linux-kernel - Re: [PATCH v2] watchdog: wdat_wdt: Set the min and max timeout values properly

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Date:   Mon, 8 Aug 2022 04:42:27 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Jean Delvare <jdelvare@...e.de>, linux-watchdog@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH v2] watchdog: wdat_wdt: Set the min and max timeout values
 properly

On 8/5/22 23:15, Jean Delvare wrote:
> The wdat_wdt driver is misusing the min_hw_heartbeat_ms field. This
> field should only be used when the hardware watchdog device should not
> be pinged more frequently than a specific period. The ACPI WDAT
> "Minimum Count" field, on the other hand, specifies the minimum
> timeout value that can be set. This corresponds to the min_timeout
> field in Linux's watchdog infrastructure.
> 
> Setting min_hw_heartbeat_ms instead can cause pings to the hardware
> to be delayed when there is no reason for that, eventually leading to
> unexpected firing of the watchdog timer (and thus unexpected reboot).
> 
> I'm also changing max_hw_heartbeat_ms to max_timeout for symmetry,
> although the use of this one isn't fundamentally wrong, but there is
> also no reason to enable the software-driven ping mechanism for the
> wdat_wdt driver.
> 

I dislike this part because it changes behavior and is unrelated to
the problem at hand, but I assume Mike knows the actual hardware limits
and understands the implications (ie that there is indeed no need to
enable the software-driven ping mechanism).

> Signed-off-by: Jean Delvare <jdelvare@...e.de>
> Fixes: 058dfc767008 ("ACPI / watchdog: Add support for WDAT hardware watchdog")
> Reviewed-by! Mika Westerberg <mika.westerberg@...ux.intel.com>
> Cc: Wim Van Sebroeck <wim@...ux-watchdog.org>
> Cc: Guenter Roeck <linux@...ck-us.net>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Reviewed-by: Guenter Roeck <linux@...ck-us.net>

> ---
> Changes since v1:
>   * Fix a stupid typo which broke the build. Apparently I shouldn't be
>     sending out patches after midnight, sorry.
> 
>   drivers/watchdog/wdat_wdt.c |    8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> --- linux-5.18.orig/drivers/watchdog/wdat_wdt.c	2022-07-27 07:32:33.336928967 +0200
> +++ linux-5.18/drivers/watchdog/wdat_wdt.c	2022-08-06 08:09:49.235935543 +0200
> @@ -342,8 +342,8 @@ static int wdat_wdt_probe(struct platfor
>   		return -EINVAL;
>   
>   	wdat->period = tbl->timer_period;
> -	wdat->wdd.min_hw_heartbeat_ms = wdat->period * tbl->min_count;
> -	wdat->wdd.max_hw_heartbeat_ms = wdat->period * tbl->max_count;
> +	wdat->wdd.min_timeout = DIV_ROUND_UP(wdat->period * tbl->min_count, 1000);
> +	wdat->wdd.max_timeout = wdat->period * tbl->max_count / 1000;
>   	wdat->stopped_in_sleep = tbl->flags & ACPI_WDAT_STOPPED;
>   	wdat->wdd.info = &wdat_wdt_info;
>   	wdat->wdd.ops = &wdat_wdt_ops;
> @@ -450,8 +450,8 @@ static int wdat_wdt_probe(struct platfor
>   	 * watchdog properly after it has opened the device. In some cases
>   	 * the BIOS default is too short and causes immediate reboot.
>   	 */
> -	if (timeout * 1000 < wdat->wdd.min_hw_heartbeat_ms ||
> -	    timeout * 1000 > wdat->wdd.max_hw_heartbeat_ms) {
> +	if (timeout < wdat->wdd.min_timeout ||
> +	    timeout > wdat->wdd.max_timeout) {
>   		dev_warn(dev, "Invalid timeout %d given, using %d\n",
>   			 timeout, WDAT_DEFAULT_TIMEOUT);
>   		timeout = WDAT_DEFAULT_TIMEOUT;
> 
>