Re: [PATCH 2/8] watchdog: Introduce hardware maximum timeout in watchdog core

[Guenter Roeck]

Hi Uwe,

On 08/04/2015 08:52 AM, Uwe Kleine-König wrote:
> On Tue, Aug 04, 2015 at 08:31:43AM -0700, Guenter Roeck wrote:
> > Hi Uwe,
> >
> > On 08/04/2015 05:18 AM, Uwe Kleine-König wrote:
> > > On Mon, Aug 03, 2015 at 07:13:28PM -0700, Guenter Roeck wrote:
> > > > Introduce an optional hardware maximum timeout in the watchdog core.
> > > > The hardware maximum timeout can be lower than the maximum timeout.
> > > Is this only until all drivers are converted to make use of the central
> > > worker? Otherwise this doesn't make sense, right?
> > >
> > > > Drivers can set the maximum hardare timeout value in the watchdog data
> > > s/hardare/hardware/
> > Always those fat fingers ;-)
> >
> > > > structure. If the configured timeout exceeds half the value of the
> > > > maximum hardware timeout, the watchdog core enables a timer function
> > > > to assist sending keepalive requests to the watchdog driver.
> > > I don't understand why you want to halve the maximum hw-timeout. If my
> > > watchdog has hw-max-timeout = 5s and userspace sets it to 3s there
> > > should be no need for assistance?! I think the implementation is the
> > > other way round?
> > It is supposed to reflect the _maximum_ timeout. That is different to
> > the time between heartbeats, which is supposed to be less; using half
> > the value of the maximum hardware timeout seemed to be a safe number.
> Right, I got that. With hw-max-timeout = 5s the machine resets after 5s
> not caring for the device. And so pinging repeatedly after 2.5s is fine.
> But if userspace sets a timeout of 3s (probably with the intention to
> ping with a frequency of 1/1.5s) there is no need for worker-assistance,
> because the pings coming in each 1.5s provided by userspace are good
> enough.
Yes, that is how it is supposed to work.

> > > > +static inline bool watchdog_need_worker(struct watchdog_device *wdd)
> > > > +{
> > > > +	unsigned int hm = wdd->max_hw_timeout_ms;
> > > > +	unsigned int m = wdd->max_timeout * 1000;
> > > > +
> > > > +	return watchdog_active(wdd) && hm && hm != m &&
> > > > +		wdd->timeout * 500 > hm;
> > >
> > > I don't understand what max_timeout is now that there is max_hw_timeout.
> > > So I don't understand why you need hm != m either.
> >
> > Backward compatibility. A driver which does not set max_hw_timeout_ms,
> > or sets both to the same value, by definition expects to handle everything
> > internally, and thus no worker is configured.
> And a driver that does
>
> 	max_timeout = 5
> 	max_hw_timeout = 5125
>
> falls through the cracks.
Hmm - not that this configuration makes any sense, but you are right.
I'll make it "hm < m".

Thanks,
Guenter

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