Re: [PATCH 3/3] watchdog: qcom: register a restart notifier

Josh Cartwright
2014-09-19T03:38:18Z

On Thu, Sep 18, 2014 at 07:47:54PM -0700, Guenter Roeck wrote:
> On 09/18/2014 03:27 PM, Josh Cartwright wrote:
> >The WDT's BITE_TIME warm-reset behavior can be leveraged as a last
> >resort mechanism for triggering chip reset. Usually, other restart
> >methods (such as PS_HOLD) are preferrable for issuing a more complete
> >reset of the chip. As such, keep the priority of the watchdog notifier
> >low.
> >
> >Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
[..]
> >+static int qcom_wdt_restart(struct notifier_block *nb, unsigned long action,
> >+ void *data)
> >+{
> >+ struct qcom_wdt *wdt = container_of(nb, struct qcom_wdt, restart_nb);
> >+
> >+ /*
> >+ * Trigger watchdog bite:
> >+ * Setup BITE_TIME to be very low, and enable WDT.
> >+ */
> >+ mutex_lock(&wdt->wdd.lock);
>
> At this time you don't need to worry about locks.
>
> Actually, this might be dangerous if the lock happens to be taken,
> as it won't be released (there is no other code running anymore
> when this function is called).

Ah, great. I'll drop the locking.

> >+ writel_relaxed(0, wdt->base + WDT_EN);
> >+ writel_relaxed(1, wdt->base + WDT_RST);
> >+ writel_relaxed(0x31F3, wdt->base + WDT_BITE_TIME);
>
> What is the magic here, ie what does 0x31F3 stand for ?

Nothing magic, it's just a reasonably low value to set the bite time
counter at. It also happens to be the value at reset.

> >+ writel_relaxed(1, wdt->base + WDT_EN);
> >+ mutex_unlock(&wdt->wdd.lock);
> >+ return NOTIFY_DONE;
> >+}
> >+
> > static int qcom_watchdog_probe(struct platform_device *pdev)
> > {
> > struct qcom_wdt *wdt;
> >@@ -121,6 +141,17 @@ static int qcom_watchdog_probe(struct platform_device *pdev)
> > return ret;
> > }
> >
> >+ /*
> >+ * WDT restart notifier has priority 0 (use as a last resort)
> >+ */
> >+ wdt->restart_nb.notifier_call = qcom_wdt_restart;
> >+ ret = register_restart_handler(&wdt->restart_nb);
> >+ if (ret) {
> >+ dev_err(&pdev->dev, "failed to setup restart handler\n");
> >+ watchdog_unregister_device(&wdt->wdd);
> >+ return ret;
>
> Sure you want to return an error here ? The watchdog itself is still working,
> and this is supposed to be a restart method of last resort. Causing the driver
> to fail loading because it can not register its restart handler seems to be
> a bit aggressive.

It is a bit aggressive. I'll at least drop it to a dev_warn(); even
though it is a "last resort", on some boards it's the only available
mechanism for reliable restart.

Thanks!
Josh