> -----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 Thanks, Ken Sloat
