[PATCH] cpufreq: suspend cpufreq governors on shutdown

[Doug Anderson <dianders@chromium.org>]

We should stop cpufreq governors when we shut down the system.  If we
don't do this, we can end up with this deadlock:

1. cpufreq governor may be running on a CPU other than CPU0.
2. In machine_restart() we call smp_send_stop() which stops CPUs.
   If one of these CPUs was actively running a cpufreq governor
   then it may have the mutex / spinlock needed to access the main
   PMIC in the system (perhaps over I2C)
3. If a machine needs access to the main PMIC in order to shutdown
   then it will never get it since the mutex was lost when the other
   CPU stopped.

Let's avoid the race by stopping the cpufreq governor at shutdown,
which is a sensible thing to do anyway.

Signed-off-by: Doug Anderson <dianders@chromium.org>
---
NOTE: This was developed / tested / on a 3.14 kernel with backports.
I have confirmed that it compiles on a mainline kernel and doesn't
crash, but I haven't verified that there isn't some other fix in
mainline that also fixes this problem.  If you are aware of such a fix
then please drop this patch.

 drivers/cpufreq/cpufreq.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index a09a29c..bd89721 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -27,6 +27,7 @@
 #include <linux/mutex.h>
 #include <linux/slab.h>
 #include <linux/suspend.h>
+#include <linux/syscore_ops.h>
 #include <linux/tick.h>
 #include <trace/events/power.h>
 
@@ -2550,6 +2551,15 @@ int cpufreq_unregister_driver(struct cpufreq_driver *driver)
 }
 EXPORT_SYMBOL_GPL(cpufreq_unregister_driver);
 
+static void cpufreq_shutdown(void)
+{
+	cpufreq_suspend();
+}
+
+static struct syscore_ops cpufreq_syscore_ops = {
+	.shutdown = cpufreq_shutdown,
+};
+
 static int __init cpufreq_core_init(void)
 {
 	if (cpufreq_disabled())
@@ -2558,6 +2568,8 @@ static int __init cpufreq_core_init(void)
 	cpufreq_global_kobject = kobject_create();
 	BUG_ON(!cpufreq_global_kobject);
 
+	register_syscore_ops(&cpufreq_syscore_ops);
+
 	return 0;
 }
 core_initcall(cpufreq_core_init);
-- 
2.2.0.rc0.207.ga3a616c