[PATCH v4 1/6] Documentation: arm: define DT idle states bindings
Santosh Shilimkar
santosh.shilimkar at ti.com
Wed Jun 18 16:13:27 PDT 2014
On Wednesday 18 June 2014 05:09 PM, Nicolas Pitre wrote:
> On Wed, 18 Jun 2014, Santosh Shilimkar wrote:
>
>> On Wednesday 18 June 2014 04:51 PM, Nicolas Pitre wrote:
>>> On Wed, 18 Jun 2014, Santosh Shilimkar wrote:
>>>
>>>> On Wednesday 18 June 2014 01:36 PM, Lorenzo Pieralisi wrote:
>>>> [..]
>>>>> + To correctly specify idle states timing and energy related properties,
>>>>> + the following definitions identify the different execution phases
>>>>> + a CPU goes through to enter and exit idle states and the implied
>>>>> + energy metrics:
>>>>> +
>>>>> + ..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]__..
>>>>> + | | | | |
>>>>> +
>>>>> + |<------ entry ------->|
>>>>> + | latency |
>>>>> + |<- exit ->|
>>>>> + | latency |
>>>>> + |<-------- min-residency -------->|
>>>>> + |<------- wakeup-latency ------->|
>>>>> +
>>>> I don't know the wakeup latency makes much sense and also correct.
>>>> Hardware wakeup latency is actually exit latency. Is it for failed
>>>> or abort-able ilde case ? We are adding this as a new parameter
>>>> at least from idle states perspective. I think we should just
>>>> avoid it.
>>>
>>> I explained the rationale for this parameter in a previous email but
>>> Lorenzo didn't carry it over. To be clearer, this should be "worst case
>>> wake-up latency". It is of interest for PMQOS. This is the maximum
>>> delay that can be expected from the moment a wake-up event is signaled
>>> and the moment the CPU is back operational. This is more than just exit
>>> latency. By default this is entry_latency + exit_latency but when there
>>> is an abortable PREP phase then it may be shorter than that.
>>>
>> PMQOS angle is right. It is just that the idle code is not
>> going to do anything with this value. But I see a value adding it
>> instead of some one doing calculation.
>
> The idle code should take it into account when a PMQOS restriction is in
> effect i.e. avoid using those modes whose worst case wake-up latency is
> too large.
>
> And cpuidle is being migrated into the scheduler as we speak. So some
> of the values there, namely entry_latency and exit_latency (taken
> separately for timing purposes) will be directly used by the scheduler
> to decide which CPU to wake up for example.
>
> So there is fundamentally 4 parameters if we want to comprehensively
> support all pertinent use cases.
>
Fair enough.
regards,
Santosh
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