linux-kernel - Re: [PATCH v2 5/6] watchdog: at91sam9: request the irq with IRQF_NO

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Date:	Sat, 7 Mar 2015 12:06:45 +0100
From:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Sylvain Rochet <gradator@...dator.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	Mike Turquette <mturquette@...aro.org>,
	Jason Cooper <jason@...edaemon.net>,
	"rtc-linux@...glegroups.com" <rtc-linux@...glegroups.com>,
	Len Brown <len.brown@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Wim Van Sebroeck <wim@...ana.be>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jiri Slaby <jslaby@...e.cz>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>
Subject: Re: [PATCH v2 5/6] watchdog: at91sam9: request the irq with
 IRQF_NO_SUSPEND

On 07/03/2015 at 11:39:39 +0100, Pavel Machek wrote :
> > The Atmel watchdog can't be stopped once it's started. This is actually 
> > very useful so we can reset if suspend or resume failed, the only 
> > drawback is that you have to wake up from time to time (e.g. by using 
> > the RTC/RTT) to clear the watchdog and then go back to sleep ASAP.
> 
> Yeah. So you do "echo mem > /sys/power/state", and few seconds/minutes
> after watchdog kills the system. But you did not ask for dead system,
> you asked for suspend.
> 
> And while that behaviour is useful for you, I don't think it is
> exactly useful behaviour, nor it is the behaviour user would expect.
> 

I think you misunderstood, that is exactly the expected behaviour. This
is hardware defined. Once the watchdog is started, nobody can stop it.
Trying to change the mode register will result in a reset of the SoC.

It is documented in the datasheet and any user wanting another behaviour
is out of luck.

So basically, when using a watchdog, you have to wake up every 15-16s to
restart it.

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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