Date:	Wed, 07 Jan 2015 13:53:42 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	unlisted-recipients:; (no To-header on input)
CC:	mtk.manpages@...il.com, lkml <linux-kernel@...r.kernel.org>,
	Petr Gajdos <PGajdos@...e.cz>,
	Laurent Georget <laurent.georget@...elec.fr>,
	John Stultz <john.stultz@...aro.org>,
	Richard Cochran <richardcochran@...il.com>,
	Jeff Epler <jepler@...ythonic.net>
Subject: Request for review of adjtimex(2) man page

Hello all,

Recently, I made made a number of changes to the adjtimex(2)
man page, to try and add a bit more detail, since the page
was formerly in a very sorry state.

I would be happy if some NTP/time-knowledgeable folk (John, Richard,
I'm kind of hoping you) would review the page to see if I've injected 
any errors. Furthermore, notwithstanding my attempt to improve the page, 
there remain many gaps in details in the page. I've added a large number
of FIXMEs in the draft below, and would be happy if anyone can supply
some content to fill any of the gaps.

Cheers,

Michael

.\" Copyright (c) 1995 Michael Chastain (mec@...ll.portal.com), 15 April 1995.
.\" and Copyright (C) 2014 Michael Kerrisk <mtk.manpages@...il.com>
.\"
.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
.\" This is free documentation; you can redistribute it and/or
.\" modify it under the terms of the GNU General Public License as
.\" published by the Free Software Foundation; either version 2 of
.\" the License, or (at your option) any later version.
.\"
.\" The GNU General Public License's references to "object code"
.\" and "executables" are to be interpreted as the output of any
.\" document formatting or typesetting system, including
.\" intermediate and printed output.
.\"
.\" This manual is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\" GNU General Public License for more details.
.\"
.\" You should have received a copy of the GNU General Public
.\" License along with this manual; if not, see
.\" <http://www.gnu.org/licenses/>.
.\" %%%LICENSE_END
.\"
.\" Modified 1997-01-31 by Eric S. Raymond <esr@...rsus.com>
.\" Modified 1997-07-30 by Paul Slootman <paul@...tel.demon.nl>
.\" Modified 2004-05-27 by Michael Kerrisk <mtk.manpages@...il.com>
.\"
.TH ADJTIMEX 2 2014-12-31 "Linux" "Linux Programmer's Manual"
.SH NAME
adjtimex \- tune kernel clock
.SH SYNOPSIS
.nf
.BR "#define _BSD_SOURCE" "      /* See feature_test_macros(7) */"
.B #include <sys/timex.h>

