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

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Message-ID: <20150306131025.GH8700@leverpostej>
Date:	Fri, 6 Mar 2015 13:10:26 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Wim Van Sebroeck <wim@...ana.be>,
	"linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	"rtc-linux@...glegroups.com" <rtc-linux@...glegroups.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	Mike Turquette <mturquette@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 5/6] watchdog: at91sam9: request the irq with
 IRQF_NO_SUSPEND

> >> > We seem to be conflating some related properties:
> >> >
> >> > [a] The IRQ will be left unmasked.
> >> > [b] The IRQ will be handled immediately when taken.
> >> > [c] The IRQ will wake the system from suspend.

[...]

> > Considering that the use-case of a watchdog is to alert us to something
> > going hideously wrong in the kernel, we want to handle the IRQ after
> > executing the smallest amount of kernel code possible. For that, they
> > need to have their handlers to be called "immediately" outside of the
> > arch_suspend_disable_irqs() ... arch_suspend_enable_irqs() window, and
> > need to be enabled during suspend to attempt to catch bad wakeup device
> > configuration.
> >
> > I think it's possible (assuming the caveats on [b] above) to provide
> > [a,b,c] for this case.
> 
> OK
> 
> But in this case the request_irq() passing IRQF_NO_SUSPEND *and* requiring
> enable_irq_wake() in addition to that needs a big fat comment explaining the
> whole thing or we'll forget about the gory details at one point and no one will
> know what's going on in there.

Agreed.

I'd expect an IRQF_SW_WATCHDOG or something to that effect should also
be required for that case.

Thanks,
Mark.
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