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

[Tomeu Vizoso]

On 17 April 2015 at 17:02, Tomeu Vizoso <tomeu.vizoso@xxxxxxxxxxxxx> wrote:
> On 17 April 2015 at 16:08, Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx> wrote:
>> 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.
>
> Sure, that's why I proposed doing it in arch_cpu_idle_enter/exit.
>
>> I would ask Russell opinion on this, before making any decision.
>
> Sure, that would be very welcome.

Hi Russell,

do you think FIQs should be disabled when the tick is frozen when
going to suspend to idle?

Thanks,

Tomeu
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