Re: [PATCH] thermal: power_allocate: add upper and lower limits

[Lukasz Luba <lukasz.luba@arm.com>]

On 4/29/20 11:39 AM, Michael Kao wrote:
> On Fri, 2020-04-24 at 10:22 +0100, Lukasz Luba wrote:
>> Hi Michael,
>>
>> On 4/24/20 8:16 AM, Michael Kao wrote:
>>> The upper and lower limits of thermal throttle state in the
>>> device tree do not apply to the power_allocate governor.
>>> Add the upper and lower limits to the power_allocate governor.
>>>
>>> Signed-off-by: Michael Kao <michael.kao@mediatek.com>
>>> ---
>>>    drivers/thermal/thermal_core.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/thermal/thermal_core.c b/drivers/thermal/thermal_core.c
>>> index 9a321dc548c8..f6feed2265bd 100644
>>> --- a/drivers/thermal/thermal_core.c
>>> +++ b/drivers/thermal/thermal_core.c
>>> @@ -598,7 +598,7 @@ int power_actor_set_power(struct thermal_cooling_device *cdev,
>>>    	if (ret)
>>>    		return ret;
>>>    
>>> -	instance->target = state;
>>> +	instance->target = clamp_val(state, instance->lower, instance->upper);
>>>    	mutex_lock(&cdev->lock);
>>>    	cdev->updated = false;
>>>    	mutex_unlock(&cdev->lock);
>>>
>>
>> Thank you for the patch and having to look at it. I have some concerns
>> with this approach. Let's analyze it further.
>>
>> In default the cooling devices in the thermal zone which is used by IPA
>> do not have this 'lower' and 'upper' limits. They are set to
>> THERMAL_NO_LIMIT in DT to give full control to IPA over the states.
>>
>> This the function 'power_actor_set_power' actually translates granted
>> power to the state that device will run for the next period.
>> The IPA algorithm has already split the power budget.
>> Now what happen when the 'lower' value will change the state to a state
>> which consumes more power than was calculated in the IPA alg... It will
>> became unstable.
>>
>> I would rather see a change which uses these 'lower' and 'upper' limits
>> before the IPA do the calculation of the power budget. But this wasn't
>> a requirement and we assumed that IPA has full control over the cooling
>> device (which I described above with this DT THERMAL_NO_LIMIT).
>>
>> Is there a problem with your platform that it has to provide some
>> minimal performance, so you tried to introduce this clamping?
>>
>> Regards,
>> Lukasz
> 
> 
> Hi Lukasz,
> 
> I refer to the documentation settings of the thermal device tree
> (Documentation / devicetree / bindings / thermal / thermal.txt).
> 
> It shows that cooling-device is a mandatory property, so max/min cooling
> state should be able to support in framework point of view.
> Otherwise, the limitation should be added in binding document.
> 
> Different hardware mechanisms have different heat dissipation
> capabilities.
> Limiting the input heat source can slow down the heat accumulation and
> temperature burst.
> We want to reduce the accumulation of heat at high temperature by
> limiting the minimum gear of thermal throttle.

I agree that these 'lower' and 'upper' limits shouldn't be just
ignored as is currently. This patch clamps the value at late stage,
though.

Let me have a look how it could be taken into account in the early
stage, before the power calculation and split are done. Maybe there
is a clean way to inject this.

Regards,
Lukasz