at91: watchdog: Can't get watchdog to trigger

[Yang, Wenyou]

> -----Original Message-----
> From: Bryan Evenson [mailto:bevenson at melinkcorp.com]
> Sent: 2013年8月15日 9:42
> To: linux-arm-kernel at lists.infradead.org
> Cc: Ferre, Nicolas; Yang, Wenyou
> Subject: at91: watchdog: Can't get watchdog to trigger
> 
> All,
> 
> I am using Atmel's stable branch of the Linux kernel
> (https://github.com/linux4sam/linux-at91.git, branch linux-3.6.9-at91, back at
> commit 5fe012d4aee98ef82df66e6763882c735902ed0f) on a AT91SAM9G25
> based board.  I am having issues getting the watchdog to trigger with this
> version of the kernel.
> 
> First, I've verified that the watchdog is enabled in the bootstrap and U-Boot.  I
> see the line:
> 
> at91sam9_wdt: enabled (heartbeat=15 sec, nowayout=0)
> 
> during boot showing that the watchdog is enabled.  I've tried triggering the
> watchdog by writing a single character to it from the command line (echo "0" >
> /dev/watchdog) and by using the two watchdog sample applications under
> Documentation/watchdog.  In every case I see the line:
> 
> watchdog watchdog0: watchdog did not stop!
> 
> But the board does not reset.  Just to make sure, I also set
> CONFIG_WATCHDOG_NOWAYOUT in the kernel configuration and I still have
> the same results.
> 
> I have my kernel configuration and the device tree files for my build over at
> https://github.com/evensonbryan/at91sam9g25-dt.git.  Other than the
> device tree files and the configuration, I have not made any kernel
> modifications.  What do I have setup wrong?

This is the at91_wdt controller inherent property, once the watchdog is started, it can't be stopped.
Because the Watchdog Mode Register (WDT_MR) can be written only once. Only a processor reset resets it, as said in the datasheet.

So the driver is implemented as following,
1./ If the \dev\watchdog device is opened by the user space application, then it is the application's responsibility to feed the watchdog. If the application does not feed the watchdog, it will trigger RESET.
2./ If the device isn't opened or close by the application, The responsibility to feed the watchdog is turned to the driver.

For your case, 
echo "0" > /dev/watchdog, it will arise the following actions
1./ open \dev\watchdog device, -->The responsibility to feed the watchdog is turned to the user space application
2./ start the watchdog controller
3./ ping the watchdog controller
4./ stop the watchdog controller --> Since the watchdog can't be stopped, so output: watchdog watchdog0: watchdog did not stop!
5./ close \dev\watchdog device --> The responsibility to feed the watchdog is turned to the Kernel Space

So, the watchdog doesn't trigger the RESET.

Documentation/watchdog/src/ watchdog-simple.c: after opening \dev\watchdog device, it is always feed the watchdog with the 10 seconds interval.
Documentation/watchdog/src/ watchdog-test.c: after opening \dev\watchdog device, it is always ping the watchdog with the 1 seconds interval.

Bye

Wenyou Yang

> 
> Thanks,
> Bryan