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Subject: Re: [PATCH] thermal: power_allocate: add upper and lower limits
To:     Michael Kao <michael.kao@mediatek.com>
Cc:     Zhang Rui <rui.zhang@intel.com>,
        Eduardo Valentin <edubezval@gmail.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Matthias Brugger <matthias.bgg@gmail.com>, hsinyi@chromium.org,
        linux-pm@vger.kernel.org, srv_heupstream@mediatek.com,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
References: <20200424071601.2636-1-michael.kao@mediatek.com>
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 <03286571-c110-7f5e-a911-24f8c3e4fd42@arm.com>
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From:   Lukasz Luba <lukasz.luba@arm.com>
Message-ID: <25cb2d5b-0d87-8a22-a881-a5b6e46307ec@arm.com>
Date:   Tue, 6 Oct 2020 15:39:57 +0100
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Hi Michael,

On 8/25/20 10:29 AM, Michael Kao wrote:
> On Wed, 2020-04-29 at 21:24 +0100, Lukasz Luba wrote:
>>
>> 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
> Hi Lukasz,
> 
> After the research, do you have any ideas or suggestions?
> 
> Best Regards,
> Michael
> 

My apologies for the delay. I have done some experiments.
Could you resend the patch, please make sure it is not encoded in base64
like this one.

I am going to take your patch together with some other changes.

Regards,
Lukasz