linux-kernel - Re: [PATCH 2/8] watchdog: Introduce hardware maximum timeout in watchdog core

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Date:	Tue, 04 Aug 2015 09:03:27 -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>,
	kernel@...gutronix.de
Subject: Re: [PATCH 2/8] watchdog: Introduce hardware maximum timeout in watchdog
 core

Hi Uwe,

On 08/04/2015 08:52 AM, Uwe Kleine-König wrote:
> On Tue, Aug 04, 2015 at 08:31:43AM -0700, Guenter Roeck wrote:
>> Hi Uwe,
>>
>> On 08/04/2015 05:18 AM, Uwe Kleine-König wrote:
>>> On Mon, Aug 03, 2015 at 07:13:28PM -0700, Guenter Roeck wrote:
>>>> Introduce an optional hardware maximum timeout in the watchdog core.
>>>> The hardware maximum timeout can be lower than the maximum timeout.
>>> Is this only until all drivers are converted to make use of the central
>>> worker? Otherwise this doesn't make sense, right?
>>>
>>>> Drivers can set the maximum hardare timeout value in the watchdog data
>>> s/hardare/hardware/
>>>
>> Always those fat fingers ;-)
>>
>>>> structure. If the configured timeout exceeds half the value of the
>>>> maximum hardware timeout, the watchdog core enables a timer function
>>>> to assist sending keepalive requests to the watchdog driver.
>>> I don't understand why you want to halve the maximum hw-timeout. If my
>>> watchdog has hw-max-timeout = 5s and userspace sets it to 3s there
>>> should be no need for assistance?! I think the implementation is the
>>> other way round?
>>>
>> It is supposed to reflect the _maximum_ timeout. That is different to
>> the time between heartbeats, which is supposed to be less; using half
>> the value of the maximum hardware timeout seemed to be a safe number.
> Right, I got that. With hw-max-timeout = 5s the machine resets after 5s
> not caring for the device. And so pinging repeatedly after 2.5s is fine.
> But if userspace sets a timeout of 3s (probably with the intention to
> ping with a frequency of 1/1.5s) there is no need for worker-assistance,
> because the pings coming in each 1.5s provided by userspace are good
> enough.
>
Yes, that is how it is supposed to work.

>>>> +static inline bool watchdog_need_worker(struct watchdog_device *wdd)
>>>> +{
>>>> +	unsigned int hm = wdd->max_hw_timeout_ms;
>>>> +	unsigned int m = wdd->max_timeout * 1000;
>>>> +
>>>> +	return watchdog_active(wdd) && hm && hm != m &&
>>>> +		wdd->timeout * 500 > hm;
>>>
>>> I don't understand what max_timeout is now that there is max_hw_timeout.
>>> So I don't understand why you need hm != m either.
>>>
>>
>> Backward compatibility. A driver which does not set max_hw_timeout_ms,
>> or sets both to the same value, by definition expects to handle everything
>> internally, and thus no worker is configured.
> And a driver that does
>
> 	max_timeout = 5
> 	max_hw_timeout = 5125
>
> falls through the cracks.
>
Hmm - not that this configuration makes any sense, but you are right.
I'll make it "hm < m".

Thanks,
Guenter

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