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* [BUG] omap_wdt.c: Watchdog not serviced by kernel
@ 2021-09-17  8:36 Walter Stoll
  2021-09-17 13:47 ` Guenter Roeck
  0 siblings, 1 reply; 4+ messages in thread
From: Walter Stoll @ 2021-09-17  8:36 UTC (permalink / raw)
  To: wim, linux; +Cc: linux-watchdog

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);

^ permalink raw reply	[flat|nested] 4+ messages in thread

end of thread, other threads:[~2021-09-17 15:08 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-09-17  8:36 [BUG] omap_wdt.c: Watchdog not serviced by kernel Walter Stoll
2021-09-17 13:47 ` Guenter Roeck
2021-09-17 15:00   ` AW: " Walter Stoll
2021-09-17 15:08     ` Guenter Roeck

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