Re: [PATCH v1] watchdog: sama5d4_wdt: Reset delay on start

[Guenter Roeck]

On Thu, Mar 03, 2016 at 01:53:47PM +0100, Romain Izard wrote:
> Hi Guenter,
> 
> 2016-03-03 13:10 GMT+01:00 Guenter Roeck <linux@xxxxxxxxxxxx>:
> > On 03/03/2016 02:29 AM, Romain Izard wrote:
> >>
> >> If the internal counter is not refreshed when the watchdog is started
> >> for the first time, the watchdog will trigger very rapidly. For
> >> example, opening /dev/watchdog without writing in it will immediately
> >> trigger a reboot, instead of waiting for the delay to expire.
> >>
> >> To avoid this problem, reload the timer on opening the watchdog
> >> device.
> >>
> >> Command: "while sleep 5; do echo 1; done > /dev/watchdog"
> >> Before: system reset
> >> After: the watchdog runs correctly
> >>
> >> Signed-off-by: Romain Izard <romain.izard.pro@xxxxxxxxx>
> >
> >
> > Subject might better read "ping watchdog on start" or similar.
> >
> OK. I'll change it for a v2.
> 
> > Does the watchdog have to be pinged before it is enabled ?  I am a bit
> > concerned that there may still be a 125 uS window during which the
> > system could restart.
> >
> 
> According to the SAMA5D2 & SAMA5D4 datasheets, the timer ought to be
> reloaded when the watchdog is enabled by a write in the MR register.
> Unfortunately, it does not work as described, as I encountered the
> problem on a SAMA5D2 Xplained board.
> 
> The 4 clock delay is not in the datasheet either, but without any delay
> the timer is clearly not reloaded, as my issue stays the same. As there
> is a required delay before writing to MR after writing to CR, I applied
> the same type of delay in the reverse case.
> 
Question is if there is now a 4 clock window where the watchdog can
still reset the system. Can you reload the timer before enabling
the watchdog ?

Thanks,
Guenter

> Perhaps Nicolas or Wenyou have more information on this.
> 
> Best regards,
> -- 
> Romain Izard