RE: [RFE]: watchdog: atmel: atmel-sama5d4-wdt

From: Ken Sloat
Date: Mon Jun 10 2019 - 13:44:59 EST

> -----Original Message-----
> From: Guenter Roeck <groeck7@xxxxxxxxx> On Behalf Of Guenter Roeck
> Sent: Monday, June 10, 2019 12:28 PM
> To: Ken Sloat <KSloat@xxxxxxxxxxxxxx>
> Cc: Nicolas.Ferre@xxxxxxxxxxxxx; alexandre.belloni@xxxxxxxxxxxxxxxxxx;
> wim@xxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-
> watchdog@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [RFE]: watchdog: atmel: atmel-sama5d4-wdt
> [This is an EXTERNAL EMAIL]
> ________________________________
> On Mon, Jun 10, 2019 at 03:51:52PM +0000, Ken Sloat wrote:
> > Hello Nicolas,
> >
> > I wanted to open a discussion proposing new functionality to allow
> > disabling of the watchdog timer upon entering suspend in the SAMA5D2/4.
> >
> > Typical use case of a hardware watchdog timer in the kernel is a
> > userspace application opens the watchdog timer and periodically
> > "kicks" it. If the application hits a deadlock somewhere and is no
> > longer able to kick it, then the watchdog intervenes and often resets
> > the processor. Such is the case for the Atmel driver (which also
> > allows a watchdog interrupt to be asserted in lieu of a system reset). In
> most use cases, upon entering a low power/suspend state, the application
> will no longer be able to "kick" the watchdog. If the watchdog is not disabled
> or kicked via another method, then it will reset the system. This is the current
> behavior of the Atmel driver as of today.
> >
> > The watchdog peripheral itself does have a "WDIDLEHLT" bit however,
> > and this is enabled via the "atmel,idle-halt" dt property. However,
> > this is not very useful, as it literally only makes the watchdog count
> > when the CPU is active. This results in non-deterministic triggering
> > of the WDT and means that if a critical application were to crash, it
> > may be quite a long time before the WDT would ever trigger. Below is a
> > similar statement made in the device-tree doc for this
> > peripheral:
> >
> > - atmel,idle-halt: present if you want to stop the watchdog when the CPU is
> > in idle state.
> > CAUTION: This property should be used with care, it actually makes the
> > watchdog not counting when the CPU is in idle state, therefore the
> > watchdog reset time depends on mean CPU usage and will not reset at
> all
> > if the CPU stop working while it is in idle state, which is probably
> > not what you want.
> >
> > It seems to me, that it would be logical and useful to introduce a new
> > property that would cause the Atmel WDT to disable on suspend and
> > re-enable on resume. It also appears that the WDT is re-initialized anyways
> upon resume, so the only piece missing here would really be a dt flag and a
> call to disable.
> >
Hello Guenter,

> Wondering - why would this need a dt property ? That would be quite
> unusual. Is there a condition where one would _not_ want the watchdog to
> stop on suspend ?

Good point, not sure there would be.

> If anything I would suggest to drop atmel,idle-halt completely; it really looks
> like it would make the watchdog unreliable.
I agree, while it is a function of the SAMA5, it seems to me that it is not very
useful in Linux, unless I am missing something. I will wait for Nicolas to chime
in before I submit anything. I can certainly submit separate patches for each,
I already have something working for this.

> Thanks,
> Guenter

Ken Sloat