[RFC] ARM: sched_clock: update epoch_cyc on resume

[Colin Cross]

Many clocks that are used to provide sched_clock will reset during
suspend.  If read_sched_clock returns 0 after suspend, sched_clock will
appear to jump forward.  This patch resets cd.epoch_cyc to the current
value of read_sched_clock during resume, which causes sched_clock() just
after suspend to return the same value as sched_clock() just before
suspend.

In addition, during the window where epoch_ns has been updated before
suspend, but epoch_cyc has not been updated after suspend, it is unknown
whether the clock has reset or not, and sched_clock() could return a
bogus value.  Add a suspended flag, and return the pre-suspend epoch_ns
value during this period.

This will have a side effect of causing SoCs that have clocks that
continue to count in suspend to appear to stop counting, reporting the
same sched_clock() value before and after suspend.

Signed-off-by: Colin Cross <ccross at android.com>
---
 arch/arm/kernel/sched_clock.c |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/arch/arm/kernel/sched_clock.c b/arch/arm/kernel/sched_clock.c
index 27d186a..46c7d32 100644
--- a/arch/arm/kernel/sched_clock.c
+++ b/arch/arm/kernel/sched_clock.c
@@ -21,6 +21,7 @@ struct clock_data {
 	u32 epoch_cyc_copy;
 	u32 mult;
 	u32 shift;
+	bool suspended;
 };
 
 static void sched_clock_poll(unsigned long wrap_ticks);
@@ -49,6 +50,9 @@ static unsigned long long cyc_to_sched_clock(u32 cyc, u32 mask)
 	u64 epoch_ns;
 	u32 epoch_cyc;
 
+	if (cd.suspended)
+		return cd.epoch_ns;
+
 	/*
 	 * Load the epoch_cyc and epoch_ns atomically.  We do this by
 	 * ensuring that we always write epoch_cyc, epoch_ns and
@@ -169,11 +173,20 @@ void __init sched_clock_postinit(void)
 static int sched_clock_suspend(void)
 {
 	sched_clock_poll(sched_clock_timer.data);
+	cd.suspended = true;
 	return 0;
 }
 
+static void sched_clock_resume(void)
+{
+	cd.epoch_cyc = read_sched_clock();
+	cd.epoch_cyc_copy = cd.epoch_cyc;
+	cd.suspended = false;
+}
+
 static struct syscore_ops sched_clock_ops = {
 	.suspend = sched_clock_suspend,
+	.resume = sched_clock_resume,
 };
 
 static int __init sched_clock_syscore_init(void)
-- 
1.7.7.3