linux-stable/kernel/time
John Stultz cb33217b1b time: Avoid accumulating time drift in suspend/resume
Because the read_persistent_clock interface is usually backed by
only a second granular interface, each time we read from the persistent
clock for suspend/resume, we introduce a half second (on average) of error.

In order to avoid this error accumulating as the system is suspended
over and over, this patch measures the time delta between the persistent
clock and the system CLOCK_REALTIME.

If the delta is less then 2 seconds from the last suspend, we compensate
by using the previous time delta (keeping it close). If it is larger
then 2 seconds, we assume the clock was set or has been changed, so we
do no correction and update the delta.

Note: If NTP is running, ths could seem to "fight" with the NTP corrected
time, where as if the system time was off by 1 second, and NTP slewed the
value in, a suspend/resume cycle could undo this correction, by trying to
restore the previous offset from the persistent clock.  However, without
this patch, since each read could cause almost a full second worth of
error, its possible to get almost 2 seconds of error just from the
suspend/resume cycle alone, so this about equal to any offset added by
the compensation.

Further on systems that suspend/resume frequently, this should keep time
closer then NTP could compensate for if the errors were allowed to
accumulate.

Credits to Arve Hjønnevåg for suggesting this solution.

CC: Arve Hjønnevåg <arve@android.com>
CC: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>
2011-06-21 16:55:37 -07:00
..
alarmtimer.c hrtimers: Avoid touching inactive timer bases 2011-05-23 13:59:54 +02:00
clockevents.c clockevents: Handle empty cpumask gracefully 2011-06-03 11:13:33 +02:00
clocksource.c clocksource: Make watchdog robust vs. interruption 2011-06-16 19:30:53 +02:00
jiffies.c Fix common misspellings 2011-03-31 11:26:23 -03:00
Kconfig time: Kill off CONFIG_GENERIC_TIME 2010-07-27 12:40:54 +02:00
Makefile timers: Introduce in-kernel alarm-timer interface 2011-04-26 14:01:44 -07:00
ntp.c ntp: fix non privileged system time shifting 2011-04-04 08:31:23 -07:00
posix-clock.c posix clocks: Replace mutex with reader/writer semaphore 2011-04-18 10:39:38 +02:00
tick-broadcast.c Merge branch 'timers/urgent' into timers/core 2011-05-20 20:08:05 +02:00
tick-common.c Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip 2011-03-15 18:53:35 -07:00
tick-internal.h Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip 2011-03-15 18:53:35 -07:00
tick-oneshot.c time: Make do_timer() and xtime_lock local to kernel/time/ 2011-01-31 19:26:50 +01:00
tick-sched.c time: Make do_timer() and xtime_lock local to kernel/time/ 2011-01-31 19:26:50 +01:00
timecompare.c time: Use ARRAY_SIZE macro in timecompare.c 2010-10-21 17:30:06 +02:00
timeconv.c time: add function to convert between calendar time and broken-down time for universal use 2009-09-24 07:20:56 -07:00
timekeeping.c time: Avoid accumulating time drift in suspend/resume 2011-06-21 16:55:37 -07:00
timer_list.c timer debug: Hide kernel addresses via %pK in /proc/timer_list 2011-02-12 14:11:56 +01:00
timer_stats.c Fix common misspellings 2011-03-31 11:26:23 -03:00