linux-kernel - Re: [PATCH] ARM: tegra: cpuidle: implement cpuidle_state.enter

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Date:	Sat, 16 May 2015 00:30:41 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: tegra: cpuidle: implement
 cpuidle_state.enter_freeze()

On Fri, May 15, 2015 at 11:03:35AM +0200, Tomeu Vizoso wrote:
> I have been looking and asking around, and seems like we should
> actually leave FIQs enabled when the tick is frozen and only disable
> them within enter_freeze.
> 
> My understanding is that FIQ handlers are extremely limited in what
> they can do, with the main use being NMI-like functionality. There are
> a handful of other FIQ handlers in mainline, but they don't call into
> any other kernel code and limit themselves to doing some basic I/O.
> 
> The only reason why they should be disabled in enter_freeze is that
> the context of the CPU that is going to a lower power state could
> become corrupted if a FIQ gets fired during a save or restore
> operation.

Right, that makes total sense.

(And yes, FIQ handlers are very limited in what they're allowed to do,
since they can't take any locks without the potential for deadlock.)

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