Re: [RFC 1/3] watchdog: change watchdog_need_worker logic

[Guenter Roeck]

On 07/17/2016 12:24 PM, Wim Van Sebroeck wrote:
> Hi Rasmus,
>
> > If the driver indicates that the watchdog is running, the framework
> > should feed it until userspace opens the device, regardless of whether
> > the driver has set max_hw_heartbeat_ms.
> >
> > This patch only affects the case where wdd->max_hw_heartbeat_ms is
> > zero, wdd->timeout is non-zero, the watchdog is not active and the
> > hardware device is running (*):
> >
> > - If wdd->timeout is zero, watchdog_need_worker() returns false both
> > before and after this patch, and watchdog_next_keepalive() is not
> > called.
> >
> > - If watchdog_active(wdd), the return value from watchdog_need_worker
> > is also the same as before (namely, hm && t > hm). Hence in that case,
> > watchdog_next_keepalive() is only called if hm == max_hw_heartbeat_ms
> > is non-zero, so the change to min_not_zero there is a no-op.
> >
> > - If the watchdog is not active and the device is not running, we
> > return false from watchdog_need_worker just as before.
> >
> > That leaves the watchdog_hw_running(wdd) && !watchdog_active(wdd) &&
> > wdd->timeout case. Again, it's easy to see that if
> > wdd->max_hw_heartbeat_ms is non-zero, we return true from
> > watchdog_need_worker with and without this patch, and the logic in
> > watchdog_next_keepalive is unchanged. Finally, if
> > wdd->max_hw_heartbeat_ms is 0, we used to end up in the
> > cancel_delayed_work branch, whereas with this patch we end up
> > scheduling a ping timeout_ms/2 from now.
> >
> > (*) This should imply that no current kernel drivers are affected,
> > since the only drivers which explicitly set WDOG_HW_RUNNING are
> > imx2_wdt.c and dw_wdt.c, both of which also provide a non-zero value
> > for max_hw_heartbeat_ms. The watchdog core also sets WDOG_HW_RUNNING,
> > but only when the driver doesn't provide ->stop, in which case it
> > must, according to Documentation/watchdog/watchdog-kernel-api.txt, set
> > max_hw_heartbeat_ms.
>
> This isn't completely true. We will have the following in the linux-watchdog tree:
> drivers/watchdog/aspeed_wdt.c:		set_bit(WDOG_HW_RUNNING, &wdt->wdd.status);
> drivers/watchdog/dw_wdt.c:	set_bit(WDOG_HW_RUNNING, &wdd->status);
> drivers/watchdog/dw_wdt.c:		set_bit(WDOG_HW_RUNNING, &wdd->status);
> drivers/watchdog/imx2_wdt.c:	set_bit(WDOG_HW_RUNNING, &wdog->status);
> drivers/watchdog/imx2_wdt.c:		set_bit(WDOG_HW_RUNNING, &wdog->status);
> drivers/watchdog/max77620_wdt.c:		set_bit(WDOG_HW_RUNNING, &wdt_dev->status);
> drivers/watchdog/sbsa_gwdt.c:		set_bit(WDOG_HW_RUNNING, &wdd->status);
> drivers/watchdog/tangox_wdt.c:		set_bit(WDOG_HW_RUNNING, &dev->wdt.status);
>
> I checked the ones that aren't mentioned and aspeed_wdt, max77620_wdt and sbsa_gwdt.c
> also have a non-zero value for max_hw_heartbeat_ms. But tangox_wdt.c doesn't set it.
> This one will need to be looked at closer.

I had a brief look; the tangox_wdt problem is my fault. I overlooked that with
my commit 'watchdog: tangox: Mark running watchdog correctly'.

We have a number of options: Set max_hw_heartbeat_ms in tangox_wdt.c,
accept this patch, or both. I think we should accept this patch.

Thanks,
Guenter

> > Signed-off-by: Rasmus Villemoes <rasmus.villemoes@xxxxxxxxx>
> > ---
> >   drivers/watchdog/watchdog_dev.c | 10 +++++++---
> >   1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
> > index 3595cff..14f8a92 100644
> > --- a/drivers/watchdog/watchdog_dev.c
> > +++ b/drivers/watchdog/watchdog_dev.c
> > @@ -92,9 +92,13 @@ static inline bool watchdog_need_worker(struct watchdog_device *wdd)
> >   	 *   thus is aware that the framework supports generating heartbeat
> >   	 *   requests.
> >   	 * - Userspace requests a longer timeout than the hardware can handle.
> > +	 *
> > +	 * Alternatively, if userspace has not opened the watchdog
> > +	 * device, we take care of feeding the watchdog if it is
> > +	 * running.
> >   	 */
> > -	return hm && ((watchdog_active(wdd) && t > hm) ||
> > -		      (t && !watchdog_active(wdd) && watchdog_hw_running(wdd)));
> > +	return (hm && watchdog_active(wdd) && t > hm) ||
> > +		(t && !watchdog_active(wdd) && watchdog_hw_running(wdd));
> >   }
> >
> >   static long watchdog_next_keepalive(struct watchdog_device *wdd)
> > @@ -107,7 +111,7 @@ static long watchdog_next_keepalive(struct watchdog_device *wdd)
> >   	unsigned int hw_heartbeat_ms;
> >
> >   	virt_timeout = wd_data->last_keepalive + msecs_to_jiffies(timeout_ms);
> > -	hw_heartbeat_ms = min(timeout_ms, wdd->max_hw_heartbeat_ms);
> > +	hw_heartbeat_ms = min_not_zero(timeout_ms, wdd->max_hw_heartbeat_ms);
> >   	keepalive_interval = msecs_to_jiffies(hw_heartbeat_ms / 2);
> >
> >   	if (!watchdog_active(wdd))
> > --
> > 2.5.0
>
> Kind regards,
> Wim.