[PATCH 1/2] watchdog: NXP LPC18XX Windowed Watchdog Timer Driver

Ezequiel Garcia ezequiel at vanguardiasur.com.ar
Wed Jul 1 08:22:12 PDT 2015

Hi Guenter,

First of all, thanks a lot for your feedback.

On 1 July 2015 at 10:54, Guenter Roeck <linux at roeck-us.net> wrote:
> On 07/01/2015 05:02 AM, Ariel D'Alessandro wrote:
>> (Sorry, I sent the last mail with an incorrect mail account)
>> El 01/07/15 a las 08:30, adalessandro escibió:
>>> El 29/06/15 a las 01:47, Guenter Roeck escibió:
>>>> On 06/28/2015 11:13 AM, Ariel D'Alessandro wrote:
>>>>>>> +/* Timeout values in seconds */
>>>>>>> +#define LPC_WDT_DEF_TIMEOUT        1
>>>>>>> +
>>>>>> One second ? This is highly unusual. 30 or 60 seconds is more common,
>>>>>> and one second would be very challenging for user space.
>>>>>> Any special reason for using such a tight default ?
>>>>> Considering that LPC18xx Watchdog has a fixed divide-by-4 clock
>>>>> pre-scaler and a 24-bit counter and that Watchdog clock runs at a fixed
>>>>> frequency of 12MHz, timeout range goes from 1 to 5 seconds.
>>>>> I think you're right, 1 sec is very challenging, so it's 5 secs then.
>>>> Ultimately you might want to consider a soft timer as backup to the
>>>> system
>>>> timeout. But that can be done later if/when needed.
>>> I understand your point, but just to be sure, what do mean by soft timer?
> A kernel function which pings the watchdog periodically even if the
> watchdog is open.
> Example: Timeout is set to 30 seconds. Since the HW watchdog times out
> earlier than that, it needs to be pinged regularly (eg every 2.5 seconds).
> The kernel does that with a timer unless user space does not ping the
> watchdog within the configured interval of 30 seconds.

Do we really need this? It sounds like bloat to me. Considering this watchdog
controller is included in cortex-M MCUs, you wouldn't expect the
scheduler to be under so much pressure.

I realize that 1-5 seconds is challenging for userspace, but having a soft timer
in the kernel side sounds like making the system heavier instead of lighter.

Not sure I'm making sense here, but I would think twice before adding bloat.

Maybe Ariel can submit his v2 (with basic watchdog support, which BTW also
provides a reset handler) now and we can pospone this discussion.
Ezequiel García, VanguardiaSur

More information about the linux-arm-kernel mailing list