seems to increase convergence time much too long.
  79 *
  80 * https://lists.ntp.org/pipermail/hackers/2008-January/003487.html
  81 *
  82 * In the above mailing list discussion, it seems the value of 4
  83 * was appropriate for other Unix systems with HZ=100, and that
  84 * SHIFT_PLL should be decreased as HZ increases. However, Linux's
  85 * clock steering implementation is HZ independent.
  86 *
  87 * Through experimentation, a SHIFT_PLL value of 2 was found to allow
  88 * for fast convergence (very similar to the NTPv3 code used prior to
  89 * v2.6.19), with good clock stability.
  90 *
  91 *
  92 * SHIFT_FLL is used as a dampening factor to define how much we
  93 * adjust the frequency correction for a given offset in FLL mode.
  94 * In David Mills' nanokernel reference implementation SHIFT_FLL is 2.
  95 *
  96 * MAXTC establishes the maximum time constant of the PLL.
  97 */
  98#define SHIFT_PLL       2       /* PLL frequency factor (shift) */
  99#define SHIFT_FLL       2       /* FLL frequency factor (shift) */
 100#define MAXTC           10      /* maximum time constant (shift) */
 101
 102/*
 103 * SHIFT_USEC defines the scaling (shift) of the time_freq and
 104 * time_tolerance variables, which represent the current frequency
 105 * offset and maximum frequency tolerance.
 106 */
 107#define SHIFT_USEC 16           /* frequency offset scale (shift) */
 108#define PPM_SCALE ((s64)NSEC_PER_USEC << (NTP_SCALE_SHIFT - SHIFT_USEC))
 109#define PPM_SCALE_INV_SHIFT 19
 110#define PPM_SCALE_INV ((1LL << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \
 111                       PPM_SCALE + 1)
 112
 113#define MAXPHASE 500000000L     /* max phase error (ns) */
 114#define MAXFREQ 500000          /* max frequency error (ns/s) */
 115#define MAXFREQ_SCALED ((s64)MAXFREQ << NTP_SCALE_SHIFT)
 116#define MINSEC 256              /* min interval between updates (s) */
 117#define MAXSEC 2048             /* max interval between updates (s) */
 118#define NTP_PHASE_LIMIT ((MAXPHASE / NSEC_PER_USEC) << 5) /* beyond max. dispersion */
 119
 120/*
 121 * kernel variables
 122 * Note: maximum error = NTP synch distance = dispersion + delay / 2;
 123 * estimated error = NTP dispersion.
 124 */
 125extern unsigned long tick_usec;         /* USER_HZ period (usec) */
 126extern unsigned long tick_nsec;         /* SHIFTED_HZ period (nsec) */
 127
 128extern void ntp_init(void);
 129extern void ntp_clear(void);
 130
 131/* Required to safely shift negative values */
 132#define shift_right(x, s) ({    \
 133        __typeof__(x) __x = (x);        \
 134        __typeof__(s) __s = (s);        \
 135        __x < 0 ? -(-__x >> __s) : __x >> __s;  \
 136})
 137
 138#define NTP_SCALE_SHIFT         32
 139
 140#define NTP_INTERVAL_FREQ  (HZ)
 141#define NTP_INTERVAL_LENGTH (NSEC_PER_SEC/NTP_INTERVAL_FREQ)
 142
 143/* Returns how long ticks are at present, in ns / 2^NTP_SCALE_SHIFT. */
 144extern u64 ntp_tick_length(void);
 145
 146extern int second_overflow(unsigned long secs);
 147extern int do_adjtimex(struct timex *);
 148extern void hardpps(const struct timespec *, const struct timespec *);
 149
 150int read_current_timer(unsigned long *timer_val);
 151
 152/* The clock frequency of the i8253/i8254 PIT */
 153#define PIT_TICK_RATE 1193182ul
 154
 155#endif /* LINUX_TIMEX_H */
 156
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