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watchdog: update the documentation
While at it, split the watchdog section into a few paragraphs to make it easier to read as it becomes lengthy.
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Franck Bui
@@ -133,33 +133,46 @@
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<term><varname>RebootWatchdogSec=</varname></term>
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<term><varname>KExecWatchdogSec=</varname></term>
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<listitem><para>Configure the hardware watchdog at runtime and at reboot. Takes a timeout value in seconds (or
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in other time units if suffixed with <literal>ms</literal>, <literal>min</literal>, <literal>h</literal>,
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<literal>d</literal>, <literal>w</literal>). If <varname>RuntimeWatchdogSec=</varname> is set to a non-zero
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value, the watchdog hardware (<filename>/dev/watchdog</filename> or the path specified with
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<varname>WatchdogDevice=</varname> or the kernel option <varname>systemd.watchdog-device=</varname>) will be
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programmed to automatically reboot the system if it is not contacted within the specified timeout interval. The
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system manager will ensure to contact it at least once in half the specified timeout interval. This feature
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requires a hardware watchdog device to be present, as it is commonly the case in embedded and server
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systems. Not all hardware watchdogs allow configuration of all possible reboot timeout values, in which case
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the closest available timeout is picked. <varname>RebootWatchdogSec=</varname> may be used to configure the
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hardware watchdog when the system is asked to reboot. It works as a safety net to ensure that the reboot takes
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place even if a clean reboot attempt times out. Note that the <varname>RebootWatchdogSec=</varname> timeout
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applies only to the second phase of the reboot, i.e. after all regular services are already terminated, and
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after the system and service manager process (PID 1) got replaced by the <filename>systemd-shutdown</filename>
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binary, see system <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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for details. During the first phase of the shutdown operation the system and service manager remains running
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and hence <varname>RuntimeWatchdogSec=</varname> is still honoured. In order to define a timeout on this first
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phase of system shutdown, configure <varname>JobTimeoutSec=</varname> and <varname>JobTimeoutAction=</varname>
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in the [Unit] section of the <filename>shutdown.target</filename> unit. By default
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<varname>RuntimeWatchdogSec=</varname> defaults to 0 (off), and <varname>RebootWatchdogSec=</varname> to
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10min. <varname>KExecWatchdogSec=</varname> may be used to additionally enable the watchdog when kexec
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is being executed rather than when rebooting. Note that if the kernel does not reset the watchdog on kexec (depending
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on the specific hardware and/or driver), in this case the watchdog might not get disabled after kexec succeeds
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and thus the system might get rebooted, unless <varname>RuntimeWatchdogSec=</varname> is also enabled at the same time.
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For this reason it is recommended to enable <varname>KExecWatchdogSec=</varname> only if
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<varname>RuntimeWatchdogSec=</varname> is also enabled.
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These settings have no effect if a hardware watchdog is not available.</para></listitem>
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<listitem><para>Configure the hardware watchdog at runtime and at reboot. Takes a timeout value in
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seconds (or in other time units if suffixed with <literal>ms</literal>, <literal>min</literal>,
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<literal>h</literal>, <literal>d</literal>, <literal>w</literal>). If set to zero the watchdog logic
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is disabled: no watchdog device is opened, configured, or pinged. If set to the special string
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<literal>infinity</literal> the watchdog is opened and pinged in regular intervals, but the timeout
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is not changed from the default. If set to any other time value the watchdog timeout is configured to
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the specified value (or a value close to it, depending on hardware capabilities).</para>
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<para>If <varname>RuntimeWatchdogSec=</varname> is set to a non-zero value, the watchdog hardware
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(<filename>/dev/watchdog</filename> or the path specified with <varname>WatchdogDevice=</varname> or
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the kernel option <varname>systemd.watchdog-device=</varname>) will be programmed to automatically
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reboot the system if it is not contacted within the specified timeout interval. The system manager
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will ensure to contact it at least once in half the specified timeout interval. This feature requires
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a hardware watchdog device to be present, as it is commonly the case in embedded and server
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systems. Not all hardware watchdogs allow configuration of all possible reboot timeout values, in
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which case the closest available timeout is picked.</para>
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<para><varname>RebootWatchdogSec=</varname> may be used to configure the hardware watchdog when the
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system is asked to reboot. It works as a safety net to ensure that the reboot takes place even if a
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clean reboot attempt times out. Note that the <varname>RebootWatchdogSec=</varname> timeout applies
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only to the second phase of the reboot, i.e. after all regular services are already terminated, and
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after the system and service manager process (PID 1) got replaced by the
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<filename>systemd-shutdown</filename> binary, see system
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<citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
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details. During the first phase of the shutdown operation the system and service manager remains
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running and hence <varname>RuntimeWatchdogSec=</varname> is still honoured. In order to define a
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timeout on this first phase of system shutdown, configure <varname>JobTimeoutSec=</varname> and
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<varname>JobTimeoutAction=</varname> in the [Unit] section of the
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<filename>shutdown.target</filename> unit. By default <varname>RuntimeWatchdogSec=</varname> defaults
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to 0 (off), and <varname>RebootWatchdogSec=</varname> to 10min.</para>
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<para><varname>KExecWatchdogSec=</varname> may be used to additionally enable the watchdog when kexec
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is being executed rather than when rebooting. Note that if the kernel does not reset the watchdog on
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kexec (depending on the specific hardware and/or driver), in this case the watchdog might not get
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disabled after kexec succeeds and thus the system might get rebooted, unless
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<varname>RuntimeWatchdogSec=</varname> is also enabled at the same time. For this reason it is
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recommended to enable <varname>KExecWatchdogSec=</varname> only if
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<varname>RuntimeWatchdogSec=</varname> is also enabled.</para>
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<para>These settings have no effect if a hardware watchdog is not available.</para></listitem>
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</varlistentry>
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<varlistentry>
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