AW: [BUG] omap_wdt.c: Watchdog not serviced by kernel

[Walter Stoll <Walter.Stoll@duagon.com>]

> -----Ursprüngliche Nachricht-----
> Von: Guenter Roeck <groeck7@gmail.com> Im Auftrag von Guenter Roeck
> Gesendet: Freitag, 17. September 2021 15:47
> An: Walter Stoll <Walter.Stoll@duagon.com>; wim@linux-watchdog.org
> Cc: linux-watchdog@vger.kernel.org
> Betreff: Re: [BUG] omap_wdt.c: Watchdog not serviced by kernel
> 
> On 9/17/21 1:36 AM, Walter Stoll wrote:
> > 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???
> >
> It is primarily a watchdog core feature. Handling running watchdogs in the core
> used to be enabled by default. Not everyone was happy with that, so
> WATCHDOG_HANDLE_BOOT_ENABLED was added to be able to _disable_ that functionality.
> It was never intended to be a driver feature. The STM driver (mis)uses it
> because it wants to be able to support WDOG_HW_RUNNING, but the HW has no means
> to detect if it is running. That doesn't mean that other drivers need to do
> the same.
> 
> > 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);
> > +	}
> > +
> 
> No, this is wrong. The driver should set WDOG_HW_RUNNING if the watchdog is running,
> independently of CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED.
> 
> The omap_wdt driver has a boot option "early_enable" which does start the watchdog
> during probe, but it doesn't set WDOG_HW_RUNNING (which is the real problem).
> Plus, if early_enable is not set, the driver explicitly disables the watchdog
> (see code above), and setting WDOG_HW_RUNNING would be wrong in that case.
> 
> The fix would be something like
> 
>          if (early_enable) {
>                  omap_wdt_start(&wdev->wdog);
> 		set_bit(WDOG_HW_RUNNING, &wdev->wdog.status);
> 	} else {
> 		omap_wdt_disable(wdev);
> 	}
> 
> That needs to be ahead of watchdog_register_device(), and is independent
> of CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED.
> 
> Guenter
> 
> >   	ret = watchdog_register_device(&wdev->wdog);
> >   	if (ret) {
> >   		pm_runtime_disable(wdev->dev);
> >

Hello Guenter

Thank you very much for your fast response. I checked your fix with all
combinations of early_enable and CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED.

From my point of view, this works exactly as expected. Any chance to get
that mainline ?

Walter