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
