Clocks in a Linux System
There are two types of date-time clocks:
The Hardware Clock: This clock is an independent hardware device,
with its own power domain (battery, capacitor, etc), that
operates when the machine is powered off, or even unplugged.
On an ISA compatible system, this clock is specified as part of
the ISA standard. A control program can read or set this clock
only to a whole second, but it can also detect the edges of the 1
second clock ticks, so the clock actually has virtually infinite
precision.
This clock is commonly called the hardware clock, the real time
clock, the RTC, the BIOS clock, and the CMOS clock. Hardware
Clock, in its capitalized form, was coined for use by hwclock.
The Linux kernel also refers to it as the persistent clock.
Some non-ISA systems have a few real time clocks with only one of
them having its own power domain. A very low power external I2C
or SPI clock chip might be used with a backup battery as the
hardware clock to initialize a more functional integrated
real-time clock which is used for most other purposes.
The System Clock: This clock is part of the Linux kernel and is
driven by a timer interrupt. (On an ISA machine, the timer
interrupt is part of the ISA standard.) It has meaning only while
Linux is running on the machine. The System Time is the number of
seconds since 00:00:00 January 1, 1970 UTC (or more succinctly,
the number of seconds since 1969 UTC). The System Time is not an
integer, though. It has virtually infinite precision.
The System Time is the time that matters. The Hardware Clock's
basic purpose is to keep time when Linux is not running so that
the System Clock can be initialized from it at boot. Note that in
DOS, for which ISA was designed, the Hardware Clock is the only
real time clock.
It is important that the System Time not have any discontinuities
such as would happen if you used the date(1) program to set it
while the system is running. You can, however, do whatever you
want to the Hardware Clock while the system is running, and the
next time Linux starts up, it will do so with the adjusted time
from the Hardware Clock. Note: currently this is not possible on
most systems because hwclock --systohc is called at shutdown.
The Linux kernel's timezone is set by hwclock. But don't be
misled — almost nobody cares what timezone the kernel thinks it
is in. Instead, programs that care about the timezone (perhaps
because they want to display a local time for you) almost always
use a more traditional method of determining the timezone: They
use the TZ environment variable or the /etc/localtime file, as
explained in the man page for tzset(3). However, some programs
and fringe parts of the Linux kernel such as filesystems use the
kernel's timezone value. An example is the vfat filesystem. If
the kernel timezone value is wrong, the vfat filesystem will
report and set the wrong timestamps on files. Another example is
the kernel's NTP '11 minute mode'. If the kernel's timezone value
and/or the persistent_clock_is_local variable are wrong, then the
Hardware Clock will be set incorrectly by '11 minute mode'. See
the discussion below, under Automatic Hardware Clock
Synchronization by the Kernel.
hwclock sets the kernel's timezone to the value indicated by TZ
or /etc/localtime with the --hctosys or --systz functions.
The kernel's timezone value actually consists of two parts: 1) a
field tz_minuteswest indicating how many minutes local time (not
adjusted for DST) lags behind UTC, and 2) a field tz_dsttime
indicating the type of Daylight Savings Time (DST) convention
that is in effect in the locality at the present time. This
second field is not used under Linux and is always zero. See also
settimeofday(2).
Hardware Clock Access Methods
hwclock uses many different ways to get and set Hardware Clock
values. The most normal way is to do I/O to the rtc device
special file, which is presumed to be driven by the rtc device
driver. Also, Linux systems using the rtc framework with udev,
are capable of supporting multiple Hardware Clocks. This may
bring about the need to override the default rtc device by
specifying one with the --rtc option.
However, this method is not always available as older systems do
not have an rtc driver. On these systems, the method of accessing
the Hardware Clock depends on the system hardware.
On an ISA compatible system, hwclock can directly access the
"CMOS memory" registers that constitute the clock, by doing I/O
to Ports 0x70 and 0x71. It does this with actual I/O instructions
and consequently can only do it if running with superuser
effective userid. This method may be used by specifying the
--directisa option.
This is a really poor method of accessing the clock, for all the
reasons that userspace programs are generally not supposed to do
direct I/O and disable interrupts. hwclock provides it for
testing, troubleshooting, and because it may be the only method
available on ISA systems which do not have a working rtc device
driver.
The Adjust Function
The Hardware Clock is usually not very accurate. However, much of
its inaccuracy is completely predictable - it gains or loses the
same amount of time every day. This is called systematic drift.
hwclock's --adjust function lets you apply systematic drift
corrections to the Hardware Clock.
It works like this: hwclock keeps a file, /etc/adjtime, that
keeps some historical information. This is called the adjtime
file.
