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+Last reviewed: 10/05/2007
+
+
+The Linux Watchdog driver API.
+
+Copyright 2002 Christer Weingel <wingel@nano-system.com>
+
+Some parts of this document are copied verbatim from the sbc60xxwdt
+driver which is (c) Copyright 2000 Jakob Oestergaard <jakob@ostenfeld.dk>
+
+This document describes the state of the Linux 2.4.18 kernel.
+
+Introduction:
+
+A Watchdog Timer (WDT) is a hardware circuit that can reset the
+computer system in case of a software fault.  You probably knew that
+already.
+
+Usually a userspace daemon will notify the kernel watchdog driver via the
+/dev/watchdog special device file that userspace is still alive, at
+regular intervals.  When such a notification occurs, the driver will
+usually tell the hardware watchdog that everything is in order, and
+that the watchdog should wait for yet another little while to reset
+the system.  If userspace fails (RAM error, kernel bug, whatever), the
+notifications cease to occur, and the hardware watchdog will reset the
+system (causing a reboot) after the timeout occurs.
+
+The Linux watchdog API is a rather ad-hoc construction and different
+drivers implement different, and sometimes incompatible, parts of it.
+This file is an attempt to document the existing usage and allow
+future driver writers to use it as a reference.
+
+The simplest API:
+
+All drivers support the basic mode of operation, where the watchdog
+activates as soon as /dev/watchdog is opened and will reboot unless
+the watchdog is pinged within a certain time, this time is called the
+timeout or margin.  The simplest way to ping the watchdog is to write
+some data to the device.  So a very simple watchdog daemon would look
+like this source file:  see Documentation/watchdog/src/watchdog-simple.c
+
+A more advanced driver could for example check that a HTTP server is
+still responding before doing the write call to ping the watchdog.
+
+When the device is closed, the watchdog is disabled, unless the "Magic
+Close" feature is supported (see below).  This is not always such a
+good idea, since if there is a bug in the watchdog daemon and it
+crashes the system will not reboot.  Because of this, some of the
+drivers support the configuration option "Disable watchdog shutdown on
+close", CONFIG_WATCHDOG_NOWAYOUT.  If it is set to Y when compiling
+the kernel, there is no way of disabling the watchdog once it has been
+started.  So, if the watchdog daemon crashes, the system will reboot
+after the timeout has passed. Watchdog devices also usually support
+the nowayout module parameter so that this option can be controlled at
+runtime.
+
+Magic Close feature:
+
+If a driver supports "Magic Close", the driver will not disable the
+watchdog unless a specific magic character 'V' has been sent to
+/dev/watchdog just before closing the file.  If the userspace daemon
+closes the file without sending this special character, the driver
+will assume that the daemon (and userspace in general) died, and will
+stop pinging the watchdog without disabling it first.  This will then
+cause a reboot if the watchdog is not re-opened in sufficient time.
+
+The ioctl API:
+
+All conforming drivers also support an ioctl API.
+
+Pinging the watchdog using an ioctl:
+
+All drivers that have an ioctl interface support at least one ioctl,
+KEEPALIVE.  This ioctl does exactly the same thing as a write to the
+watchdog device, so the main loop in the above program could be
+replaced with:
+
+	while (1) {
+		ioctl(fd, WDIOC_KEEPALIVE, 0);
+		sleep(10);
+	}
+
+the argument to the ioctl is ignored.
+
+Setting and getting the timeout:
+
+For some drivers it is possible to modify the watchdog timeout on the
+fly with the SETTIMEOUT ioctl, those drivers have the WDIOF_SETTIMEOUT
+flag set in their option field.  The argument is an integer
+representing the timeout in seconds.  The driver returns the real
+timeout used in the same variable, and this timeout might differ from
+the requested one due to limitation of the hardware.
+
+    int timeout = 45;
+    ioctl(fd, WDIOC_SETTIMEOUT, &timeout);
+    printf("The timeout was set to %d seconds\n", timeout);
+
+This example might actually print "The timeout was set to 60 seconds"
+if the device has a granularity of minutes for its timeout.
+
+Starting with the Linux 2.4.18 kernel, it is possible to query the
+current timeout using the GETTIMEOUT ioctl.
+
+    ioctl(fd, WDIOC_GETTIMEOUT, &timeout);
+    printf("The timeout was is %d seconds\n", timeout);
+
+Pretimeouts:
+
+Some watchdog timers can be set to have a trigger go off before the
+actual time they will reset the system.  This can be done with an NMI,
+interrupt, or other mechanism.  This allows Linux to record useful
+information (like panic information and kernel coredumps) before it
+resets.
