[PATCH v2] backlight: report error on failure

[Sudip Mukherjee <sudipm.mukherjee@gmail.com>]

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.

This is not a change of ABI as the userspace expects an error of ENXIO,
after this patch the range of errors that are returned to the userspace
will increase.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
---

v2: update power with old value on failure.

Copy-pasting from the last patch conversation:

The problem that prompted me to send this patch is the current project
that I am working on now. And we faced this there. The userspace code is
writing to the sysfs node to poweron the backlight and reported success.
But sometimes we noticed that backlight was not actally powered on. And
that lead me to check the code and noticed that it is swallowing all the
errors.

 drivers/video/backlight/backlight.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
index 288318a..ad0aa6a 100644
--- a/drivers/video/backlight/backlight.c
+++ b/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)
@@ -143,12 +143,16 @@ static ssize_t bl_power_store(struct device *dev, struct device_attribute *attr,
 	rc = -ENXIO;
 	mutex_lock(&bd->ops_lock);
 	if (bd->ops) {
+		rc = 0;
 		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;
 		}
-		rc = count;
+		rc = rc ? rc : count;
 	}
 	mutex_unlock(&bd->ops_lock);
 
@@ -176,8 +180,7 @@ 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);
-- 
1.9.1