[PATCH] backlight: propagate errors from get_brightness()

backlight.h documents "struct backlight_ops->get_brightness()" to return
a negative errno on failure.
So far these errors have not been handled in the backlight core.
This leads to negative values being exposed through sysfs although only
positive values are documented to be reported.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
 drivers/video/backlight/backlight.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
index 537fe1b376ad..d681962f8509 100644
--- a/drivers/video/backlight/backlight.c
+++ b/drivers/video/backlight/backlight.c

@@ -292,10 +292,13 @@ static ssize_t actual_brightness_show(struct device *dev,
 	struct backlight_device *bd = to_backlight_device(dev);
 
 	mutex_lock(&bd->ops_lock);
-	if (bd->ops && bd->ops->get_brightness)
-		rc = sprintf(buf, "%d\n", bd->ops->get_brightness(bd));
-	else
+	if (bd->ops && bd->ops->get_brightness) {
+		rc = bd->ops->get_brightness(bd);
+		if (rc >= 0)
+			rc = sprintf(buf, "%d\n", rc);
+	} else {
 		rc = sprintf(buf, "%d\n", bd->props.brightness);
+	}
 	mutex_unlock(&bd->ops_lock);
 
 	return rc;

@@ -381,9 +384,18 @@ ATTRIBUTE_GROUPS(bl_device);
 void backlight_force_update(struct backlight_device *bd,
 			    enum backlight_update_reason reason)
 {
+	int brightness;
+
 	mutex_lock(&bd->ops_lock);
-	if (bd->ops && bd->ops->get_brightness)
-		bd->props.brightness = bd->ops->get_brightness(bd);
+	if (bd->ops && bd->ops->get_brightness) {
+		brightness = bd->ops->get_brightness(bd);
+		if (brightness >= 0)
+			bd->props.brightness = brightness;
+		else
+			dev_warn(&bd->dev,
+				 "Could not update brightness from device: errno = %d",
+				 -brightness);
+	}
 	mutex_unlock(&bd->ops_lock);
 	backlight_generate_event(bd, reason);
 }

base-commit: 79fad92f2e596f5a8dd085788a24f540263ef887
-- 
2.33.0

On Mon, Sep 06, 2021 at 11:55:25PM +0200, Thomas Weißschuh wrote:

backlight.h documents "struct backlight_ops->get_brightness()" to return
a negative errno on failure.
So far these errors have not been handled in the backlight core.
This leads to negative values being exposed through sysfs although only
positive values are documented to be reported.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

Change looks good overall but I've raised a few quibbles about the new
error message below.

The format string is missing a \n and should it really be a dev_warn()?
Is dev_err() more appropriate?

Also please print the error symbolically (which is self explaining
meaning we don't need the errno prefix):

		"Could not update brightness from device: %pe\n",
		ERR_PTR(brightness)



Daniel.

+	}
 	mutex_unlock(&bd->ops_lock);
 	backlight_generate_event(bd, reason);
 }

base-commit: 79fad92f2e596f5a8dd085788a24f540263ef887
-- 
2.33.0