Message-ID: <55558EBB.30502@roeck-us.net>
Date:	Thu, 14 May 2015 23:14:19 -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 10:43 PM, Andreas Werner wrote:
> 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.
>
You could try adding a sysfs attribute.

> 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?.
>
The ABI is the ABI, it has been there for a long time, and it only
supports second intervals.

> In my case I can only set 2 timouts (1sec and 2sec) but I need to support all 8 timeout
> values.
>
Kind of strict for a Linux watchdog. Most if not all other timeouts are much higher.
The drivers for hardware with low maximum timeout values often implement
a two-stage timeout, one handled in the driver that pings the actual hardware
timeout, and a soft-timeout to be triggered from user space with a more relaxed
timing.

> 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.
>
Normally you'd expect such platform specific details to be configured via devicetree
or platform data if that is not available (or ACPI, of course).

The userspace/kernel ABI/API needs to be standardized, so that user space doesn't
need to know implementation details.

> This is why i had the idea with the table to only set some "indexes" for the timout
> to handle all the cases.
>
Such an "index" would not be a well defined number. A standard application,
such as watchdogd or systemd, would not know what to do with it.

After all, the common use case of a watchdog driver is for it to interface with
a standard userspace application, so its interface to userspace needs to be well
defined. We can not permit "wildcards" such as redefining the meaning of a time
interval from seconds to something driver-specific; that would break all kinds
of applications.

Guenter

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