linux-kernel - Re: [PATCH v3] backlight: report error on failure

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Date:   Mon, 8 May 2017 16:45:17 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Cc:     linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [PATCH v3] backlight: report error on failure

On 06/05/17 19:00, Sudip Mukherjee wrote:
> 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.

This comment is wrong, no code path through 
backlight_device_set_brightness() can possibly return ENXIO.

My review comment to v1 was:
 > Strictly speaking this is an ABI change. Its probably a harmless one
 > making it ok to change but I'm interested what testing or code review
 > you've done to be sure the userspace doesn't do odd things if the
 > kernel starts to pass through errors.

I find myself somewhat surprised to find the above review comment 
addressed by adding text to the patch header denying that there is a 
change of ABI...


Daniel.


> 
> Signed-off-by: Sudip Mukherjee <sudip.mukherjee@...ethink.co.uk>
> ---
> 
> v3: remove rc = 0, and update rc with count in else condition.
> 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 | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
> index 288318a..0289107 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)
> @@ -145,10 +145,15 @@ 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;
>   		}
> -		rc = count;
> +		rc = rc ? rc : count;
>   	}
>   	mutex_unlock(&bd->ops_lock);
>   
> @@ -176,8 +181,7 @@ 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);
>