Documentation: add watchdog documentation

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 85 insertions, 0 deletions

diff --git a/Documentation/user/watchdog.rst b/Documentation/user/watchdog.rst
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+Watchdog Support
+================
+
+Barebox Watchdog Functionality
+------------------------------
+
+In some cases we are not able to influence the hardware design anymore or while
+developing one needs to be able to feed the watchdog to disable it from within
+the bootloader. For these scenarios barebox provides the watchdog framework
+with the following functionality and at least ``CONFIG_WATCHDOG`` should be
+enabled.
+
+Polling
+~~~~~~~
+
+Watchdog polling/feeding allows to feed the watchdog and keep it running on one
+side and to not reset the system on the other side. It is needed on hardware
+with short-time watchdogs. For example the Atheros ar9331 watchdog has a
+maximal timeout of 7 seconds, so it may reset even on netboot.
+Or it can be used on systems where the watchdog is already running and can't be
+disabled, an example for that is the watchdog of the i.MX2 series.
+This functionally can be seen as a threat, since in error cases barebox will
+continue to feed the watchdog even if that is not desired. So, depending on
+your needs ``CONFIG_WATCHDOG_POLLER`` can be enabled or disabled at compile
+time. Even if barebox was built with watchdog polling support, it is not
+enabled by default. To start polling from command line run:
+
+.. code-block:: console
+
+  wdog0.autoping=1
+
+The poller interval is not configurable, but fixed at 500ms and the watchdog
+timeout is configured by default to the maximum of the supported values by
+hardware. To change the timeout used by the poller, run:
+
+.. code-block:: console
+
+  wdog0.timeout_cur=7
+
+To read the current watchdog's configuration, run:
+
+.. code-block:: console
+
+  devinfo wdog0
+
+The output may look as follows where ``timeout_cur`` and ``timeout_max`` are
+measured in seconds:
+
+.. code-block:: console
+
+  barebox@DPTechnics DPT-Module:/ devinfo wdog0
+  Parameters:
+    autoping: 1 (type: bool)
+    timeout_cur: 7 (type: uint32)
+    timeout_max: 10 (type: uint32)
+
+Use barebox' environment to persist these changes between reboots:
+
+.. code-block:: console
+
+  nv dev.wdog0.autoping=1
+  nv dev.wdog0.timeout_cur=7
+
+Boot Watchdog Timeout
+~~~~~~~~~~~~~~~~~~~~~
+
+With this functionality barebox may start a watchdog or update the timeout of
+an already-running one, just before kicking the boot image. It can be
+configured temporarily via
+
+.. code-block:: console
+
+  global boot.watchdog_timeout=10
+
+or persistently by
+
+.. code-block:: console
+
+  nv boot.watchdog_timeout=10
+
+where the used value again is measured in seconds.
+
+On a system with multiple watchdogs, the watchdog with the highest positive
+priority is the one affected by the ``boot.watchdog_timeout`` parameter.  If
+multiple watchdogs share the same priority, only one will be started.