jz4740-battery: Protect against concurrent battery readings

We can not handle more then one ADC request at a time to the battery.
The patch adds a mutex around the ADC read code to ensure this.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: stable@kernel.org
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>

drivers/power/jz4740-battery.c

@@ -47,6 +47,8 @@ struct jz_battery {
 
 	struct power_supply battery;
 	struct delayed_work work;
+
+	struct mutex lock;
 };
 
 static inline struct jz_battery *psy_to_jz_battery(struct power_supply *psy)
@@ -68,6 +70,8 @@ static long jz_battery_read_voltage(struct jz_battery *battery)
 	unsigned long val;
 	long voltage;
 
+	mutex_lock(&battery->lock);
+
 	INIT_COMPLETION(battery->read_completion);
 
 	enable_irq(battery->irq);
@@ -91,6 +95,8 @@ static long jz_battery_read_voltage(struct jz_battery *battery)
 	battery->cell->disable(battery->pdev);
 	disable_irq(battery->irq);
 
+	mutex_unlock(&battery->lock);
+
 	return voltage;
 }
 
@@ -291,6 +297,7 @@ static int __devinit jz_battery_probe(struct platform_device *pdev)
 	jz_battery->pdev = pdev;
 
 	init_completion(&jz_battery->read_completion);
+	mutex_init(&jz_battery->lock);
 
 	INIT_DELAYED_WORK(&jz_battery->work, jz_battery_work);