linux-kernel - Re: [PATCH v2 3/8] watchdog: Introduce WDOG

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Date:	Sat, 15 Aug 2015 16:38:26 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
CC:	linux-watchdog@...r.kernel.org, Wim Van Sebroeck <wim@...ana.be>,
	linux-kernel@...r.kernel.org,
	Timo Kokkonen <timo.kokkonen@...code.fi>,
	linux-doc@...r.kernel.org, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v2 3/8] watchdog: Introduce WDOG_RUNNING flag

On 08/14/2015 12:04 PM, Uwe Kleine-König wrote:
> Hello Guenter,
>
>> diff --git a/Documentation/watchdog/watchdog-kernel-api.txt b/Documentation/watchdog/watchdog-kernel-api.txt
>> index 25b00b878a7b..6a54dc15a556 100644
>> --- a/Documentation/watchdog/watchdog-kernel-api.txt
>> +++ b/Documentation/watchdog/watchdog-kernel-api.txt
>> [...]
>> @@ -193,9 +194,12 @@ they are supported. These optional routines/operations are:
>>   The status bits should (preferably) be set with the set_bit and clear_bit alike
>>   bit-operations. The status bits that are defined are:
>>   * WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
>> -  is active or not. When the watchdog is active after booting, then you should
>> -  set this status bit (Note: when you register the watchdog timer device with
>> -  this bit set, then opening /dev/watchdog will skip the start operation)
>> +  is active or not from user perspective. User space is expected to send
>> +  heartbeat requests to the driver while this flag is set. If the watchdog
>> +  is active after booting, and you don't want the infrastructure to send
>> +  heartbeats to the watchdog driver, then you should set this status bit.
>
> IMHO this should not be the driver author's choice! If you implement
> policy in the kernel it should at least be implemented in the framework
> and preferably easily changeable. (At least with Kconfig, but better use
> a kernel parameter (or both, the latter overriding the former).)
>
Agreed. I'll change that and simply drop that part. After all,
we now have WDOG_RUNNING to indicate that the watchdog is running.
I'll just have to make sure that there are no drivers which set
WDOG_ACTIVE at boot (afaics there are none).

>> +  Note: when you register the watchdog timer device with this bit set,
>> +  then opening /dev/watchdog will skip the start operation.
>>   * WDOG_DEV_OPEN: this status bit shows whether or not the watchdog device
>>     was opened via /dev/watchdog.
>>     (This bit should only be used by the WatchDog Timer Driver Core).
>> @@ -209,6 +213,11 @@ bit-operations. The status bits that are defined are:
>>     any watchdog_ops, so that you can be sure that no operations (other then
>>     unref) will get called after unregister, even if userspace still holds a
>>     reference to /dev/watchdog
>> +* WDOG_RUNNING: Set by the watchdog driver if the hardware watchdog is running.
>> +  The bit must be set if the watchdog timer hardware can not be stopped.
>> +  The bit may also be set if the watchdog timer is running aftyer booting,
>> +  before the watchdog device is opened. If set, the watchdog infrastructure
>> +  will send keepalives to the watchdog hardware while WDOG_ACTIVE is not set.
>>
>>     To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
>>     timer device) you can either:
>> [...]
>> diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
>> index c04ba1a98cc8..676e233d5e7b 100644
>> --- a/drivers/watchdog/watchdog_dev.c
>> +++ b/drivers/watchdog/watchdog_dev.c
>> @@ -59,7 +59,8 @@ static inline bool watchdog_need_worker(struct watchdog_device *wdd)
>>   	unsigned int m = wdd->max_timeout * 1000;
>>   	unsigned int t = wdd->timeout * 1000;
>>
>> -	return watchdog_active(wdd) && hm && (!m || hm < m) && t > hm;
>> +	return (watchdog_active(wdd) && hm && (!m || hm < m) && t > hm) ||
>> +	       (t && !watchdog_active(wdd) && watchdog_running(wdd));
>
> What is the meaning of
>
> 	!t && !watchdog_active(wdd) && watchdog_running(wdd)
>
> ? Can this happen at all? If not, drop "t && "?
>
t can be 0, meaning "the watchdog timeout is unknown", unless a driver sets min_timeout
to a value larger than 0. Unfortunately, that is not explicitly specified.

Thanks,
Guenter

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