linux-kernel - Re: Window watchdog driver design

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Date:	Thu, 14 May 2015 17:52:38 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Andreas Werner <andy@...nerandy.de>
CC:	linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Window watchdog driver design

On 05/14/2015 07:09 AM, Andreas Werner wrote:
> On Thu, May 14, 2015 at 06:30:05AM -0700, Guenter Roeck wrote:
>> On 05/14/2015 04:56 AM, Andreas Werner wrote:
>>> Hi,
>>> in the next few weeks I need to write a driver for a window wachtdog
>>> implemented in a CPLD. I have some questions about the design
>>> of the driver and the best way to write this driver to also be able
>>> to submit it.
>>>
>>> The triggering and configuration of the Watchdog is done by several GPIOs which
>>> are connected to the CPLD watchdog device. The correct GPIOs are configurable
>>> using the Device Tree.
>>>
>>> 1. Timeout
>>> 	The timeout values are defined in ms and start from 20ms to 2560ms.
>>> 	The timout is set by 3 GPIOs this means we have only 8 different
>>> 	timout values. It is also possible that a future Watchdog CPLD device
>>> 	does have different timeout values.
>>>
>>> 	Is it possible to set ms timeouts? It seems that the WDT API does
>>> 	only support a resolution of 1sec.
>>>
>>> 	One idea would be to use the API timeout as something like a timeout
>>> 	index to set the different values. Of course this needs to be documented.
>>>
>>> 	e.g.
>>> 	timeout	(API)	timeout in device
>>> 	1 		20ms
>>> 	2		100ms
>>> 	3		500ms
>>> 	...		...		
>>>
>>> 2. Upper/Lower Window
>>> 	There is currently no support for a windowed watchdog in the wdt core.
>>> 	The lower window can be activated by a gpio and its timeout is defined
>>> 	as "upper windows timeout/4"	
>>>
>>> 	What is the best way to implement those additional settings? Adding additional
>>> 	ioctl or export these in sysfs?
>>> --
>>
>> Sorry for the maybe dumb question, but what is a window watchdog,
>> and what is the lower window timeout for (assuming the upper window
>> timeout causes the watchdog to expire) ?
>>
>> Guenter
>>
>
> Oh sorry forgot to describe it in more detail.
>
> If you have a watchdog window you do not have just one timeout where the watchdog can expire.
> You have a so called "window" to trigger it within.
>
> 		|<----trig---->|
> ---lower timeout----------------upper timeout
>
> This means you have to trigger the watchdog not to late and not to early.
> This kind of watchdog is often used in embedded applications or more often
> in safety cases to fullfil requirements given e.g. by SIL1-SIL4 certifications.
>
> The lower timeout is set by a dedicated GPIO and the value will then "Upper timeout / 4". The
> upper timeout is set by 3 GPIOs to get different timeout values.
>

Thanks a lot for the explanation.

I would suggest to use a module parameter to enable the "lower timeout" functionality.

Timeouts have to be specified in seconds.

Hope this helps,
Guenter

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