Date:   Wed, 15 Jul 2020 19:10:43 -0400
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Zhang Rui <rui.zhang@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>, robh+dt@...nel.org
Cc:     linux-pm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/4] thermal: Introduce support for monitoring falling
 temperature



On 7/15/20 4:27 AM, Zhang Rui wrote:
> Hi, Thara,
> 
> On Tue, 2020-07-14 at 17:39 -0400, Thara Gopinath wrote:
>>
>>>
>>> For example, to support this, we can
>>> either
>>> introduce both "cold" trip points and "warming devices", and
>>> introduce
>>> new logic in thermal framework and governors to handle them,
>>> Or
>>> introduce "cold" trip point and "warming" device, but only
>>> semantically, and treat them just like normal trip points and
>>> cooling
>>> devices. And strictly define cooling state 0 as the state that
>>> generates most heat, and define max cooling state as the state that
>>> generates least heat. Then, say, we have a trip point at -10C, the
>>> "warming" device is set to cooling state 0 when the temperature is
>>> lower than -10C, and in most cases, this thermal zone is always in
>>> a
>>> "overheating" state (temperature higher than -10C), and the
>>> "warming"
>>> device for this thermal zone is "throttled" to generate as least
>>> heat
>>> as possible. And this is pretty much what the current code has
>>> always
>>> been doing, right?
>>
>>
>> IMHO, thermal framework should move to a direction where the term
>> "mitigation" is used rather than cooling or warming. In this case
>> "cooling dev" and "warming dev" should will become
>> "temp-mitigating-dev". So going by this, I think what you mention as
>> option 1 is more suitable where new logic is introduced into the
>> framework and governors to handle the trip points marked as "cold".
>>
>> Also in the current set of requirements, we have a few power domain
>> rails and other resources that are used exclusively in the thermal
>> framework for warming alone as in they are not used ever for cooling
>> down a zone. But then one of the requirements we have discussed is
>> for cpufreq and gpu scaling to be behave as warming devices where
>> the minimum operating point/ voltage of the relevant cpu/gpu is
>> restricted.
>> So in this case, Daniel had this suggestion of introducing negative
>> states for presently what is defined as cooling devices. So cooling
>> dev
>> / temp-mitigation-dev states can range from say -3 to 5 with 0 as
>> the
>> good state where no mitigation is happening. This is an interesting
>> idea
>> though I have not proto-typed it yet.
> 
> Agreed. If some devices support both "cooling" and "warning", we should
> have only one "temp-mitigating-dev" instead.
>>
>>>
>>> I can not say which one is better for now as I don't have the
>>> background of this requirement. It's nice that Thara sent this RFC
>>> series for discussion, but from upstream point of view, I'd prefer
>>> to
>>> see a full stack solution, before taking any code.
>>
>> We had done a session at ELC on this requirement. Here is the link
>> to
>> the presentation. Hopefully it gives you some back ground on this.
> 
> yes, it helps. :)
>>
>>
> https://elinux.org/images/f/f7/ELC-2020-Thara-Ram-Linux-Kernel-Thermal-Warming.pdf
>>
>> I have sent across some patches for introducing a generic power
>> domain
>> warming device which is under review by Daniel.
>>
>> So how do you want to proceed on this? Can you elaborate a bit more
>> on
>> what you mean by a full stack solution.
> 
> I mean, the patches, and the idea look good to me, just with some minor
> comments. But applying this patch series, alone, does not bring us
> anything because we don't have a thermal zone driver that supports cold
> trip point, right?
> I'd like to see this patch series together with the support in
> thermal_core/governors and real users like updated/new thermal
> zone/cdev drivers that supports the cold trip point and warming
> actions.
> Or else I've the concern that this piece of code may be changed back
> and forth when prototyping the rest of the support.

Got it! I will try to include more pieces in the next version.

> 
> thanks,
> rui
> 

-- 
Warm Regards
Thara

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