[ 78/79] timekeeping: Correct run-time detection of persistent_clock.

[Greg Kroah-Hartman]

3.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Zoran Markovic <zoran.markovic@xxxxxxxxxx>

commit 0d6bd9953f739dad96d9a0de65383e479ab4e10d upstream.

Since commit 31ade30692dc9680bfc95700d794818fa3f754ac, timekeeping_init()
checks for presence of persistent clock by attempting to read a non-zero
time value. This is an issue on platforms where persistent_clock (instead
is implemented as a free-running counter (instead of an RTC) starting
from zero on each boot and running during suspend. Examples are some ARM
platforms (e.g. PandaBoard).

An attempt to read such a clock during timekeeping_init() may return zero
value and falsely declare persistent clock as missing. Additionally, in
the above case suspend times may be accounted twice (once from
timekeeping_resume() and once from rtc_resume()), resulting in a gradual
drift of system time.

This patch does a run-time correction of the issue by doing the same check
during timekeeping_suspend().

A better long-term solution would have to return error when trying to read
non-existing clock and zero when trying to read an uninitialized clock, but
that would require changing all persistent_clock implementations.

This patch addresses the immediate breakage, for now.

Signed-off-by: Zoran Markovic <zoran.markovic@xxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Feng Tang <feng.tang@xxxxxxxxx>
[jstultz: Tweaked commit message and subject]
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
[zoran.markovic@xxxxxxxxxx: reworked patch to fit 3.9-stable.]
Signed-off-by: Zoran Markovic <zoran.markovic@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/time/timekeeping.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -826,6 +826,14 @@ static int timekeeping_suspend(void)
 
 	read_persistent_clock(&timekeeping_suspend_time);
 
+	/*
+	 * On some systems the persistent_clock can not be detected at
+	 * timekeeping_init by its return value, so if we see a valid
+	 * value returned, update the persistent_clock_exists flag.
+	 */
+	if (timekeeping_suspend_time.tv_sec || timekeeping_suspend_time.tv_nsec)
+		persistent_clock_exist = true;
+
 	write_seqlock_irqsave(&tk->lock, flags);
 	timekeeping_forward_now(tk);
 	timekeeping_suspended = 1;


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