backlight: Report error on failure

It is possible to update the backlight power and the brightness using
the sysfs and on writing it either returns the count or if the callback
function does not exist then returns the error code 'ENXIO'.

We have a situation where the userspace client is writing to the sysfs
to update the power and since the callback function exists the client
receives the return value as count and considers the operation to be
successful. That is correct as the write to the sysfs was successful.
But there is no way to know if the actual operation was done or not.

backlight_update_status() returns the error code if it fails. Pass that
to the userspace client who is trying to update the power so that the
client knows that the operation failed.
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/video/backlight/backlight.c

@@ -134,7 +134,7 @@ static ssize_t bl_power_store(struct device *dev, struct device_attribute *attr,
 {
 	int rc;
 	struct backlight_device *bd = to_backlight_device(dev);
-	unsigned long power;
+	unsigned long power, old_power;
 
 	rc = kstrtoul(buf, 0, &power);
 	if (rc)
@@ -145,11 +145,17 @@ static ssize_t bl_power_store(struct device *dev, struct device_attribute *attr,
 	if (bd->ops) {
 		pr_debug("set power to %lu\n", power);
 		if (bd->props.power != power) {
+			old_power = bd->props.power;
 			bd->props.power = power;
-			backlight_update_status(bd);
-		}
+			rc = backlight_update_status(bd);
+			if (rc)
+				bd->props.power = old_power;
+			else
+				rc = count;
+		} else {
 			rc = count;
 		}
+	}
 	mutex_unlock(&bd->ops_lock);
 
 	return rc;
@@ -176,8 +182,7 @@ int backlight_device_set_brightness(struct backlight_device *bd,
 		else {
 			pr_debug("set brightness to %lu\n", brightness);
 			bd->props.brightness = brightness;
-			backlight_update_status(bd);
-			rc = 0;
+			rc = backlight_update_status(bd);
 		}
 	}
 	mutex_unlock(&bd->ops_lock);