linux-kernel - Re: [PATCH] thermal/core: Introduce user trip points

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Message-ID: <1eb7eb88-4230-4803-83fe-415ce0745951@linaro.org>
Date: Tue, 2 Jul 2024 12:56:20 +0200
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Rob Herring <robh@...nel.org>, linux-pm@...r.kernel.org,
 Zhang Rui <rui.zhang@...el.com>, Lukasz Luba <lukasz.luba@....com>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>,
 "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
 <devicetree@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] thermal/core: Introduce user trip points

On 02/07/2024 12:22, Rafael J. Wysocki wrote:
> On Tue, Jul 2, 2024 at 11:29 AM Daniel Lezcano
> <daniel.lezcano@...aro.org> wrote:
>>
>> On 01/07/2024 18:26, Rob Herring wrote:
>>> On Thu, Jun 27, 2024 at 10:54:50AM +0200, Daniel Lezcano wrote:
>>>> Currently the thermal framework has 4 trip point types:
>>>>
>>>> - active : basically for fans (or anything requiring energy to cool
>>>>     down)
>>>>
>>>> - passive : a performance limiter
>>>>
>>>> - hot : for a last action before reaching critical
>>>>
>>>> - critical : a without return threshold leading to a system shutdown
>>>>
>>>> A thermal zone monitors the temperature regarding these trip
>>>> points. The old way to do that is actively polling the temperature
>>>> which is very bad for embedded systems, especially mobile and it is
>>>> even worse today as we can have more than fifty thermal zones. The
>>>> modern way is to rely on the driver to send an interrupt when the trip
>>>> points are crossed, so the system can sleep while the temperature
>>>> monitoring is offloaded to a dedicated hardware.
>>>>
>>>> However, the thermal aspect is also managed from userspace to protect
>>>> the user, especially tracking down the skin temperature sensor. The
>>>> logic is more complex than what we found in the kernel because it
>>>> needs multiple sources indicating the thermal situation of the entire
>>>> system.
>>>>
>>>> For this reason it needs to setup trip points at different levels in
>>>> order to get informed about what is going on with some thermal zones
>>>> when running some specific application.
>>>>
>>>> For instance, the skin temperature must be limited to 43°C on a long
>>>> run but can go to 48°C for 10 minutes, or 60°C for 1 minute.
>>>>
>>>> The thermal engine must then rely on trip points to monitor those
>>>> temperatures. Unfortunately, today there is only 'active' and
>>>> 'passive' trip points which has a specific meaning for the kernel, not
>>>> the userspace. That leads to hacks in different platforms for mobile
>>>> and embedded systems where 'active' trip points are used to send
>>>> notification to the userspace. This is obviously not right because
>>>> these trip are handled by the kernel.
>>>>
>>>> This patch introduces the 'user' trip point type where its semantic is
>>>> simple: do nothing at the kernel level, just send a notification to
>>>> the user space.
>>>
>>> Sounds like OS behavior/policy though I guess the existing ones kind are
>>> too. Maybe we should have defined *what* action to take and then the OS
>>> could decide whether what actions to handle vs. pass it up a level.
>>
>> Right
>>
>>> Why can't userspace just ask to be notified at a trip point it
>>> defines?
>>
>> Yes I think it is possible to create a netlink message to create a trip
>> point which will return a trip id.
>>
>> Rafael what do you think ?
> 
> Trips cannot be created on the fly ATM.
> 
> What can be done is to create trips that are invalid to start with and
> then set their temperature via sysfs.  This has been done already for
> quite a while AFAICS.

Yes, I remember that.

I would like to avoid introducing more weirdness in the thermal 
framework which deserve a clear ABI.

What is missing to create new trip points on the fly ?





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