[PATCH v3] Documentation: add watchdog documentation

[Oleksij Rempel <o.rempel@pengutronix.de>]

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 Documentation/user/user-manual.rst |   1 +
 Documentation/user/watchdog.rst    | 116 +++++++++++++++++++++++++++++
 2 files changed, 117 insertions(+)
 create mode 100644 Documentation/user/watchdog.rst

diff --git a/Documentation/user/user-manual.rst b/Documentation/user/user-manual.rst
index 516b760b1b..d5526de285 100644
--- a/Documentation/user/user-manual.rst
+++ b/Documentation/user/user-manual.rst
@@ -33,6 +33,7 @@ Contents:
    system-reset
    state
    random
+   watchdog
 
 * :ref:`search`
 * :ref:`genindex`
diff --git a/Documentation/user/watchdog.rst b/Documentation/user/watchdog.rst
new file mode 100644
index 0000000000..87c63aa078
--- /dev/null
+++ b/Documentation/user/watchdog.rst
@@ -0,0 +1,116 @@
+Watchdog Support
+================
+
+Warnings and Design Consideration
+---------------------------------
+
+A watchdog is the last line of defense on misbehaving systems. Thus, proper
+hardware and watchdog design considerations should be made to be able to reduce
+the impact of failing systems in the field. In the best case, the bootloader
+should not touch it at all. No watchdog feeding should be done until
+application-critical software (or a userspace service manager such as
+'systemd') was started.
+
+In case the bootloader is responsible for watchdog activation, the system can
+be considered as failed by design. The following threats can affect the system
+which are mostly addressable by properly designed watchdog and watchdog
+strategy:
+
+- software-based misconfigurations or bugs prevent the system from starting.
+- glitches caused by under-voltage, inappropriate power-on sequence or noisy
+  power supply.
+- physical damages caused by humidity, vibration or temperature.
+- temperature-based misbehavior of the system, e.g. clock is not running or
+  running with wrong frequency.
+- chemical reactions, e.g. some clock crystals will stop to work in contact
+  with Helium, see for example:
+  https://ifixit.org/blog/11986/iphones-are-allergic-to-helium/
+- failed storage prevents booting. NAND, SD, SSD, HDD, SPI-flash all of this
+  some day stop to work because their read/write cycles are exceeded.
+
+In all these cases, the bootloader won't be able to start and a properly
+designed watchdog may take some action. For example: recover the system by
+resetting it, or power it off to reduce the damage.
+
+Barebox Watchdog Functionality
+------------------------------
+
+Nevertheless, 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, only the first one (wdog0) is affected by
+the ``boot.watchdog_timeout`` parameter.
+
-- 
2.20.1


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