Re: [PATCH V3 2/4] thermal/drivers/cpu_cooling: Add idle cooling device documentation

[Daniel Lezcano]

Hi Amit,

thanks for the review.


On 04/12/2019 05:24, Amit Kucheria wrote:

[ ... ]

>> +the CPUs will have to wakeup from a deep sleep state.
>> +
>> +     ^
>> +     |
>> +     |
>> +     |-------       -------       -------
>> +     |_______|_____|_______|_____|_______|___________
>> +
>> +      <----->
>> +       idle  <---->
>> +              running
>> +
>> +With the fixed idle injection duration, we can give a value which is
>> +an acceptable performance drop off or latency when we reach a specific
>> +temperature and we begin to mitigate by varying the Idle injection
>> +period.
>> +
> 
> I'm not sure what it the purpose of this statement. You've described
> how the period value starts at a maximum and is adjusted dynamically
> below.

We can have different way to inject idle periods. We can increase the
idle duration and/or keep this duration constant but make a variation of
the period. This statement clarify the method which is the latter
because we want to have a constant latency per period easier to deal with.

>> +The mitigation begins with a maximum period value which decrease when
> 
> Shouldn't the idle injection period increase to get more cooling effect?

The period is the opposite of the frequency. The highest the period, the
lowest the frequency, thus less idle cycles and lesser cooling effect.

>> +more cooling effect is requested. When the period duration is equal to
>> +the idle duration, then we are in a situation the platform canât
>> +dissipate the heat enough and the mitigation fails. In this case the
>> +situation is considered critical and there is nothing to do. The idle
>> +injection duration must be changed by configuration and until we reach
>> +the cooling effect, otherwise an additionnal cooling device must be
> 
> typo: additional
> 
>> +used or ultimately decrease the SoC performance by dropping the
>> +highest OPP point of the SoC.
>> +
>> +The idle injection duration value must comply with the constraints:
>> +
>> +- It is lesser or equal to the latency we tolerate when the mitigation
> 
> s/lesser/less than/
> 
>> +  begins. It is platform dependent and will depend on the user
>> +  experience, reactivity vs performance trade off we want. This value
>> +  should be specified.
>> +
>> +- It is greater than the idle stateâs target residency we want to go
>> +  for thermal mitigation, otherwise we end up consuming more energy.
>> +
>> +Minimum period
>> +--------------
>> +
>> +The idle injection duration being fixed, it is obvious the minimum
> 
> Change to:
> When the idle injection duration is fixed,
> 

The idle duration is always fixed in the cpuidle cooling device, why do
you want to add the sentence above?


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