Patch "backlight: Report error on failure" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

backlight: Report error on failure

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
backlight-report-error-on-failure.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Sudip Mukherjee <[email protected]>
Date: Wed, 17 May 2017 21:55:07 +0100
Subject: backlight: Report error on failure

From: Sudip Mukherjee <[email protected]>

[ Upstream commit 7e715c2d9c27c23f3187454157c58cf292ed103e ]

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.

---
 drivers/video/backlight/backlight.c |   15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

@@ -134,7 +134,7 @@ static ssize_t bl_power_store(struct dev
 {
 	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)

@@ -145,10 +145,16 @@ static ssize_t bl_power_store(struct dev
 	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;
 		}
-		rc = count;
 	}
 	mutex_unlock(&bd->ops_lock);
 

@@ -176,8 +182,7 @@ int backlight_device_set_brightness(stru
 		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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/backlight-report-error-on-failure.patch