.BI "int adjtimex(struct timex *" "buf" );
.fi
.SH DESCRIPTION
Linux uses David L. Mills' clock adjustment algorithm (see RFC\ 5905).
The system call
.BR adjtimex ()
reads and optionally sets adjustment parameters for this algorithm.
It takes a pointer to a
.I timex
structure, updates kernel parameters from field values,
and returns the same structure with current kernel values.
This structure is declared as follows:
.PP
.in +4n
.nf
struct timex {
    int  modes;      /* Mode selector */
    long offset;     /* Time offset; nanoseconds, if STA_NANO
                        status flag is set, otherwise microseconds */
    long freq;       /* Frequency offset, in units of 2^-16 ppm
                        (parts per million, see NOTES below) */
    long maxerror;   /* Maximum error (microseconds) */
    long esterror;   /* Estimated error (microseconds) */
    int  status;     /* Clock command/status */
    long constant;   /* PLL (phase-locked loop) time constant */
    long precision;  /* Clock precision (microseconds, read-only) */
    long tolerance;  /* Clock frequency tolerance (ppm, read-only) */
    struct timeval time;
                     /* Current time (read-only, except for
                        ADJ_SETOFFSET); upon return, time.tv_usec
                        contains nanoseconds, if STA_NANO status
                        flag is set, otherwise microseconds */
    long tick;       /* Microseconds between clock ticks */
    long ppsfreq;    /* PPS (pulse per second) frequency (in units
                        of 2^-16 ppm\-\-see NOTES, read-only) */
    long jitter;     /* PPS jitter (read-only); nanoseconds, if
                        STA_NANO status flag is set, otherwise
                        microseconds */
    int  shift;      /* PPS interval duration (seconds, read-only) */
    long stabil;     /* PPS stability (2^-16 ppm\-\-see NOTES,
                        read-only) */
    long jitcnt;     /* PPS jitter limit exceeded (read-only) */
    long calcnt;     /* PPS calibration intervals (read-only) */
    long errcnt;     /* PPS calibration errors (read-only) */
    long stbcnt;     /* PPS stability limit exceeded (read-only) */
    int tai;         /* TAI offset, as set by previous ADJ_TAI
                        operation (seconds, read-only,
                        since Linux 2.6.26) */
    /* Further padding bytes to allow for future expansion */
};
.fi
.in
.PP
The
.I modes
field determines which parameters, if any, to set.
It is a bit mask containing a
.RI bitwise- or
combination of zero or more of the following bits:
.TP
.BR ADJ_OFFSET
Set time offset from
.IR buf.offset .
.TP
.BR ADJ_FREQUENCY
Set frequency offset from
.IR buf.freq .
.TP
.BR ADJ_MAXERROR
Set maximum time error from
.IR buf.maxerror .
.TP
.BR ADJ_ESTERROR
Set estimated time error from
.IR buf.esterror .
.TP
.BR ADJ_STATUS
Set clock status from
.IR buf.status .
.TP
.BR ADJ_TIMECONST
Set PLL time constant from
.IR buf.constant .
If the
.B STA_NANO
status flag (see below) is clear, the kernel adds 4 to this value.
.TP
.BR ADJ_SETOFFSET " (since Linux 2.6.29)"
.\" commit 094aa1881fdc1b8889b442eb3511b31f3ec2b762
.\" Author: Richard Cochran <richardcochran@...il.com>
Add
.I buf.time
to the current time.
If
.I buf.status
includes the
.B ADJ_NANO
flag, then
.I buf.time.tv_usec
is interpreted as a nanosecond value;
otherwise it is interpreted as microseconds.
.TP
.BR ADJ_MICRO " (since Linux 2.6.36)"
.\" commit eea83d896e318bda54be2d2770d2c5d6668d11db
.\" Author: Roman Zippel <zippel@...ux-m68k.org>
Select microsecond resolution.
.TP
.BR ADJ_NANO " (since Linux 2.6.36)"
.\" commit eea83d896e318bda54be2d2770d2c5d6668d11db
.\" Author: Roman Zippel <zippel@...ux-m68k.org>
Select nanosecond resolution.
Only one of
.BR ADJ_MICRO
and
.BR ADJ_NANO
should be specified.
.TP
.BR ADJ_TAI " (since Linux 2.6.26)"
.\" commit 153b5d054ac2d98ea0d86504884326b6777f683d
Set TAI (Atomic International Time) offset from
.IR buf->constant .

.BR ADJ_TAI
should not be used in conjunction with
.BR ADJ_TIMECONST ,
since the latter mode also employs the
.IR buf->constant
field.

For a complete explanation of TAI
and the difference between TAI and UTC, see
.UR http://www.bipm.org/en/bipm/tai/tai.html
.I BIPM
.UE
.TP
.BR ADJ_TICK
Set tick value from
.IR buf.tick .
.PP
Alternatively,
.I modes
can be specified as either of the following (multibit mask) values,
in which case other bits should not be specified in
.IR modes :
.\" In general, the other bits are ignored, but ADJ_OFFSET_SINGLESHOT 0x8001
.\" ORed with ADJ_NANO (0x2000) gives 0xa0001 == ADJ_OFFSET_SS_READ!!
.TP
.BR ADJ_OFFSET_SINGLESHOT
.\" In user space, ADJ_OFFSET_SINGLESHOT is 0x8001
.\" In kernel space it is 0x0001, and must be ANDed with ADJ_ADJTIME (0x8000)
Old-fashioned
.BR adjtime ():
(gradually) adjust time by value specified in
.IR buf.offset ,
which specifies an adjustment in microseconds.
.TP
.BR ADJ_OFFSET_SS_READ " (functional since Linux 2.6.28)"
.\" In user space, ADJ_OFFSET_SS_READ is 0xa001
.\" In kernel space there is ADJ_OFFSET_READONLY (0x2000) anded with
.\" ADJ_ADJTIME (0x8000) and ADJ_OFFSET_SINGLESHOT (0x0001) to give 0xa001)
Return (in
.BR buf.offset )
the remaining amount of time to be adjusted after an earlier
.BR ADJ_OFFSET_SINGLESHOT
operation.
This feature was added in Linux 2.6.24,
.\" commit 52bfb36050c8529d9031d2c2513b281a360922ec
but did not work correctly
.\" commit 916c7a855174e3b53d182b97a26b2e27a29726a1
until Linux 2.6.28.
.PP
Ordinary users are restricted to a value of either 0 or
.B ADJ_OFFSET_SS_READ
for
.IR modes .
Only the superuser may set any parameters.

