Re: [PATCH] backlight: propagate errors from get_brightness()

[Daniel Thompson <daniel.thompson@linaro.org>]

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.


> ---
>  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);

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
>