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