Re: [PATCH] ARM: tegra: cpuidle: implement cpuidle_state.enter_freeze()

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

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