Re: SP805 driver

[Leif Lindholm <leif.lindholm@linaro.org>]

On Tue, Dec 18, 2018 at 05:20:59AM +0000, Udit Kumar wrote:
> Thanks for note Leif,
> 
> I am trying to understand this behavior  of specification.
> 
> >"If no handler has been registered, or the registered handler
> >returns, then the system will be reset by calling the Runtime
> >Service ResetSystem()."
> I believe,  this handler needs to be called by wdt driver after wdt
> is expired. Meaning , we want wdt to work without resetting the
> system.
> Therefore a mask is needed with wdt, which will prevent to reset the
> system.   I see it working, at least for SP805, because of PMU mask
> bits.

Yes, if the WDT can be configured to generate an interrupt instead of
a hardware reset, it can be used to implement this protocol.

> Also ArmPkg/Drivers/GenericWatchdogDxe/GenericWatchdogDxe.c has this limitation.
> I haven’t read spec of this wdt.  I hope there should a mask around.

No, the situation for the GenericWatchdogDxe is not as dire.

Correct: it does not permit registering software handlers (which
perhaps we should do something about, but is ... acceptable).

_But_, it still conforms to the above text; when the timer expires,
it resets the system by calling the ResetSystem() service. It does not
directly force a hardware reset.

The severe problem is not the lack of the ability to register the
software handler (which does remove much of the utility), but the
removal of reset control from the system firmware.

> Coming back to hardware, which does not have mask around wdt, how to
> implement this feature.

Simple - you can't.

You can absolutely implement exactly the functionality you have today,
with minimal changes to the protocol - it just should not be registered
as an implementation of EFI_WATCHDOG_TIMER_ARCH_PROTOCOL.

Then your platform code can invoke this custom protocol as needed.

Regards,

Leif