leds: implement sysfs interface locking mechanism

Add a mechanism for locking LED subsystem sysfs interface.
This patch prepares ground for addition of LED Flash Class
extension, whose API will be integrated with V4L2 Flash API.
Such a fusion enforces introducing a locking scheme, which
will secure consistent access to the LED Flash Class device.

The mechanism being introduced allows for disabling LED
subsystem sysfs interface by calling led_sysfs_disable function
and enabling it by calling led_sysfs_enable. The functions
alter the LED_SYSFS_DISABLE flag state and must be called
under mutex lock. The state of the lock is checked with use
of led_sysfs_is_disabled function. Such a design allows for
providing immediate feedback to the user space on whether
the LED Flash Class device is available or is under V4L2 Flash
sub-device control.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Bryan Wu <cooloney@gmail.com>

drivers/leds/led-class.c

@@ -40,17 +40,27 @@ static ssize_t brightness_store(struct device *dev,
 {
 	struct led_classdev *led_cdev = dev_get_drvdata(dev);
 	unsigned long state;
-	ssize_t ret = -EINVAL;
+	ssize_t ret;
+
+	mutex_lock(&led_cdev->led_access);
+
+	if (led_sysfs_is_disabled(led_cdev)) {
+		ret = -EBUSY;
+		goto unlock;
+	}
 
 	ret = kstrtoul(buf, 10, &state);
 	if (ret)
-		return ret;
+		goto unlock;
 
 	if (state == LED_OFF)
 		led_trigger_remove(led_cdev);
 	__led_set_brightness(led_cdev, state);
 
-	return size;
+	ret = size;
+unlock:
+	mutex_unlock(&led_cdev->led_access);
+	return ret;
 }
 static DEVICE_ATTR_RW(brightness);
 
@@ -214,6 +224,7 @@ int led_classdev_register(struct device *parent, struct led_classdev *led_cdev)
 #ifdef CONFIG_LEDS_TRIGGERS
 	init_rwsem(&led_cdev->trigger_lock);
 #endif
+	mutex_init(&led_cdev->led_access);
 	/* add to the list of leds */
 	down_write(&leds_list_lock);
 	list_add_tail(&led_cdev->node, &leds_list);
@@ -267,6 +278,8 @@ void led_classdev_unregister(struct led_classdev *led_cdev)
 	down_write(&leds_list_lock);
 	list_del(&led_cdev->node);
 	up_write(&leds_list_lock);
+
+	mutex_destroy(&led_cdev->led_access);
 }
 EXPORT_SYMBOL_GPL(led_classdev_unregister);
 

drivers/leds/led-core.c

@@ -143,3 +143,21 @@ int led_update_brightness(struct led_classdev *led_cdev)
 	return ret;
 }
 EXPORT_SYMBOL(led_update_brightness);
+
+/* Caller must ensure led_cdev->led_access held */
+void led_sysfs_disable(struct led_classdev *led_cdev)
+{
+	lockdep_assert_held(&led_cdev->led_access);
+
+	led_cdev->flags |= LED_SYSFS_DISABLE;
+}
+EXPORT_SYMBOL_GPL(led_sysfs_disable);
+
+/* Caller must ensure led_cdev->led_access held */
+void led_sysfs_enable(struct led_classdev *led_cdev)
+{
+	lockdep_assert_held(&led_cdev->led_access);
+
+	led_cdev->flags &= ~LED_SYSFS_DISABLE;
+}
+EXPORT_SYMBOL_GPL(led_sysfs_enable);

drivers/leds/led-triggers.c

@@ -37,6 +37,14 @@ ssize_t led_trigger_store(struct device *dev, struct device_attribute *attr,
 	char trigger_name[TRIG_NAME_MAX];
 	struct led_trigger *trig;
 	size_t len;
+	int ret = count;
+
+	mutex_lock(&led_cdev->led_access);
+
+	if (led_sysfs_is_disabled(led_cdev)) {
+		ret = -EBUSY;
+		goto unlock;
+	}
 
 	trigger_name[sizeof(trigger_name) - 1] = '\0';
 	strncpy(trigger_name, buf, sizeof(trigger_name) - 1);
@@ -47,7 +55,7 @@ ssize_t led_trigger_store(struct device *dev, struct device_attribute *attr,
 
 	if (!strcmp(trigger_name, "none")) {
 		led_trigger_remove(led_cdev);
-		return count;
+		goto unlock;
 	}
 
 	down_read(&triggers_list_lock);
@@ -58,12 +66,14 @@ ssize_t led_trigger_store(struct device *dev, struct device_attribute *attr,
 			up_write(&led_cdev->trigger_lock);
 
 			up_read(&triggers_list_lock);
-			return count;
+			goto unlock;
 		}
 	}
 	up_read(&triggers_list_lock);
 
-	return -EINVAL;
+unlock:
+	mutex_unlock(&led_cdev->led_access);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(led_trigger_store);