linux-kernel - Re: [RFC 1/3] watchdog: change watchdog_need

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Message-ID: <20160717192407.GA32415@spo001.leaseweb.nl>
Date:	Sun, 17 Jul 2016 21:24:07 +0200
From:	Wim Van Sebroeck <wim@...ana.be>
To:	Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc:	Guenter Roeck <linux@...ck-us.net>, linux-watchdog@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/3] watchdog: change watchdog_need_worker logic

Hi Rasmus,

> If the driver indicates that the watchdog is running, the framework
> should feed it until userspace opens the device, regardless of whether
> the driver has set max_hw_heartbeat_ms.
> 
> This patch only affects the case where wdd->max_hw_heartbeat_ms is
> zero, wdd->timeout is non-zero, the watchdog is not active and the
> hardware device is running (*):
> 
> - If wdd->timeout is zero, watchdog_need_worker() returns false both
> before and after this patch, and watchdog_next_keepalive() is not
> called.
> 
> - If watchdog_active(wdd), the return value from watchdog_need_worker
> is also the same as before (namely, hm && t > hm). Hence in that case,
> watchdog_next_keepalive() is only called if hm == max_hw_heartbeat_ms
> is non-zero, so the change to min_not_zero there is a no-op.
> 
> - If the watchdog is not active and the device is not running, we
> return false from watchdog_need_worker just as before.
> 
> That leaves the watchdog_hw_running(wdd) && !watchdog_active(wdd) &&
> wdd->timeout case. Again, it's easy to see that if
> wdd->max_hw_heartbeat_ms is non-zero, we return true from
> watchdog_need_worker with and without this patch, and the logic in
> watchdog_next_keepalive is unchanged. Finally, if
> wdd->max_hw_heartbeat_ms is 0, we used to end up in the
> cancel_delayed_work branch, whereas with this patch we end up
> scheduling a ping timeout_ms/2 from now.
> 
> (*) This should imply that no current kernel drivers are affected,
> since the only drivers which explicitly set WDOG_HW_RUNNING are
> imx2_wdt.c and dw_wdt.c, both of which also provide a non-zero value
> for max_hw_heartbeat_ms. The watchdog core also sets WDOG_HW_RUNNING,
> but only when the driver doesn't provide ->stop, in which case it
> must, according to Documentation/watchdog/watchdog-kernel-api.txt, set
> max_hw_heartbeat_ms.

This isn't completely true. We will have the following in the linux-watchdog tree:
drivers/watchdog/aspeed_wdt.c:		set_bit(WDOG_HW_RUNNING, &wdt->wdd.status);
drivers/watchdog/dw_wdt.c:	set_bit(WDOG_HW_RUNNING, &wdd->status);
drivers/watchdog/dw_wdt.c:		set_bit(WDOG_HW_RUNNING, &wdd->status);
drivers/watchdog/imx2_wdt.c:	set_bit(WDOG_HW_RUNNING, &wdog->status);
drivers/watchdog/imx2_wdt.c:		set_bit(WDOG_HW_RUNNING, &wdog->status);
drivers/watchdog/max77620_wdt.c:		set_bit(WDOG_HW_RUNNING, &wdt_dev->status);
drivers/watchdog/sbsa_gwdt.c:		set_bit(WDOG_HW_RUNNING, &wdd->status);
drivers/watchdog/tangox_wdt.c:		set_bit(WDOG_HW_RUNNING, &dev->wdt.status);

I checked the ones that aren't mentioned and aspeed_wdt, max77620_wdt and sbsa_gwdt.c
also have a non-zero value for max_hw_heartbeat_ms. But tangox_wdt.c doesn't set it.
This one will need to be looked at closer.

> 
> Signed-off-by: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
> ---
>  drivers/watchdog/watchdog_dev.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
> index 3595cff..14f8a92 100644
> --- a/drivers/watchdog/watchdog_dev.c
> +++ b/drivers/watchdog/watchdog_dev.c
> @@ -92,9 +92,13 @@ static inline bool watchdog_need_worker(struct watchdog_device *wdd)
>  	 *   thus is aware that the framework supports generating heartbeat
>  	 *   requests.
>  	 * - Userspace requests a longer timeout than the hardware can handle.
> +	 *
> +	 * Alternatively, if userspace has not opened the watchdog
> +	 * device, we take care of feeding the watchdog if it is
> +	 * running.
>  	 */
> -	return hm && ((watchdog_active(wdd) && t > hm) ||
> -		      (t && !watchdog_active(wdd) && watchdog_hw_running(wdd)));
> +	return (hm && watchdog_active(wdd) && t > hm) ||
> +		(t && !watchdog_active(wdd) && watchdog_hw_running(wdd));
>  }
>  
>  static long watchdog_next_keepalive(struct watchdog_device *wdd)
> @@ -107,7 +111,7 @@ static long watchdog_next_keepalive(struct watchdog_device *wdd)
>  	unsigned int hw_heartbeat_ms;
>  
>  	virt_timeout = wd_data->last_keepalive + msecs_to_jiffies(timeout_ms);
> -	hw_heartbeat_ms = min(timeout_ms, wdd->max_hw_heartbeat_ms);
> +	hw_heartbeat_ms = min_not_zero(timeout_ms, wdd->max_hw_heartbeat_ms);
>  	keepalive_interval = msecs_to_jiffies(hw_heartbeat_ms / 2);
>  
>  	if (!watchdog_active(wdd))
> -- 
> 2.5.0
> 

Kind regards,
Wim.