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

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Date:	Wed, 11 Mar 2015 12:17:01 +0100
From:	Nicolas Ferre <nicolas.ferre@...el.com>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Pavel Machek <pavel@....cz>,
	Sylvain Rochet <gradator@...dator.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Mark Rutland <mark.rutland@....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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	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

Le 11/03/2015 09:38, Boris Brezillon a écrit :
> On Sun, 08 Mar 2015 02:11:45 +0100
> "Rafael J. Wysocki" <rjw@...ysocki.net> wrote:
> 
>> On Saturday, March 07, 2015 12:06:45 PM Alexandre Belloni wrote:
>>> 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.
>>
>> So question is if we need a separate interrupt handler for that, expecially
>> since it is shared with the PIT timer anyway.
>>
>> Seems to me that the simplest way out of this conundrum would be to simply
>> make the timer's interrupt handler kick the watchdog every once a while and
>> get rid of the separate watchdog interrupt handler entirely.
> 
> The watchdog interrupt handler is not here to ping the watchdog, it's
> here to reset the platform if the watchdog hasn't been refreshed
> appropriately.
> 
> IOW, it's a software watchdog using at91 WDT capabilities to determine
> when it should reboot the system.
> IIRC, we need this on some at91 platforms to fix a HW bug (maybe
> Nicolas can confirm this).

Yes, the HW bug that we address in these functions:
at91sam9260_restart() and at91sam9g45_restart().

We have this issue because of NAND flash lines shared with DDR that are
driven during product reboot on old products (Cf. these functions
comments). This bug would kick-in when doing "software reset"/"watchdog
reset"/"push button reset". Only the "software reset" is handled by the
functions above.

So, yes, this "software watchdog" is there for this purpose IIRC.

Bye,
-- 
Nicolas Ferre
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