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<!--     Synthetic target watchdog device                            -->
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<!-- Copyright (C) 2002 Bart Veer                                    -->
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<!-- Author(s):   bartv                                              -->
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<!-- Date:        2002/09/09                                         -->
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<reference id="devs-watchdog-synth-ref">
  <title>Synthetic Target Watchdog Device</title>

<refentry id="devs-watchdog-synth">
  <refmeta>
    <refentrytitle>Synthetic Target Watchdog Device</refentrytitle>
  </refmeta>
  <refnamediv>
    <refname>Synthetic Target Watchdog Device</refname>
    <refpurpose>Emulate watchdog hardware in the synthetic target</refpurpose> 
  </refnamediv>

  <refsect1><title>Overview</title>
    <para>
Some target hardware comes equipped with a watchdog timer. Application
code can start this timer and after a certain period of time,
typically a second, the watchdog will trigger. Usually this causes the
hardware to reboot. The application can prevent this by regularly
resetting the watchdog. An automatic reboot can be very useful when
deploying hardware in the field: a hardware glitch could cause the
unit to hang; or the software could receive an unexpected sequence of
inputs, never seen in the laboratory, causing the system to lock up.
Often the hardware is still functional, and a reboot sorts out the
problem with only a brief interruption in service.
    </para>
    <para>
The synthetic target watchdog package emulates watchdog hardware.
During system initialization watchdog device will be instantiated,
and the <filename>watchdog.tcl</filename> script will be loaded by the
I/O auxiliary. When the eCos application starts the watchdog device,
the <filename>watchdog.tcl</filename> script will start checking the
state of the eCos application at one second intervals. A watchdog
reset call simply involves a message to the I/O auxiliary. If the
<filename>watchdog.tcl</filename> script detects that a second has
<link linkend="synth-watchdog-wallclock-elapsed">elapsed</link>
without a reset then it will send a <literal>SIGPWR</literal> signal
to the eCos application, causing the latter to terminate. If gdb is
being used to run the application, the user will get a chance to
investigate what is happening. This behaviour is different from real
hardware in that there is no automatic reboot, but the synthetic
target is used only for development purposes, not deployment in the
field: if a reboot is desired then this can be achieved very easily
by using gdb commands to run another instance of the application.
    </para>
  </refsect1>

  <refsect1 id="devs-watchdog-synth-install"><title>Installation</title>
    <para>
Before a synthetic target eCos application can use a watchdog device
it is necessary to build and install host-side support. The relevant
code resides in the <filename class="directory">host</filename>
subdirectory of the synthetic target watchdog package, and building it
involves the standard <command>configure</command>,
<command>make</command> and <command>make install</command> steps. The
implementation of the watchdog support does not require any
executables, just a Tcl script <filename>watchdog.tcl</filename> and
some support files, so the <command>make</command> step is a no-op.
    </para>
    <para>
There are two main ways of building the host-side software. It is
possible to build both the generic host-side software and all
package-specific host-side software, including the watchdog support,
in a single build tree. This involves using the
<command>configure</command> script at the toplevel of the eCos
repository. For more information on this, see the
<filename>README.host</filename> file at the top of the repository.
Note that if you have an existing build tree which does not include
the synthetic target watchdog support then it will be necessary to
rerun the toplevel configure script: the search for appropriate
packages happens at configure time.
    </para>
    <para>
The alternative is to build just the host-side for this package.
This requires a separate build directory, building directly in the
source tree is disallowed. The <command>configure</command> options
are much the same as for a build from the toplevel, and the
<filename>README.host</filename> file can be consulted for more
details. It is essential that the watchdog support be configured with
the same <option>--prefix</option> option as other eCos host-side
software, especially the I/O auxiliary provided by the architectural
synthetic target HAL package, otherwise the I/O auxiliary will be
unable to locate the watchdog support.
    </para>
  </refsect1>

  <refsect1 id="synth-watchdog-target-config"><title>Target-side
Configuration</title>
    <para>
The watchdog device depends on the generic watchdog support,
<varname>CYGPKG_IO_WATCHDOG</varname>: if the generic support is
absent then the watchdog device will be inactive. Some templates
include this generic package by default, but not all. If the
configuration does not include the generic package then it can be
added using the eCos configuration tools, for example:
    </para>
    <screen>
$ ecosconfig add CYGPKG_IO_WATCHDOG
</screen>
    <para>
By default the configuration will use the hardware-specific support,
i.e. this package. However the generic watchdog package contains an
alternative implementation using the kernel alarm facility, and that
implementation can be selected if desired. However usually it will be
better to rely on an external watchdog facility as provided by the I/O
auxiliary and the <filename>watchdog.tcl</filename> script: if there
are serious problems within the application, for example memory
corruption, then an internal software-only implementation will not be
reliable.
    </para>
    <para>
The watchdog resolution is currently fixed to one second: if the
device does not receive a reset signal at least once a second then
the watchdog will trigger and the eCos application will be terminated
with a <literal>SIGPWR</literal> signal. The current implementation
does not allow this resolution to be changed.
    </para>
    <para>
On some targets the watchdog device does not perform a hard reset.
Instead the device works more or less via the interrupt subsystem,
allowing application code to install action routines that will be
called when the watchdog triggers. The synthetic target watchdog
support effectively does perform a hard reset, by sending a
<literal>SIGPWR</literal> signal to the eCos application, and there is
no support for action routines.
    </para>
    <para>
The synthetic target watchdog package provides some configuration
options for manipulating the compiler flags used for building the
target-side code. That code is fairly simple, so for nearly all
applications the default flags will suffice.
    </para>
    <para>
It should be noted that the watchdog device is subject to selective
linking. Unless some code explicitly references the device, for
example by calling the start and reset functions, the watchdog support
will not appear in the final executable. This is desirable because a
watchdog device has no effect until started.
    </para>
  </refsect1>

