Effect observed --------------- We use the watchdog timer on a AM335x controller. When U-Boot runs, we enable the watchdog and want the kernel to service the watchdog until userspace takes it over. We compile the watchdog directly into the kernel and add the parameter "omap_wdt.early_enable=1" to the kernel command line. We furthermore set "CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED=y" in the kernel configuration. Our expectation is, that the watchdog is serviced by the kernel as long as userspace does not touch the device /dev/watchdog. However, this is not the case. The watchdog always expires. It is obviously not serviced by the kernel. We observed the effect with kernel version v5.4.138-rt62. However, we think that the most recent kernel exhibits the same behavior because the structure of the sources in question (see below) did not change. This also holds for the non realtime kernel. Root cause ---------- The CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED configuration option is not implemented in omap_wdt.c. Fix proposal ------------ Interestingly we found only one single driver that implements this featrue, namely the driver from STM, see https://elixir.bootlin.com/linux/v5.4.138/source/drivers/watchdog/stm32_iwdg.c#L274 This makes us wonder if there might be a good reason not to implement it??? However we think this feature should be available. Our use case is to make software updates more robust. If an updated kernel hangs for whatever reason, then U-Boot gets the chance to boot the old one provided there is a reboot. Based on the STM implementation, we created a patch (see below) which resolves the issue. The watchdog is now correctly handled by the kernel until userspace first accesses it. diff --git a/drivers/watchdog/omap_wdt.c b/drivers/watchdog/omap_wdt.c index 9b91882fe3c4..94e2e1b494d2 100644 --- a/drivers/watchdog/omap_wdt.c +++ b/drivers/watchdog/omap_wdt.c @@ -271,6 +271,11 @@ static int omap_wdt_probe(struct platform_device *pdev) if (!early_enable) omap_wdt_disable(wdev); + if (IS_ENABLED(CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED)) { + /* Make sure the watchdog is serviced */ + set_bit(WDOG_HW_RUNNING, &wdev->wdog.status); + } + ret = watchdog_register_device(&wdev->wdog); if (ret) { pm_runtime_disable(wdev->dev);
