cpufreq: suspend cpufreq governors on shutdown

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.
4. We'll hang (possibly eventually hitting the hard lockup detector).

Let's avoid the problem by stopping the cpufreq governor at shutdown,
which is a sensible thing to do anyway.
Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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>
 
@@ -2556,6 +2557,14 @@ int cpufreq_unregister_driver(struct cpufreq_driver *driver)
 }
 EXPORT_SYMBOL_GPL(cpufreq_unregister_driver);
 
+/*
+ * Stop cpufreq at shutdown to make sure it isn't holding any locks
+ * or mutexes when secondary CPUs are halted.
+ */
+static struct syscore_ops cpufreq_syscore_ops = {
+	.shutdown = cpufreq_suspend,
+};
+
 static int __init cpufreq_core_init(void)
 {
 	if (cpufreq_disabled())
@@ -2564,6 +2573,8 @@ 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);