[PATCH/RFC v6 1/3] leds: implement sysfs interface locking mechanism

[Jacek Anaszewski <j.anaszewski@samsung.com>]

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: Bryan Wu <cooloney@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
---
 drivers/leds/led-class.c    |   19 ++++++++++++++++---
 drivers/leds/led-core.c     |   18 ++++++++++++++++++
 drivers/leds/led-triggers.c |   16 +++++++++++++---
 include/linux/leds.h        |   32 ++++++++++++++++++++++++++++++++
 4 files changed, 79 insertions(+), 6 deletions(-)

diff --git a/drivers/leds/led-class.c b/drivers/leds/led-class.c
index 2e124aa2..a39ca8f 100644
--- a/drivers/leds/led-class.c
+++ b/drivers/leds/led-class.c
@@ -39,17 +39,27 @@ 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);
 
@@ -213,6 +223,7 @@ 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);
@@ -266,6 +277,8 @@ 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);
 
diff --git a/drivers/leds/led-core.c b/drivers/leds/led-core.c
index 0d15aa9..cca86ab 100644
--- a/drivers/leds/led-core.c
+++ b/drivers/leds/led-core.c
@@ -142,3 +142,21 @@ 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);
diff --git a/drivers/leds/led-triggers.c b/drivers/leds/led-triggers.c
index c3734f1..e8b1120 100644
--- a/drivers/leds/led-triggers.c
+++ b/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);
 
diff --git a/include/linux/leds.h b/include/linux/leds.h
index f8b2f58..44c8a98 100644
--- a/include/linux/leds.h
+++ b/include/linux/leds.h
@@ -13,6 +13,7 @@
 #define __LINUX_LEDS_H_INCLUDED
 
 #include <linux/list.h>
+#include <linux/mutex.h>
 #include <linux/rwsem.h>
 #include <linux/spinlock.h>
 #include <linux/timer.h>
@@ -42,6 +43,7 @@ struct led_classdev {
 #define LED_BLINK_ONESHOT	(1 << 17)
 #define LED_BLINK_ONESHOT_STOP	(1 << 18)
 #define LED_BLINK_INVERT	(1 << 19)
+#define LED_SYSFS_DISABLE	(1 << 20)
 
 	/* Set LED brightness level */
 	/* Must not sleep, use a workqueue if needed */
@@ -85,6 +87,9 @@ struct led_classdev {
 	/* true if activated - deactivate routine uses it to do cleanup */
 	bool			activated;
 #endif
+
+	/* Ensures consistent access to the LED Flash Class device */
+	struct mutex		led_access;
 };
 
 extern int led_classdev_register(struct device *parent,
@@ -151,6 +156,33 @@ extern void led_set_brightness(struct led_classdev *led_cdev,
  */
 extern int led_update_brightness(struct led_classdev *led_cdev);
 
+/**
+ * led_sysfs_disable - disable LED sysfs interface
+ * @led_cdev: the LED to set
+ *
+ * Disable the led_cdev's sysfs interface.
+ */
+extern void led_sysfs_disable(struct led_classdev *led_cdev);
+
+/**
+ * led_sysfs_enable - enable LED sysfs interface
+ * @led_cdev: the LED to set
+ *
+ * Enable the led_cdev's sysfs interface.
+ */
+extern void led_sysfs_enable(struct led_classdev *led_cdev);
+
+/**
+ * led_sysfs_is_disabled - check if LED sysfs interface is disabled
+ * @led_cdev: the LED to query
+ *
+ * Returns: true if the led_cdev's sysfs interface is disabled.
+ */
+static inline bool led_sysfs_is_disabled(struct led_classdev *led_cdev)
+{
+	return led_cdev->flags & LED_SYSFS_DISABLE;
+}
+
 /*
  * LED Triggers
  */
-- 
1.7.9.5