android_kernel_xiaomi_sm8350/Documentation/watchdog/watchdog-api.txt

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:

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<i810_margin<64.  The
	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

w83627hf_wdt.c -- w83627hf watchdog

	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.

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.