Re: Window watchdog driver design

[Guenter Roeck <linux@roeck-us.net>]

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