The
.I buf.status
field is a bit mask that is used to set and/or retrieve status
bits associated with the NTP implementation.
Some bits in the mask are both readable and settable,
while others are read-only.
.TP
.BR STA_PLL
Enable phase-locked loop (PLL) updates (read-write) via
.\" FIXME Any pointer to further information about what this means?
.\"       (It was not immediately obvious from a scan of the RFC, whether
.\"       this is described in the RFC.)
.BR ADJ_OFFSET .
.TP
.BR STA_PPSFREQ
Enable PPS freq discipline (read-write).
.\" FIXME Any pointer to further information about what this means?
.\"       (It was not immediately obvious from a scan of the RFC, whether
.\"       this is described in the RFC.)
.TP
.BR STA_PPSTIME
Enable PPS time discipline (read-write).
.\" FIXME Any pointer to further information about what this means?
.\"       (It was not immediately obvious from a scan of the RFC, whether
.\"       this is described in the RFC.)
.TP
.BR STA_FLL
Select frequency-locked loop (FLL) mode (read-write).
.\" FIXME Any pointer to further information about what this means?
.\"       (It was not immediately obvious from a scan of the RFC, whether
.\"       this is described in the RFC.)
.TP
.BR STA_INS
Insert leap second (read-write).
.\" FIXME Is the following correct?
.\"     Insert a leap second after the last second of the day.
.\"     That is, at 24:00:00, set the clock 1 second back, thus extending
.\"     the last  minute of the day by one second.
.\"     Leap second insertion will occur each day, so long as this flag
.\"     remains set.
.TP
.BR STA_DEL
Delete leap second (read-write).
.\" FIXME Is the following correct?
.\"     Delete a leap second at the last second of the leap day.
.\"     That is, at 23:5:59, add one extra second to the clock.
.\"     Leap second deletion will occur each day, so long as this flag
.\"     remains set.
.\" FIXME Does there need to be a statement that it is nonsensical to set
.\"     to set both STA_INS and STA_DEL?
.TP
.BR STA_UNSYNC
Clock unsynchronized (read-write).
.TP
.BR STA_FREQHOLD
Hold frequency (read-write).
.\" FIXME Any pointer to further information about what this means?
.\"       (It was not immediately obvious from a scan of the RFC, whether
.\"       this is described in the RFC.)
.TP
.BR STA_PPSSIGNAL
PPS signal present (read-only).
.\" FIXME Any pointer to further information about what this means?
.\"       (It was not immediately obvious from a scan of the RFC, whether
.\"       this is described in the RFC.)
.TP
.BR STA_PPSJITTER
PPS signal jitter exceeded (read-only).
.\" FIXME Any pointer to further information about what this means?
.\"       (It was not immediately obvious from a scan of the RFC, whether
.\"       this is described in the RFC.)
.TP
.BR STA_PPSWANDER
PPS signal wander exceeded (read-only).
.\" FIXME Any pointer to further information about what this means?
.\"       (It was not immediately obvious from a scan of the RFC, whether
.\"       this is described in the RFC.)
.TP
.BR STA_PPSERROR
PPS signal calibration error (read-only).
.\" FIXME Any pointer to further information about what this means?
.\"       (It was not immediately obvious from a scan of the RFC, whether
.\"       this is described in the RFC.)
.TP
.BR STA_CLOCKERR
Clock hardware fault (read-only).
.\" FIXME Any pointer to further information about what this means?
