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

[Guenter Roeck <linux@roeck-us.net>]

On Mon, Jun 10, 2019 at 10:13:01PM +0200, Alexandre Belloni wrote:
> Hello,
> 
> On 10/06/2019 09:28:11-0700, Guenter Roeck wrote:
> > 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.
> > > 
> > 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 ?
> > 
> 
> There are customers that protects suspend/resume using the watchdog.
> They wake up their platform every 15s to ping the watchdog.
> 

Interesting use case.

> Also, I don't see why the application deciding to go to suspend wouldn't
> be able to disable the watchdog before do so if this is the wanted policy.
> 

Many watchdog drivers already implement suspend/resume support. Such a
platform specific functionality seems to be quite undesirable to me.

Besides (and pretty much all watchdog drivers implementing suspend/resume
do that wrong), you'd likely want to disable the watchog late during
suspend and early during resume to reduce the risk of a hang. I don't
think you can do that from userspace.

Thanks,
Guenter