linux-kernel - Re: [PATCH RESEND RESEND] thermal/of: support thermal zones w/o trips subnode

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Date:   Sun, 23 Jul 2023 12:12:49 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Mark Brown <broonie@...nel.org>,
        Icenowy Zheng <zhengxingda@...as.ac.cn>
Cc:     Amit Kucheria <amitk@...nel.org>, Zhang Rui <rui.zhang@...el.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        Icenowy Zheng <uwu@...nowy.me>
Subject: Re: [PATCH RESEND RESEND] thermal/of: support thermal zones w/o trips
 subnode

Hi Mark,

On 22/07/2023 22:11, Mark Brown wrote:
> On Sat, Jul 22, 2023 at 08:25:34PM +0800, Icenowy Zheng wrote:
>> From: Icenowy Zheng <uwu@...nowy.me>
>>
>> Although the current device tree binding of thermal zones require the
>> trips subnode, the binding in kernel v5.15 does not require it, and many
>> device trees shipped with the kernel, for example,
>> allwinner/sun50i-a64.dtsi and mediatek/mt8183-kukui.dtsi in ARM64, still
>> comply to the old binding and contain no trips subnode.
>>
>> Allow the code to successfully register thermal zones w/o trips subnode
>> for DT binding compatibility now.
>>
>> Furtherly, the inconsistency between DTs and bindings should be resolved
>> by either adding empty trips subnode or dropping the trips subnode
>> requirement.
> 
> This makes sense to me - it allows people to see the reported
> temperature even if there's no trips defined which seems more
> helpful than refusing to register.

The binding describes the trip points as required and that since the 
beginning.

What changed is now the code reflects the required property while before 
it was permissive, that was an oversight.

Just a reminder about the thermal framework goals:

   1. It protects the silicon (thus critical and hot trip points)

   2. It mitigates the temperature (thus cooling device bound to trip 
points)

   3. It notifies the userspace when a trip point is crossed

So if the thermal zone is described but without any of this goal above, 
it is pointless.

If the goal is to report the temperature only, then hwmon should be used 
instead.

If the goal is to mitigate by userspace, then the trip point *must* be 
used to prevent the userspace polling the temperature. With the trip 
point the sensor will be set to fire an interrupt at the given trip 
temperature.

IOW, trip points are not optional

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