Re: [RFC] Using a watchdog as system reset

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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Hello Guenter,

On Tue, Oct 06, 2020 at 07:29:11AM -0700, Guenter Roeck wrote:
> On 10/6/20 4:56 AM, Guenter Roeck wrote:
> > On 10/6/20 3:29 AM, Uwe Kleine-König wrote:
> >> Hello,
> >>
> >> I have an i.MX25 system here with an external watchdog (using the
> >> gpio_wdt driver). So the internal watchdog (imx2_wdt) is unused.
> >>
> >> The problem with the unused imx2_wdt is that this usually provides the
> >> restart handler and now a reboot ends with
> >>
> >> 	reboot: Restarting system
> >> 	Reboot failed -- System halted
> >>
> >> until eventually the watchdog bites and resets the machine.
> >>
> >> I imagine that this is a common enough issue to warrant a generic
> >> solution. My suggestion is to formalize and implement something like:
> >>
> >> 	watchdog {
> >> 		compatible = "linux,wdt-gpio";
> >> 		...
> >> 		provide-system-reset;
> >> 	}
> >>
> >> with the sematic of: "This is the dedicated mechanism to reset this
> >> machine."
> >>
> > 
> > Some systems have more than one means to reset it, which is why
> > restart handlers have a priority. This in turn suggests that we should
> > maybe have a means to set that priority dynamically for the imx2_wdt driver
> > (or for watchdog drivers in general) instead of having it fixed at 128.
> > That would also solve your problem, assuming there is a different
> > (currently lower priority) means to reset the hardware in your system.
> > 
> > Alternatively, can't you just blacklist the imx2-wdt driver ?
> 
> After having another couple hours of sleep and a coffee, I wonder if
> this is already done, and the reboot just fails _because_ the imx2_wdt
> is _not_ loaded. Is that the case ?

Right, I disabled the imx2_wdt driver.
 
> If so, it looks like you want the reset functionality of the imx_wdt driver
> but not its watchdog functionality.

My thought was to use the gpio-watchdog as reset source, but using the
imx-watchdog only for reset but not watchdog is an obvious alternative I
didn't think about.

So I either want to make the gpio-watchdog provide a restart handler or
use the imx-watchdog driver to only provide a restart handler (but no
watchdog functionality).

> And the above would be a suggestion to add a "generic" restart
> functionality into the watchdog subsystem, one that uses a watchdog
> with minimum timeout to reset the system, even if its driver doesn't
> explicitly register a restart handler.  Is that what you are trying to
> suggest ?

Yes, every watchdog could provide a restart handler with just using the
watchdog callbacks.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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