linux-kernel - Re: [PATCH] watchdog: make nowayout sysfs file writable

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Message-ID: <63003ae2-a789-066d-6bb5-1136ff17acbe@roeck-us.net>
Date:   Tue, 5 Nov 2019 06:22:26 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>
Cc:     linux-kernel@...r.kernel.org, linux-watchdog@...r.kernel.org
Subject: Re: [PATCH] watchdog: make nowayout sysfs file writable

On 11/5/19 4:31 AM, Rasmus Villemoes wrote:
> It can be useful to delay setting the nowayout feature for a watchdog
> device. Moreover, not every driver (notably gpio_wdt) implements a
> nowayout module parameter/otherwise respects CONFIG_WATCHDOG_NOWAYOUT,
> and modifying those drivers carries a risk of causing a regression for
> someone who has two watchdog devices, sets CONFIG_WATCHDOG_NOWAYOUT
> and somehow relies on the gpio_wdt driver being ignorant of
> that (i.e., allowing one to gracefully close a gpio_wdt but not the
> other watchdog in the system).
> 
> So instead, simply make the nowayout sysfs file writable. Obviously,
> setting nowayout is a one-way street.
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
>   .../ABI/testing/sysfs-class-watchdog          |  9 ++++++--
>   drivers/watchdog/watchdog_dev.c               | 22 ++++++++++++++++++-
>   2 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-class-watchdog b/Documentation/ABI/testing/sysfs-class-watchdog
> index 675f9b537661..9860a8b2ba75 100644
> --- a/Documentation/ABI/testing/sysfs-class-watchdog
> +++ b/Documentation/ABI/testing/sysfs-class-watchdog
> @@ -17,8 +17,13 @@ What:		/sys/class/watchdog/watchdogn/nowayout
>   Date:		August 2015
>   Contact:	Wim Van Sebroeck <wim@...ana.be>
>   Description:
> -		It is a read only file. While reading, it gives '1' if that
> -		device supports nowayout feature else, it gives '0'.
> +		It is a read/write file. While reading, it gives '1'
> +		if the device has the nowayout feature set, otherwise
> +		it gives '0'. Writing a '1' to the file enables the
> +		nowayout feature. Once set, the nowayout feature
> +		cannot be disabled, so writing a '0' either has no
> +		effect (if the feature was already disabled) or
> +		results in a permission error.
>    >   What:		/sys/class/watchdog/watchdogn/state
>   Date:		August 2015
> diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
> index dbd2ad4c9294..0c478b8f8d5a 100644
> --- a/drivers/watchdog/watchdog_dev.c
> +++ b/drivers/watchdog/watchdog_dev.c
> @@ -452,7 +452,27 @@ static ssize_t nowayout_show(struct device *dev, struct device_attribute *attr,
>   
>   	return sprintf(buf, "%d\n", !!test_bit(WDOG_NO_WAY_OUT, &wdd->status));
>   }
> -static DEVICE_ATTR_RO(nowayout);
> +
> +static ssize_t nowayout_store(struct device *dev, struct device_attribute *attr,
> +				const char *buf, size_t len)
> +{
> +	struct watchdog_device *wdd = dev_get_drvdata(dev);
> +	unsigned int value, current;
> +	int ret;
> +
> +	ret = kstrtouint(buf, 0, &value);
> +	if (ret)
> +		return ret;
> +	if (value > 1)
> +		return -EINVAL;
> +	current = !!test_bit(WDOG_NO_WAY_OUT, &wdd->status);

"!!" on test_bit is unnecessary, and assigning the result to an unsigned int
doesn't really make sense. Might as well drop the variable and use test_bit
in the expression below directly.

Guenter

> +	/* nowayout cannot be disabled once set */
> +	if (current && !value)
> +		return -EPERM;
> +	watchdog_set_nowayout(wdd, value);
> +	return len;
> +}
> +static DEVICE_ATTR_RW(nowayout);
>   
>   static ssize_t status_show(struct device *dev, struct device_attribute *attr,
>   				char *buf)
>