+
+    pretimeout = 10;
+    ioctl(fd, WDIOC_SETPRETIMEOUT, &pretimeout);
+
+Note that the pretimeout is the number of seconds before the time
+when the timeout will go off.  It is not the number of seconds until
+the pretimeout.  So, for instance, if you set the timeout to 60 seconds
+and the pretimeout to 10 seconds, the pretimeout will go off in 50
+seconds.  Setting a pretimeout to zero disables it.
+
+There is also a get function for getting the pretimeout:
+
+    ioctl(fd, WDIOC_GETPRETIMEOUT, &timeout);
+    printf("The pretimeout was is %d seconds\n", timeout);
+
+Not all watchdog drivers will support a pretimeout.
+
+Get the number of seconds before reboot:
+
+Some watchdog drivers have the ability to report the remaining time
+before the system will reboot. The WDIOC_GETTIMELEFT is the ioctl
+that returns the number of seconds before reboot.
+
+    ioctl(fd, WDIOC_GETTIMELEFT, &timeleft);
+    printf("The timeout was is %d seconds\n", timeleft);
+
+Environmental monitoring:
+
+All watchdog drivers are required return more information about the system,
+some do temperature, fan and power level monitoring, some can tell you
+the reason for the last reboot of the system.  The GETSUPPORT ioctl is
+available to ask what the device can do:
+
+	struct watchdog_info ident;
+	ioctl(fd, WDIOC_GETSUPPORT, &ident);
+
+the fields returned in the ident struct are:
+
+        identity		a string identifying the watchdog driver
+	firmware_version	the firmware version of the card if available
+	options			a flags describing what the device supports
+
+the options field can have the following bits set, and describes what
+kind of information that the GET_STATUS and GET_BOOT_STATUS ioctls can
+return.   [FIXME -- Is this correct?]
+
+	WDIOF_OVERHEAT		Reset due to CPU overheat
+
+The machine was last rebooted by the watchdog because the thermal limit was
+exceeded
+
+	WDIOF_FANFAULT		Fan failed
+
+A system fan monitored by the watchdog card has failed
+
+	WDIOF_EXTERN1		External relay 1
+
+External monitoring relay/source 1 was triggered. Controllers intended for
+real world applications include external monitoring pins that will trigger
+a reset.
+
+	WDIOF_EXTERN2		External relay 2
+
+External monitoring relay/source 2 was triggered
+
+	WDIOF_POWERUNDER	Power bad/power fault
+
+The machine is showing an undervoltage status
+
+	WDIOF_CARDRESET		Card previously reset the CPU
+
+The last reboot was caused by the watchdog card
+
+	WDIOF_POWEROVER		Power over voltage
+
+The machine is showing an overvoltage status. Note that if one level is
+under and one over both bits will be set - this may seem odd but makes
+sense.
+
+	WDIOF_KEEPALIVEPING	Keep alive ping reply
+
+The watchdog saw a keepalive ping since it was last queried.
+
+	WDIOF_SETTIMEOUT	Can set/get the timeout
+
+The watchdog can do pretimeouts.
+
+	WDIOF_PRETIMEOUT	Pretimeout (in seconds), get/set
+
+
+For those drivers that return any bits set in the option field, the
+GETSTATUS and GETBOOTSTATUS ioctls can be used to ask for the current
+status, and the status at the last reboot, respectively.  
+
+    int flags;
+    ioctl(fd, WDIOC_GETSTATUS, &flags);
+
+    or
+
+    ioctl(fd, WDIOC_GETBOOTSTATUS, &flags);
+
+Note that not all devices support these two calls, and some only
+support the GETBOOTSTATUS call.
+
+Some drivers can measure the temperature using the GETTEMP ioctl.  The
+returned value is the temperature in degrees fahrenheit.
+
+    int temperature;
+    ioctl(fd, WDIOC_GETTEMP, &temperature);
+
+Finally the SETOPTIONS ioctl can be used to control some aspects of
+the cards operation.
+
+    int options = 0;
+    ioctl(fd, WDIOC_SETOPTIONS, &options);
+
+The following options are available:
+
+	WDIOS_DISABLECARD	Turn off the watchdog timer
+	WDIOS_ENABLECARD	Turn on the watchdog timer
+	WDIOS_TEMPPANIC		Kernel panic on temperature trip
+
+[FIXME -- better explanations]
+