Re: [PATCH v2] Documentation: add watchdog documentation

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

On 18.02.19 08:05, Oleksij Rempel wrote:
> 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..2c453d9fa5
> --- /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 miss-configurations or bugs prevent the system from starting.

gr... forgot to fix miss...

> +- 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.
> +
> 

Kind regards,
Oleksij Rempel

-- 
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