Re: Window watchdog driver design

[Guenter Roeck]

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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