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Thursday, May 23, 2013

Testing Watchdog related code without reboots

While writing some code for handling the Watchdog on the Raspberry Pi I wanted to verify what I had written so far. If using the character-based approach it's easy to simulate with any file and watch the contents, but when using IOCTL's or for a more realistic test it's better to just use a real Watchdog driver. Fortunately it's easy to do on Linux without causing any sudden reboots when the Watchdog isn't stopped when exiting your program. There's a softdog module that is providing a software-level watchdog driver, which can also be set up not to reboot at all, which makes testing really easy. Let's start by loading the module:

$ sudo modprobe softdog soft_noboot=1 soft_margin=15

dmesg | grep softdog should include a line similar to this:

softdog: Software Watchdog Timer: 0.08 initialized. soft_noboot=1 soft_margin=15 sec soft_panic=0 (nowayout=0)

Now you can run your watchdog code, for example echo a | sudo tee /dev/watchdog (just don't use V, because it stops the watchdog) and a line similar to this will show:

watchdog watchdog0: watchdog did not stop!

Now if you wait another 15 seconds (as set by the soft_margin argument to modprobe) you will see the following message:

softdog: Triggered - Reboot ignored

That's it. The watchdog was triggered, left running, and it triggered. You can run it again and again, and your system won't suddenly reboot (as it would when testing with the actual hardware watchdog).

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