[RFC] ARM: sched_clock: update epoch_cyc on resume

From: Colin Cross
Date: Tue Jul 17 2012 - 19:28:05 EST

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

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@xxxxxxxxxxx>
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)
+ 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)

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