Suppose you start with no adjtime file. You issue a hwclock --set
command to set the Hardware Clock to the true current time.
hwclock creates the adjtime file and records in it the current
time as the last time the clock was calibrated. Five days later,
the clock has gained 10 seconds, so you issue a hwclock --set
--update-drift command to set it back 10 seconds. hwclock updates
the adjtime file to show the current time as the last time the
clock was calibrated, and records 2 seconds per day as the
systematic drift rate. 24 hours go by, and then you issue a
hwclock --adjust command. hwclock consults the adjtime file and
sees that the clock gains 2 seconds per day when left alone and
that it has been left alone for exactly one day. So it subtracts
2 seconds from the Hardware Clock. It then records the current
time as the last time the clock was adjusted. Another 24 hours go
by and you issue another hwclock --adjust. hwclock does the same
thing: subtracts 2 seconds and updates the adjtime file with the
current time as the last time the clock was adjusted.
When you use the --update-drift option with --set or --systohc,
the systematic drift rate is (re)calculated by comparing the
fully drift corrected current Hardware Clock time with the new
set time, from that it derives the 24 hour drift rate based on
the last calibrated timestamp from the adjtime file. This updated
drift factor is then saved in /etc/adjtime.
A small amount of error creeps in when the Hardware Clock is set,
so --adjust refrains from making any adjustment that is less than
1 second. Later on, when you request an adjustment again, the
accumulated drift will be more than 1 second and --adjust will
make the adjustment including any fractional amount.
hwclock --hctosys also uses the adjtime file data to compensate
the value read from the Hardware Clock before using it to set the
System Clock. It does not share the 1 second limitation of
--adjust, and will correct sub-second drift values immediately.
It does not change the Hardware Clock time nor the adjtime file.
This may eliminate the need to use --adjust, unless something
else on the system needs the Hardware Clock to be compensated.
The Adjtime File
While named for its historical purpose of controlling adjustments
only, it actually contains other information used by hwclock from
one invocation to the next.
The format of the adjtime file is, in ASCII:
Line 1: Three numbers, separated by blanks: 1) the systematic
drift rate in seconds per day, floating point decimal; 2) the
resulting number of seconds since 1969 UTC of most recent
adjustment or calibration, decimal integer; 3) zero (for
compatibility with clock(8)) as a floating point decimal.
Line 2: One number: the resulting number of seconds since 1969
UTC of most recent calibration. Zero if there has been no
calibration yet or it is known that any previous calibration is
moot (for example, because the Hardware Clock has been found,
since that calibration, not to contain a valid time). This is a
decimal integer.
Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set
to Coordinated Universal Time or local time. You can always
override this value with options on the hwclock command line.
You can use an adjtime file that was previously used with the
clock(8) program with hwclock.
Automatic Hardware Clock Synchronization by the Kernel
You should be aware of another way that the Hardware Clock is
kept synchronized in some systems. The Linux kernel has a mode
wherein it copies the System Time to the Hardware Clock every 11
minutes. This mode is a compile time option, so not all kernels
will have this capability. This is a good mode to use when you
are using something sophisticated like NTP to keep your System
Clock synchronized. (NTP is a way to keep your System Time
synchronized either to a time server somewhere on the network or
to a radio clock hooked up to your system. See RFC 1305.)
If the kernel is compiled with the '11 minute mode' option it
will be active when the kernel's clock discipline is in a
synchronized state. When in this state, bit 6 (the bit that is
set in the mask 0x0040) of the kernel's time_status variable is
unset. This value is output as the 'status' line of the adjtimex
--print or ntptime commands.
It takes an outside influence, like the NTP daemon to put the
kernel's clock discipline into a synchronized state, and
therefore turn on '11 minute mode'. It can be turned off by
running anything that sets the System Clock the old fashioned
way, including hwclock --hctosys. However, if the NTP daemon is
still running, it will turn '11 minute mode' back on again the
next time it synchronizes the System Clock.
If your system runs with '11 minute mode' on, it may need to use
either --hctosys or --systz in a startup script, especially if
the Hardware Clock is configured to use the local timescale.
Unless the kernel is informed of what timescale the Hardware
Clock is using, it may clobber it with the wrong one. The kernel
uses UTC by default.
The first userspace command to set the System Clock informs the
kernel what timescale the Hardware Clock is using. This happens
via the persistent_clock_is_local kernel variable. If --hctosys
or --systz is the first, it will set this variable according to
the adjtime file or the appropriate command-line argument. Note
that when using this capability and the Hardware Clock timescale
configuration is changed, then a reboot is required to notify the
kernel.
hwclock --adjust should not be used with NTP '11 minute mode'.
ISA Hardware Clock Century value
There is some sort of standard that defines CMOS memory Byte 50
on an ISA machine as an indicator of what century it is. hwclock
does not use or set that byte because there are some machines
that don't define the byte that way, and it really isn't
necessary anyway, since the year-of-century does a good job of
implying which century it is.
If you have a bona fide use for a CMOS century byte, contact the
hwclock maintainer; an option may be appropriate.
Note that this section is only relevant when you are using the
"direct ISA" method of accessing the Hardware Clock. ACPI
provides a standard way to access century values, when they are
supported by the hardware.
