hwclock: hctosys drift compensation II

Allowing hctosys to drift compensate facilitates:

More precise setting of the System Clock early in
the boot process when --adjust cannot be used
because the file system is not writeable.

Applies sub second drift corrections immediately,
where as --adjust cannot.

Reduces boot time by not calling hwclock multiple
times, e.g., --hctosys early before fsck when the
file system is read-only, then --adjust later when
the file system is read-write and --hctosys again
for drift correction.

Use of --adjust elsewhere may no longer be
necessary.

Part II

After the original submission of this patch I
realized that now all operations except --systz
require drift corrected Hardware Clock time.
Therefore, it should be done only once early in
the process. Upon implementation of that premise
many improvements were facilitated:

* Adds drift correction to --hctosys.
* Adds setting system time with sub-second precision.
* Adds --get, a drift corrected 'show' operation.
* Improves drift factor calculation precision while
   greatly simplifying its algorithm.
* Fixes --show bug, printing integer sub-seconds, and
   now uses a more intuitive positive value.
* Fixes --predict bug, drift correction must be
   negated to predict future RTC time.
* Reduces the number of function arguments and
   lines of code.

Signed-off-by: J William Piggott <elseifthen@gmx.com>
hwclock: hctosys drift compensation II COMMENTS

Update source comments and --help output for the
hwclock: hctosys drift compensation II patch.

Signed-off-by: J William Piggott <elseifthen@gmx.com>
hwclock: hctosys drift compensation II MAN

Update hwclock man page for the
hwclock: hctosys drift compensation II patch.

Signed-off-by: J William Piggott <elseifthen@gmx.com>
hwclock: persistent_clock_is_local

When hctosys is used at boot time, making it the
first caller of settimeofday, the responsibility
of setting persistent_clock_is_local is thrust
upon it.  Currently hctosys always leaves this
variable uninitialized.  This causes a Hardware
Clock configured to use the local timescale to be
clobbered with the UTC timescale by the kernel's
NTP eleven minute mode.

This patch fixes this hctosys bug, by having it
properly set persistent_clock_is_local according
to the time scale configured for the Hardware
Clock.

It does this via the kernel warp_clock function
but this in inconsequential, because we set the
system time immediately afterward.

Signed-off-by: J William Piggott <elseifthen@gmx.com>
hwclock: persistent_clock_is_local MAN

Update hwclock man page for the
hwclock: persistent_clock_is_local patch.

Signed-off-by: J William Piggott <elseifthen@gmx.com>
hwclock: Add --update-drift option

There are cases where we need to refresh the
timestamps in the adjtime file without updating the
drift factor.

For example, with ntpd and an Eleven Minute Mode
kernel, we need to call systohc at shutdown to
facilitate drift correction.  With the current
behavior hwclock will clobber the drift factor to
near zero, because the Hardware Clock and System
Clock are synced by Eleven Minute Mode.  What
actually needs to be done is refresh the adjtime
file timestamps and not calculate a new drift
factor.

Because it is a manual process to craft a good
Hardware Clock drift factor, that is, there is no
automated method that will produce a good drift
factor, this patch changes the default drift
calculation behavior to off, and it is turned on
by using the --update-drift option. Once we have a good
drift factor for a given machine we do not want
anything clobbering it, including an administrator
forgetting to turn off recalculation. A system
administrator should make a concious effort in
telling hwclock with the --update-drift option that
(s)he wants to recalculate the drift factor.

Without using the --update-drift option with calibrate
operations only the timestamps are refreshed in
the adjtime file. With the --update-drift option the old
default behavior of refreshing the timestamps and
updating the drift factor is performed.

Signed-off-by: J William Piggott <elseifthen@gmx.com>
hwclock: Add --update-drift option MAN

Update hwclock man page for the
hwclock: Add --update-drift option patch.

Signed-off-by: J William Piggott <elseifthen@gmx.com>
Merge branch 'master' of https://github.com/jwpi/util-linux into hwclock
hwclock: cleanup man and usage()

Signed-off-by: Karel Zak <kzak@redhat.com>
hwclock: add notes about default behavior changes

Signed-off-by: Karel Zak <kzak@redhat.com>