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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 pretimout will go of 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]
-