linux-kernel - Re: Window watchdog driver design

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Message-ID: <20150515054341.GA26468@wernerandy.de>
Date:	Fri, 15 May 2015 07:43:41 +0200
From:	Andreas Werner <andy@...nerandy.de>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Window watchdog driver design

On Thu, May 14, 2015 at 05:52:38PM -0700, Guenter Roeck wrote:
> 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
> 

Thanks for the answer.

The module parameter would be ok for me, but it would be better if i can enable/disable
the lower window by the application.

I know that the API defines the timout in seconds but what about ms? Is there no
watchdog out there which has timout values < seconds?.

In my case I can only set 2 timouts (1sec and 2sec) but I need to support all 8 timeout
values.

The other thing is that my Watchdog can have differen timeout values depending
on the CPLD and the customer requirements. I can not read out this values, they are
only defined in the specification.

This is why i had the idea with the table to only set some "indexes" for the timout
to handle all the cases.

Regards
Andy

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