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The Linux Watchdog driver API.

Copyright 2002 Christer Weingel 

Some parts of this document are copied verbatim from the sbc60xxwdt

driver which is (c) Copyright 2000 Jakob Oestergaard 

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:

int main(int argc, const char *argv[]) {

        int fd=open("/dev/watchdog",O_WRONLY);

        if (fd==-1) {

                perror("watchdog");

                exit(1);

        }

        while(1) {

                write(fd, "\0", 1);

                sleep(10);

        }

}

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.  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 dameon crashes, the system

will reboot after the timeout has passed.

Some other drivers will not disable the watchdog, unless a specific

magic character 'V' has been sent /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.

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);

Envinronmental 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

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 farenheit.

    int temperature;

    ioctl(fd, WDIOC_GETTEMP, &temperature);

Finally the SETOPTIONS ioctl can be used to control some aspects of

the cards operation; right now the pcwd driver is the only one

supporting thiss ioctl.

    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]

Implementations in the current drivers in the kernel tree:

Here I have tried to summarize what the different drivers support and

where they do strange things compared to the other drivers.

acquirewdt.c -- Acquire Single Board Computer

        This driver has a hardcoded timeout of 1 minute

        Supports CONFIG_WATCHDOG_NOWAYOUT

        GETSUPPORT returns KEEPALIVEPING.  GETSTATUS will return 1 if

        the device is open, 0 if not.  [FIXME -- isn't this rather

        silly?  To be able to use the ioctl, the device must be open

        and so GETSTATUS will always return 1].

advantechwdt.c -- Advantech Single Board Computer

        Timeout that defaults to 60 seconds, supports SETTIMEOUT.

        Supports CONFIG_WATCHDOG_NOWAYOUT

        GETSUPPORT returns WDIOF_KEEPALIVEPING and WDIOF_SETTIMEOUT.

        The GETSTATUS call returns if the device is open or not.

        [FIXME -- silliness again?]

eurotechwdt.c -- Eurotech CPU-1220/1410

        The timeout can be set using the SETTIMEOUT ioctl and defaults

        to 60 seconds.

        Also has a module parameter "ev", event type which controls

        what should happen on a timeout, the string "int" or anything

        else that causes a reboot.  [FIXME -- better description]

        Supports CONFIG_WATCHDOG_NOWAYOUT

        GETSUPPORT returns CARDRESET and WDIOF_SETTIMEOUT but

        GETSTATUS is not supported and GETBOOTSTATUS just returns 0.

i810-tco.c -- Intel 810 chipset

        Also has support for a lot of other i8x0 stuff, but the

        watchdog is one of the things.

        The timeout is set using the module parameter "i810_margin",

        which is in steps of 0.6 seconds where 2

        driver supports the SETTIMEOUT ioctl.

        Supports CONFIG_WATCHDOG_NOWAYOUT.

        GETSUPPORT returns WDIOF_SETTIMEOUT.  The GETSTATUS call

        returns some kind of timer value which ist not compatible with

        the other drivers.  GETBOOT status returns some kind of

        hardware specific boot status.  [FIXME -- describe this]

ib700wdt.c -- IB700 Single Board Computer

        Default timeout of 30 seconds and the timeout is settable

        using the SETTIMEOUT ioctl.  Note that only a few timeout

        values are supported.

        Supports CONFIG_WATCHDOG_NOWAYOUT

        GETSUPPORT returns WDIOF_KEEPALIVEPING and WDIOF_SETTIMEOUT.

        The GETSTATUS call returns if the device is open or not.

        [FIXME -- silliness again?]

machzwd.c -- MachZ ZF-Logic

        Hardcoded timeout of 10 seconds

        Has a module parameter "action" that controls what happens

        when the timeout runs out which can be 0 = RESET (default),

        1 = SMI, 2 = NMI, 3 = SCI.

        Supports CONFIG_WATCHDOG_NOWAYOUT and the magic character

        'V' close handling.

        GETSUPPORT returns WDIOF_KEEPALIVEPING, and the GETSTATUS call

        returns if the device is open or not.  [FIXME -- silliness

        again?]

mixcomwd.c -- MixCom Watchdog

        [FIXME -- I'm unable to tell what the timeout is]

        Supports CONFIG_WATCHDOG_NOWAYOUT

        GETSUPPORT returns WDIOF_KEEPALIVEPING, GETSTATUS returns if

        the device is opened or not [FIXME -- I'm not really sure how

        this works, there seems to be some magic connected to

        CONFIG_WATCHDOG_NOWAYOUT]

pcwd.c -- Berkshire PC Watchdog

        Hardcoded timeout of 1.5 seconds

        Supports CONFIG_WATCHDOG_NOWAYOUT

        GETSUPPORT returns WDIOF_OVERHEAT|WDIOF_CARDRESET and both

        GETSTATUS and GETBOOTSTATUS return something useful.

        The SETOPTIONS call can be used to enable and disable the card

        and to ask the driver to call panic if the system overheats.

sbc60xxwdt.c -- 60xx Single Board Computer

        Hardcoded timeout of 10 seconds

        Does not support CONFIG_WATCHDOG_NOWAYOUT, but has the magic

        character 'V' close handling.

        No bits set in GETSUPPORT

scx200.c -- National SCx200 CPUs

        Not in the kernel yet.

        The timeout is set using a module parameter "margin" which

        defaults to 60 seconds.  The timeout can also be set using

        SETTIMEOUT and read using GETTIMEOUT.

        Supports a module parameter "nowayout" that is initialized

        with the value of CONFIG_WATCHDOG_NOWAYOUT.  Also supports the

        magic character 'V' handling.

shwdt.c -- SuperH 3/4 processors

        [FIXME -- I'm unable to tell what the timeout is]

        Supports CONFIG_WATCHDOG_NOWAYOUT

        GETSUPPORT returns WDIOF_KEEPALIVEPING, and the GETSTATUS call

        returns if the device is open or not.  [FIXME -- silliness

        again?]

softdog.c -- Software watchdog

        The timeout is set with the module parameter "soft_margin"

        which defaults to 60 seconds, the timeout is also settable

        using the SETTIMEOUT ioctl.

        Supports CONFIG_WATCHDOG_NOWAYOUT

        WDIOF_SETTIMEOUT bit set in GETSUPPORT

w83877f_wdt.c -- W83877F Computer

        Hardcoded timeout of 30 seconds

        Does not support CONFIG_WATCHDOG_NOWAYOUT, but has the magic

        character 'V' close handling.

        No bits set in GETSUPPORT

wdt.c -- ICS WDT500/501 ISA and

wdt_pci.c -- ICS WDT500/501 PCI

        Default timeout of 60 seconds.  The timeout is also settable

        using the SETTIMEOUT ioctl.

        Supports CONFIG_WATCHDOG_NOWAYOUT

        GETSUPPORT returns with bits set depending on the actual

        card. The WDT501 supports a lot of external monitoring, the

        WDT500 much less.

wdt285.c -- Footbridge watchdog

        The timeout is set with the module parameter "soft_margin"

        which defaults to 60 seconds.  The timeout is also settable

        using the SETTIMEOUT ioctl.

        Does not support CONFIG_WATCHDOG_NOWAYOUT

        WDIOF_SETTIMEOUT bit set in GETSUPPORT

wdt977.c -- Netwinder W83977AF chip

        Hardcoded timeout of 3 minutes

        Supports CONFIG_WATCHDOG_NOWAYOUT

        Does not support any ioctls at all.