The document explains the Linux watchdog timer, a hardware circuit that can reset the system in case of faults, accessed via the /dev/watchdog device. It describes the role of a user space daemon that regularly notifies the watchdog driver to prevent system resets, and discusses configurations like config_watchdog_nowayout that ensure the watchdog remains active even if the daemon crashes. Additionally, it mentions a 'magic close' feature that prevents the watchdog timer from being disabled unless a specific character is sent before closing the device file.

1. Linux Watchdog Timer
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2. Linux Watchdog Timer
 A Watchdog timer is a hardware circuit that can reset
the computer system in case of any system fault.
 Linux provides a simple watchdog interface
 /dev/watchdog device file
 Once the /dev/watchdog device is opened by any
process:
 the kernel will begin a 60 seconds countdown to reboot the
system.
 The countdown gets reset if some data is written to that
file.
 Closing the /dev/watchdog device, may or may not stop
the countdown. It depends on how kernel configured the
watchdog timer.
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3. User space daemon role
 User space daemon (or a watchdog daemon) notifies the watchdog
driver (via /dev/watchdog device file) that userspace is active.
 It sends this notification at regular intervals of time
 After receiving this notification, watchdog driver tells the hardware
watchdog that everything is in order, and should wait for yet another
little while to reset the system
 If user space daemon failed to send this notification
 The hardware watchdog resets the system (rebooting the system)
after the timeout occurs.
 Typical userspace daemon code looks like
while (1) {
ioctl(fd, WDIOC_KEEPALIVE, 0);
sleep(10);
}
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4. CONFIG_WATCHDOG_NOWAYOUT
 Watchdog will be disabled if its device file closed by
the daemon due to any bug in the daemon.
 It leads to the situation where system crash but system
will not reboot as watchdog is disabled.
 To fix this issue, CONFIG_WATCHDOG_NOWAYOUT
configuration option is introduced to disable
watchdog shutdown when device file got closed.
 If this configuration set to Y then there is no way of
disabling watchdog timer.
 If watchdog daemon crashes, then system will
reboot after the timeout has reached.
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5. Magic close feature
 If driver supports “Magic close” feature, then watchdog timer will
not be disabled unless a special character ‘V’ has been sent to the
/dev/watchdog device file just before closing the file.
 static const struct watchdog_info tegra_wdt_info = {
.options = WDIOF_SETTIMEOUT |
WDIOF_KEEPALIVEPING |
WDIOF_MAGICCLOSE,
.identity = "Tegra WDT",
};
Code reference: source/drivers/watchdog/tegra_wdt.c
 A simple ioctl() on a file descriptor to /dev/watchdog asking
WDIOC_GETSUPPORT allows one to determine if this flag is set.
Pseudo code
if (ioctl(fd, WDIOC_GETSUPPORT, &info)) {
// code;
}
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6. References
 http://stackoverflow.com/
 kernel/Documentation/watchdog/watchdog-api.txt
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7. THANK YOU 
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