The TS-7100 implements a WDT inside the supervisory microcontroller. A standard kernel WDT driver is in place that manages the WDT via the I2C bus. The WDT requires a userspace feeding system as the kernel has no provisions for self-feeding. Our stock distribution uses the 'watchdog' utility to check system health, set feed length, and perform feeds. Any arbitrary timeout value between 10 ms through 42949672.95 seconds is possible, with a resolution of 10 ms. Setting a timeout of 0 and issuing a feed will disable the WDT in hardware.
At power-on, the WDT in the microcontroller is live and running. This means that any failures in the boot process will cause a reboot and another attempt. From there, feeds must be manually done in U-Boot or the system booted to Linux within the default 60 second timeout. Once the kernel initializes the WDT driver, it will change the timeout and issue a single feed (default 5 minutes). Userspace must be fully booted and able to take over feeding within this time frame.
The default Linux timeout of 5 minutes was designed to accommodate very slow external I/O. For example, performing an 'fsck' on an external HDD of large sizes via USB mass storage interface, may take a number of minutes worst case. This timeout can be adjusted via the FDT file for the TS-7100. Setting this timeout to 0 will cause the WDT to be disabled in hardware as soon as the kernel is started.
The kernel driver supports the "Magic Close" feature of the WDT. This means that a 'V' character must be fed in to the watchdog file before the file is closed in order to disable the WDT. If this does not happen then the WDT is not stopped and it will continue it's countdown. This is the default behavior of our stock kernel.
Additionally, if the kernel is compiled with CONFIG_WATCHDOG_NOWAYOUT then the WDT can never be stopped once it is started at boot. This is not enabled by default in our stock kernel
See the Linux WDT API documentation for more information.