Messages in this thread | | | Date | Fri, 17 Apr 2015 15:08:19 +0100 | From | Lorenzo Pieralisi <> | Subject | Re: [PATCH] ARM: tegra: cpuidle: implement cpuidle_state.enter_freeze() |
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On Thu, Apr 16, 2015 at 03:37:19PM +0100, Tomeu Vizoso wrote:
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> >> 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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