[RFC PATCH] timekeeping: Correct run-time detection of real-time clock.

From: Zoran Markovic
Date: Fri May 17 2013 - 14:24:43 EST

Since commit <31ade30692dc9680bfc95700d794818fa3f754ac>, timekeeping_init()
checks for presence of persistent clock by attempting to read a non-zero
time value from real-time clock. This is an issue on platforms where
persistent_clock (instead of a RTC) is implemented as a free-running counter
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.

Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Feng Tang <feng.tang@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Zoran Markovic <zoran.markovic@xxxxxxxxxx>
kernel/time/timekeeping.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 98cd470..baeeb5c 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -975,6 +975,14 @@ static int timekeeping_suspend(void)


+ /*
+ * 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;
raw_spin_lock_irqsave(&timekeeper_lock, flags);

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