linux-kernel - Re: [PATCH v1] thermal/of: Introduce k-po, k-pu and k-i for a thermal zone

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Message-ID: <CAL_JsqJO7PFYMpO-eDaCOzz5MxyjkfOScw9Q_ZzF=bEswCweGA@mail.gmail.com>
Date:   Tue, 25 Aug 2020 09:50:12 -0600
From:   Rob Herring <robh@...nel.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Lukasz Luba <lukasz.luba@....com>,
        Finley Xiao <finley.xiao@...k-chips.com>,
        "heiko@...ech.de" <heiko@...ech.de>,
        Zhang Rui <rui.zhang@...el.com>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        devicetree@...r.kernel.org, Tao Huang <huangtao@...k-chips.com>,
        Tony Xie <tony.xie@...k-chips.com>,
        Liang Chen <cl@...k-chips.com>
Subject: Re: [PATCH v1] thermal/of: Introduce k-po, k-pu and k-i for a thermal zone

On Tue, Aug 25, 2020 at 3:44 AM Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
>
> On 25/08/2020 10:25, Lukasz Luba wrote:
> > Hi Rob,
> >
> > On 8/25/20 12:09 AM, Rob Herring wrote:
> >> On Tue, Aug 11, 2020 at 08:31:15PM +0800, Finley Xiao wrote:
> >>> The default value for k_pu is:
> >>>      2 * sustainable_power / (desired_temperature - switch_on_temp)
> >>> The default value for k_po is:
> >>>      sustainable_power / (desired_temperature - switch_on_temp)
> >>> The default value for k_i is 10.
> >>>
> >>> Even though these parameters of the PID controller can be changed
> >>> by the following sysfs files:
> >>>      /sys/class/thermal/thermal_zoneX/k_pu
> >>>      /sys/class/thermal/thermal_zoneX/k_po
> >>>      /sys/class/thermal/thermal_zoneX/k_i
> >>>
> >>> But it's still more convenient to change the default values by
> >>> devicetree,
> >>> so introduce these three optional properties. If provided these
> >>> properties,
> >>> they will be parsed and associated with the thermal zone via the thermal
> >>> zone parameters.
> >>>
> >>> Signed-off-by: Finley Xiao <finley.xiao@...k-chips.com>
> >>> ---
> >>>   Documentation/devicetree/bindings/thermal/thermal.txt | 14
> >>> ++++++++++++++
> >>
> >> Bindings should be a separate file and this one is a DT schema now.
> >>
> >>>   drivers/thermal/thermal_of.c                          |  7 +++++++
> >>>   2 files changed, 21 insertions(+)
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/thermal/thermal.txt
> >>> b/Documentation/devicetree/bindings/thermal/thermal.txt
> >>> index f78bec19ca35..ebe936b57ded 100644
> >>> --- a/Documentation/devicetree/bindings/thermal/thermal.txt
> >>> +++ b/Documentation/devicetree/bindings/thermal/thermal.txt
> >>> @@ -165,6 +165,20 @@ Optional property:
> >>>               2000mW, while on a 10'' tablet is around
> >>>               4500mW.
> >>>   +- k-po:            Proportional parameter of the PID controller when
> >>> +            current temperature is above the target.
> >>> +  Type: signed
> >>> +  Size: one cell
> >>> +
> >>> +- k-pu:            Proportional parameter of the PID controller when
> >>> +            current temperature is below the target.
> >>> +  Type: signed
> >>> +  Size: one cell
> >>> +
> >>> +- k-i:            Integral parameter of the PID controller.
> >>> +  Type: signed
> >>> +  Size: one cell
> >>
> >> What's PID?
> >>
> >> I know nothing about the sysfs params, but the binding needs to stand on
> >> it's own and needs enough detail to educate me.
> > Sorry for the delay, I missed that patch.
> > These parameters are the coefficients for the
> > Proportional-Integral-Derivative (PID) controller [1], which is the
> > core of the Intelligent Power Allocation (IPA) thermal governor.
>
> Just a few words to elaborate a bit for Rob who may not have time to
> digest the whole concept from Wikipedia :)
>
> The PID is an regulation loop where the input is compared to the output.
>
> For example when driving a car and you aim a speed cruise of 90km/h. You
> press the accelerator and watch the current speed. The smaller the
> current speed is, the stronger you will push the accelerator. And the
> closer to the cruise speed the car is, the lesser you push the
> accelerator until the car stabilize to the cruise to speed.
>
> The k-* describes how strong you push the accelerator and release it.
>
> In the thermal framework, that has an impact on how brutal the
> mitigation acts and depending on them it results in a flat temperature
> curve or a sawtooth aspect.
>
> These coefficient depends on the ambient temperature (casing, room
> temperature), the heat sink and the load. Depending on the use cases,
> you may want to change their values at runtime.
>
> From my POV, setting these values in the DT does not really make sense.

I pretty much always agree with not putting things in DT. :)

Rob