Advantech RSB-4210 Evaluation Kit Manual de usuario descargar pdf (Pagina 94)

RSB-4210 User Manual 86

3.11.8 WatchDog

Usually an 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 notifica-

tions cease to occur, and the hardware watchdog will reset the system (causing a

reboot) after the timeout occurs.

3.11.8.1 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 as below:

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <fcntl.h>

int main(void)

{

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

int ret = 0;

if (fd == -1) {

perror("watchdog");

exit(EXIT_FAILURE);

}

while (1) {

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

if (ret != 1) {

ret = -1;

break;

}

sleep(10);

}

close(fd);

return ret;

}

3.11.8.2 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.

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