.\"       (It was not immediately obvious from a scan of the RFC, whether
.\"       this is described in the RFC.)
.TP
.BR STA_NANO " (since Linux 2.6.26)"
.\" commit eea83d896e318bda54be2d2770d2c5d6668d11db
.\" Author: Roman Zippel <zippel@...ux-m68k.org>
Resolution (0 = microsecond, 1 = nanoseconds; read-only).
Set via
.BR ADJ_NANO ,
cleared via
.BR ADJ_MICRO .
.TP
.BR STA_MODE " (since Linux 2.6.26)"
.\" commit eea83d896e318bda54be2d2770d2c5d6668d11db
.\" Author: Roman Zippel <zippel@...ux-m68k.org>
Mode (0 = Phase Locked Loop, 1 = Frequency Locked Loop; read-only).
.TP
.BR STA_CLK " (since Linux 2.6.26)"
.\" commit eea83d896e318bda54be2d2770d2c5d6668d11db
.\" Author: Roman Zippel <zippel@...ux-m68k.org>
Clock source (0 = A, 1 = B; read-only).
.\" FIXME It would be helpful to have some explanation of what
.\"       "Clock source" is.
.PP
Attempts to set read-only
.I status
bits are silently ignored.
.SH RETURN VALUE
On success,
.BR adjtimex ()
returns the clock state; that is, one of the following values:
.TP 12
.BR TIME_OK
Clock synchronized.
.TP
.BR TIME_INS
Insert leap second.
.\" FIXME Is the following correct:
.\"       Indicates that a leap second will be added at the end of the day
.TP
.BR TIME_DEL
Delete leap second.
.\" FIXME Is the following correct:
.\"       Indicates that a leap second will be deleted at the end of the day
.TP
.BR TIME_OOP
Leap second in progress.
.\" FIXME Is the following correct:
.\"       Indicates that we are currently in the middle of the leap second
.\"       that is being added at the end of the day (as a result of STA_INS)
.TP
.BR TIME_WAIT
Leap second has occurred.
.\" FIXME Is the following correct:
.\"       Indicates that a leap second has just been added or deleted
.\"       during the previous second
.TP
.BR TIME_ERROR
Clock not synchronized.
.\" FIXME Should more be said about how the TIME_ERROR state can occur?
The symbolic name
.B TIME_BAD
is a synonym for
.BR TIME_ERROR ,
provided for backward compatibility.
.PP
On failure,
.BR adjtimex ()
returns \-1 and sets
.IR errno .
.SH ERRORS
.TP
.B EFAULT
.I buf
does not point to writable memory.
.TP
.B EINVAL
An attempt was made to set
.I buf.offset
to a value outside the range \-131071 to +131071,
or to set
.I buf.status
to a value other than those listed above,
or to set
.I buf.tick
to a value outside the range
.RB 900000/ HZ
to
.RB 1100000/ HZ ,
where
.B HZ
is the system timer interrupt frequency.
.TP
.B EPERM
.I buf.modes
is neither 0 nor
.BR ADJ_OFFSET_SS_READ ,
and the caller does not have sufficient privilege.
Under Linux, the
.B CAP_SYS_TIME
capability is required.
.SH NOTES
In struct
.IR timex ,
.IR freq ,
.IR ppsfreq ,
and
.I stabil
are ppm (parts per million) with a 16-bit fractional part,
which means that a value of 1 in one of those fields
actually means 2^-16 ppm, and 2^16=65536 is 1 ppm.
This is the case for both input values (in the case of
.IR freq )
and output values.
.SH CONFORMING TO
.BR adjtimex ()
is Linux-specific and should not be used in programs
intended to be portable.
See
.BR adjtime (3)
for a more portable, but less flexible,
method of adjusting the system clock.
.SH SEE ALSO
.BR settimeofday (2),
.BR adjtime (3),
.BR capabilities (7),
.BR time (7),
.BR adjtimex (8)

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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