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

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Message-ID: <20150307091204.GM23367@worktop.ger.corp.intel.com>
Date:	Sat, 7 Mar 2015 10:12:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	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

On Fri, Mar 06, 2015 at 11:06:18AM +0000, Mark Rutland wrote:
> [...]
> 
> > > The request_irq path never results in a call to chip->irq_set_wake(),
> > > even with the IRQF_NO_SUSPEND flag. So requesting an irq with
> > > IRQF_NO_SUSPEND does not guarantee wakeup; it only guarantees that the
> > > CPU can take the interrupt _around_ the suspended state, not necessarily
> > > while _in_ the suspended state.
> > 
> > Right.  "Suspended state" meaning full suspend here I suppose?
> 
> Yes; any state deeper than suspend-to-idle.

I don't think we should want to make such distinction; we should treat
all suspend states the same.

Drivers should not want to rely on the fact that one state
(suspend-to-idle) might maybe deal with interrupts while other states do
not.

> > > 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.
> > > 
> > > Requesting an IRQ with IRQF_NO_SUSPEND guarantees [a,b], but does not
> > > guarantee [c].
> > 
> > That's correct.  IRQF_NO_SUSPEND does not guarantee that interrupts from
> > that IRQ will have any effect after arch_suspend_disable_irqs() in
> > suspend_enter().
> 
> [...]
> 
> > > It sounds like for this kind of watchdog device we want [a,b,c], even if
> > > the IRQ is not shared with an IRQF_NO_SUSPEND user.
> > 
> > We can't guarantee that, though.  arch_suspend_disable_irqs() disables
> > interrupts on the last working CPU and it won't get any.  It may be
> > brought out of a low-power state by a pending interrupt, but it won't act
> > upon that interrupt immediately anyway, only after the arch_suspend_enable_irqs()
> > in suspend_enter().
> 
> Ok, so [b] needs the caveat that it's only handled "immediately" outside
> of the arch_suspend_disable_irqs() ... arch_suspend_enable_irqs()
> section.
> 
> > But then it might as well be deferred until after
> > resume_device_irqs().
> 
> That was my original line of thinking, in which case the watchdog driver
> should use IRQF_COND_SUSPEND rather than IRQF_NO_SUSPEND, with
> enable_irq_wake() if we care about the watchdog during suspend. I'm
> happy with this.

Note that COND_SUSPEND must have SHARED set.

> 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.

While I appreciate the use-case; we should be careful not to make of
mess of things either.
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