Re: [PATCH] drivers: cpuidle: assign enter_freeze to same as enter callback function

[Vincent Guittot]

On 9 November 2016 at 19:48, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
>
>
> On 09/11/16 18:39, Lorenzo Pieralisi wrote:
>>
>> On Wed, Nov 09, 2016 at 05:43:30PM +0000, Sudeep Holla wrote:
>>>
>>> enter_freeze() callback is expected atleast to do the same as enter()
>>> but it has to guarantee that interrupts aren't enabled at any point
>>> in its execution, as the tick is frozen.
>>>
>>> CPUs execute ->enter_freeze with the local tick or entire timekeeping
>>> suspended, so it must not re-enable interrupts at any point (even
>>> temporarily) or attempt to change states of clock event devices.
>>>
>>> It will be called when the system goes to suspend-to-idle and will
>>> reduce power usage because CPUs won't be awaken for unnecessary IRQs
>>> (i.e. woken up only on IRQs from "wakeup sources")
>>>
>>> Since for all the states that have CPUIDLE_FLAG_TIMER_STOP flag set,
>>> local tick is stopped, we can reuse the same code for both the enter()
>>> and enter_freeze() callbacks. Only "coupled" cpuidle mechanism enables
>>> interrupts and doing that with timekeeping suspended is generally not
>>> safe. Since this generic DT based idle driver doesn't support "coupled"
>>> states, it is safe to assume that the interrupts are not re-enabled.
>>>
>>> This patch assign enter_freeze to same as enter callback function which
>>> helps to save power without any intermittent spurious wakeups from
>>> suspend-to-idle.
>>>
>>> Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxx>
>>> ---
>>>  drivers/cpuidle/dt_idle_states.c | 11 ++++++++++-
>>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/cpuidle/dt_idle_states.c
>>> b/drivers/cpuidle/dt_idle_states.c
>>> index a5c111b67f37..5a087d108475 100644
>>> --- a/drivers/cpuidle/dt_idle_states.c
>>> +++ b/drivers/cpuidle/dt_idle_states.c
>>> @@ -79,8 +79,17 @@ static int init_state_node(struct cpuidle_state
>>> *idle_state,
>>>                 desc = state_node->name;
>>>
>>>         idle_state->flags = 0;
>>> -       if (of_property_read_bool(state_node, "local-timer-stop"))
>>> +       if (of_property_read_bool(state_node, "local-timer-stop")) {
>>>                 idle_state->flags |= CPUIDLE_FLAG_TIMER_STOP;
>>> +               /*
>>> +                * CPUIDLE_FLAG_TIMER_STOP guarantees that the local tick
>>> is
>>> +                * stopped and since this is not a "coupled" state
>>> interrupts
>>> +                * won't be enabled when it exits allowing the tick to be
>>> +                * frozen safely. So enter() can be also enter_freeze()
>>> +                * callback.
>>> +                */
>>
>>
>> I do not think that represents a guarantee for enter_freeze() to be
>> functional, we can initialize enter_freeze() with a function that
>> does _not_ enable IRQs while executing, it has not much to do with
>> the local timer losing HW state.
>>
>
> I agree, and I didn't mean that with the above comment. But reading
> again, I see your point.
>
>> I would just init the enter_freeze() pointer and be done with that,
>> adding code to check whether the idle back-end enables IRQs when it
>> enters idle is a major PITA that really is not worth the hassle and
>> apart from coupled C-states (which we do not support in DT as you said)
>> I can't find another example (and on top of that it is not even
>> something we can solve through DT since it is not a property of the idle
>> state but more related to its kernel implementation).
>>
>
> Makes sense, I was just trying to avoid setting for a state like
> CPU/Cluster retention but I agree, we need not do that.

I agree with Lorenzo and would prefer to keep it simple

Regards,
Vincent

>
>> If we wanted to do it _properly_ we have to add an arch hook to check
>> if the given idle state enter function back-end, ie cpu_ops on ARM64 or
>> or cpuidle_ops on ARM, enables IRQs while executing, I would honestly
>> avoid it but comments are nonetheless welcome.
>>
>
> Yes, that's may be unnecessary addition of some code when we can do it
> in simple ways, but I am open to suggestions.
>
> --
> Regards,
> Sudeep