  <refsect1 id="synth-watchdog-wallclock-elapsed"><title>Wallclock versus Elapsed Time</title>
    <para>
On real hardware the watchdog device uses wallclock time: if the
device does not receive a reset signal within a set period of time
then the watchdog will trigger. When developing for the synthetic
target this is not always appropriate. There may be other processes
running, using up some or most of the cpu time. For example, the
application may be written such that it will issue a reset after some
calculations which are known to complete within half a second, well
within the one-second resolution of the watchdog device. However if
other Linux processes are running then the synthetic target
application may get timesliced, and half a second of computation may
take several seconds of wallclock time.
    </para>
    <para>
Another problem with using wallclock time is that it interferes with
debugging: if the application hits a breakpoint then it is unlikely
that the user will manage to restart it in less than a second, and the
watchdog will not get reset in time.
    </para>
    <para>
To avoid these problems the synthetic target watchdog normally uses
consumed cpu time rather than wallclock time. If the application is
timesliced or if it is halted inside gdb then it does not consume any
cpu time. The application actually has to spend a whole second's worth
of cpu cycles without issuing a reset before the watchdog triggers.
    </para>
    <para>
However using consumed cpu time is not a perfect solution either. If
the application makes blocking system calls then it is not using cpu
time. Interaction with the I/O auxiliary involves system calls, but
these should take only a short amount of time so their effects can be
ignored. If the application makes direct system calls such as
<function>cyg_hal_sys_read</function> then the system behaviour
becomes undefined. In addition by default the idle thread will make
blocking <function>select</function> system calls, effectively waiting
until an interrupt occurs. If an application spends much of its time
idle then the watchdog device may take much longer to trigger than
expected. It may be desirable to enable the synthetic target HAL
configuration option <varname>CYGIMP_HAL_IDLE_THREAD_SPIN</varname>,
causing the idle thread to spin rather than block, at the cost of
wasted cpu cycles.
    </para>
    <para>
The default is to use consumed cpu time, but this can be changed in
the target definition file:
    </para>
    <programlisting>
synth_device watchdog {
    use wallclock_time
    &hellip;
}
</programlisting>
  </refsect1>

  <refsect1 id="synth-watchdog-gui"><title>User Interface</title>
    <para>
When the synthetic target is run in graphical mode the watchdog device
extends the user interface in two ways. The <guimenu>Help</guimenu>
menu is extended with an entry for the watchdog-specific
documentation. There is also a graphical display of the current state
of the watchdog. Initially the watchdog is asleep:
    </para>
    <informalfigure PgWide=1>
      <mediaobject>
        <imageobject>
          <imagedata fileref="asleep.gif" format="gif" Scalefit=1 Align="Center">
        </imageobject>
      </mediaobject>
    </informalfigure>
    <para>
When application code starts the device the watchdog will begin to
keep an eye on things (or occasionally both eyes).
    </para>
    <informalfigure PgWide=1>
      <mediaobject>
        <imageobject>
          <imagedata fileref="awake.gif" format="gif" Scalefit=1 Align="Center">
        </imageobject>
      </mediaobject>
    </informalfigure>
    <para>
If the watchdog triggers the display will change again, and optionally
the user can receive an audible alert. The location of the watchdog
display within the I/O auxiliary's window can be controlled via
a <command>watchdog_pack</command> entry in the target definition
file. For example the following can be used to put the watchdog
display to the right of the central text window:
    </para>
    <programlisting>
synth_device watchdog {
    watchdog_pack -in .main.e -side top
    &hellip;
}
</programlisting>
    <para>
The user interface section of the generic synthetic target HAL
documentation can be consulted for more information on window packing.
    </para>
    <para>
By default the watchdog support will not generate an audible alert
when the watchdog triggers, to avoid annoying colleagues. Sound can be
enabled in the target definition file, and two suitable files
<filename>sound1.au</filename> and <filename>sound2.au</filename> are
supplied as standard:
    </para>
    <programlisting>
synth_device watchdog {
    sound sound1.au
    &hellip;
}
</programlisting>
    <para>
An absolute path can be specified if desired:
    </para>
    <programlisting>
synth_device watchdog {
    sound /usr/share/emacs/site-lisp/emacspeak/sounds/default-8k/alarm.au
    &hellip;
}
</programlisting>
    <para>
Sound facilities are not built into the I/O auxiliary itself, instead
an external program is used. The default player is
<command>play</command>, a front-end to the
<application>sox</application> application shipped with some Linux
distributions. If another player should be used then this can be
specified in the target definition file:
    </para>
    <programlisting>
synth_device watchdog {
    &hellip;
    sound_player my_sound_player
</programlisting>
    <para>
The specified program will be run in the background with a single
argument, the sound file.
    </para>
  </refsect1>

  <refsect1 id="devs-watchdog-synth-args"><title>Command Line Arguments</title>
    <para>
The watchdog support does not use any command line arguments. All
configuration is handled through the target definition file.
    </para>
  </refsect1>

  <refsect1 id="devs-watchdog-synth-hooks"><title>Hooks</title>
    <para>
The watchdog support does not provide any hooks for use by other
scripts. There is rarely any need for customizing the system's
behaviour when a watchdog triggers because those should be rare
events, even during application development.
    </para>
  </refsect1>

  <refsect1 id="devs-watchdog-synth-tcl"><title>Additional Tcl Procedures</title>
    <para>
The watchdog support does not provide any additional Tcl procedures or
variables for use by other scripts.
    </para>
  </refsect1>

</refentry>
</reference>
