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This article provides information on the Linux<sup>&reg;</sup> WDT (Watchdog Timer) framework.<br/>
It explains how to activate the WDT interface and, based on examples, how to use it.<br />
<br />

== Framework purpose ==
A Watchdog Timer is a hardware circuit that can reset the computer system in case of a software fault (for example, when the software is blocked in an infinity loop and cannot reload the timer before an elapsed time).

==System overview==
[[File:watchdog_overview.png|center|link=]]

===Component description===
From client application to hardware, the components are the following:
*'''Application''' (User Space): an application that configures and regularly pings the char device<ref name="char device">[https://bootlin.com/doc/training/linux-kernel/linux-kernel-slides.pdf character device interface], ''Linux Kernel and Driver Development'' training document, see '''Character drivers''' and '''Kernel frameworks for device drivers''' chapter</ref>.
*'''User Space interface''': composed of a standard char device<ref name="char device" />.
*'''Watchdog framework''' (Kernel Space): this component provides a watchdog common interface <ref name="wdt user api">{{CodeSource | Linux kernel | Documentation/watchdog/watchdog-api.txt | kernel.org watchdog user api}}</ref>(through a char device) to configure and control a watchdog hardware block.
*'''User driver''' (Kernel Space): Linux kernel drivers that handle internal peripherals or external devices under watchdog core<ref name="wdt kernel api">{{CodeSource | Linux kernel | Documentation/watchdog/watchdog-kernl-api.txt | kernel.org watchdog user api}}</ref>.
*'''STM32 peripherals''' (Hardware): Hardware block for watchdog handling ([[IWDG internal peripheral|IWDT]]).

===API description===
A userspace daemon notifies the kernel watchdog driver that the userspace is still alive at regular intervals. This is done via the char device<ref name="char device" /> special device file ((/dev/watchdog). When such a notification occurs, the driver tells the hardware watchdog that everything is in order, and that the watchdog must wait a little more to reset the system. 
If the userspace fails (because of a RAM error or a kernel bug), the notifications stop and the hardware watchdog resets the system after the timeout occurs, thus causing a reboot.

==Configuration ==


===Kernel configuration===
By default, the Watchdog Timer (WDT<ref name="wdt user api"/>) framework is activated in STMicroelectronics deliveries. If a specific configuration is needed, this section indicates how the WDT framework can be activated/inactivated in the kernel.

The WDT framework can be activated in the kernel configuration via Linux<sup>&reg;</sup> Menuconfig tool.
Follow the steps below to configure the kernel via Menuconfig:
<pre>
[*] Watchdog Timer Support  --->
    [*] Disable watchdog shutdown on close
    [*] Update boot-enabled watchdog until userspace takes over
    <*>   STM32 Independent WatchDoG (IWDG) support
</pre>

===Device tree configuration===
DT configuration can be done thanks to STM32CubeMX.

Please refer to the [[IWDG_device_tree_configuration| IWDG device tree configuration]].

==How to use the Watchdog framework==
The Watchdog framework can be used by the userspace application:

* The kernel provides a simple code example<ref>{{CodeSource | Linux kernel | samples/watchdog/watchdog-simple.c | user example}}</ref>
* Busybox provides a watchdog daemon<ref>[https://busybox.net/downloads/BusyBox.html busybox commands]</ref>
* Systemd provides a watchdog support<ref>[http://0pointer.de/blog/projects/watchdog.html systemd watchdog configuration]</ref>

==How to trace and debug the framework==
===How to trace===
For details on dynamic trace usage, refer to How to use the kernel dynamic debug.
 {{Board$}} echo "file drivers/watchdog/* +p" > /sys/kernel/debug/dynamic_debug/control

==Source code location==
*{{CodeSource | Linux kernel | drivers/watchdog/watchdog_core.c | WatchdogAPI core}}
*{{CodeSource | Linux kernel | include/uapi/linux/watchdog.h | WatchdogAPI interface}}
*{{CodeSource | Linux kernel | drivers/watchdog/stm32_iwdg.c | stm32 iwdg drivers}}

==References==
<references />

<noinclude>
[[Category:Watchdog|0]]
{{PublicationRequestId | 10344 | 2019-01-17 | AnneJ}}
{{ArticleBasedOnModel | Framework overview article model}}
</noinclude>