Date: Sat, 16 May 2015 00:30:41 +0100
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        "linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Stephen Warren <swarren@wwwdotorg.org>,
        Thierry Reding <thierry.reding@gmail.com>,
        Alexandre Courbot <gnurou@gmail.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Kukjin Kim <kgene.kim@samsung.com>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] ARM: tegra: cpuidle: implement
 cpuidle_state.enter_freeze()
Message-ID: <20150515233041.GN2067@n2100.arm.linux.org.uk>
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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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