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diff --git a/Documentation/watchdog/watchdog-api.rst b/Documentation/watchdog/watchdog-api.rst new file mode 100644 index 0000000000..800dcd7586 --- /dev/null +++ b/Documentation/watchdog/watchdog-api.rst @@ -0,0 +1,271 @@ +============================= +The Linux Watchdog driver API +============================= + +Last reviewed: 10/05/2007 + + + +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 samples/watchdog/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. + + ================ ========================= + 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] |