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

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Date:	Fri, 17 Apr 2015 15:08:19 +0100
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:	"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>,
	Russell King <linux@....linux.org.uk>,
	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 Thu, Apr 16, 2015 at 03:37:19PM +0100, Tomeu Vizoso wrote:

[...]

> >> I don't know what FIQs are. :-)
> >
> > In short, fast IRQs, it is a separate IRQ line handled as a separate
> > exception source with some private (banked) registers that minimize registers
> > saving/restoring. They are not identical to NMI on x86, since
> > their behaviour (handling) may be overriden by platforms and they
> > can be masked.
> >
> >> ->enter_freeze is entered with interrupts disabled on the local CPU.  It is
> >> not supposed to re-enable them.  That is, while in the ->enter_freeze callback
> >> routine, the CPU must not be interrupted aby anything other than NMI.
> >
> > It boils down to what FIQs handlers are allowed to do with tick frozen
> > and what they are (may be) currently used for.
> >
> > Russell has more insights on this than I do, in particular what FIQs are
> > currently used for on ARM and if we can leave them enabled safely with tick
> > frozen.
> 
> But even if it's currently safe to leave them enabled, is there any
> reason for not disabling them?

Ok, the point here is: either it is safe, and you leave them enabled,
or it is not and we must disable them *before* enter_freeze() is entered.

Disabling them in the platform enter_freeze() hook does not make sense,
because this means we run with FIQs enabled with tick frozen, either
it is safe or it is not, it can't be both.

I would ask Russell opinion on this, before making any decision.

Lorenzo
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