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 ? If anything I would suggest to drop atmel,idle-halt completely; it really looks like it would make the watchdog unreliable. Thanks, Guenter
