Message-ID: <3b526ad3-2db6-40c3-9017-91f63d43f97d@tuxedocomputers.com>
Date: Thu, 16 Jan 2025 12:00:19 +0100
From: Werner Sembach <wse@...edocomputers.com>
To: Daniel Lezcano <daniel.lezcano@...aro.org>,
 "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
 "Rafael J . Wysocki" <rafael@...nel.org>
Cc: rui.zhang@...el.com, Hans de Goede <hdegoede@...hat.com>,
 Armin Wolf <W_Armin@....de>, Ilpo Järvinen
 <ilpo.jarvinen@...ux.intel.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 platform-driver-x86@...r.kernel.org
Subject: Re: Thermal driver with safeguards


Am 10.01.25 um 18:43 schrieb Daniel Lezcano:
> On 10/01/2025 17:56, Werner Sembach wrote:
>> Hi Daniel,
>>
>> Am 09.01.25 um 22:36 schrieb Daniel Lezcano:
>>> On 02/12/2024 15:52, Werner Sembach wrote:
>>>> Hi,
>>>>
>>>> given a pair of a temperature sensor and a fan, I want to implement a 
>>>> driver. that allows userspace to directly control the fan if it wants to. 
>>>> But have a minimum fan speed when certain high temperatures are reached to 
>>>> avoid crashes or hardware damage.
>>>
>>> From the userspace, use directly the thermal-engine which is currently under 
>>> development [1]. You can add your platform specific code in a plugin while 
>>> the thermal engine will catch all the thermal events and pass them to it [2].
>>>
>>> The thermal engine has a configuration file which will setup the thermal 
>>> framework to be woken up at different temperatures.
>> That still requires to trust userspace/the user to not write dangerous values 
>> directly to sysfs?
>
> No, it is not a trip point but temperature thresholds. So if the firmware 
> defines trip points, the userspace can not change them.
>
> Userspace thresholds are new : https://lwn.net/Articles/986009/
>
>>> The thermal engine will be proposed for a distro package, so the platform 
>>> support will be automatically supported.
>>>
>>> Beside the trip points can be setup in the device to act on higher temperature.
>> As far as i can tell these trip points only notify userspace but you can't 
>> attach code executed in kernel to it.
>
> [ ... ]
>
>>> What is unclear is how the fan is managed. I suggest to have a look at 
>>> pwm-fan.c in drivers/hwmon
>>
>> I already looked at hwmon, but that basically just writes trough values from 
>> and to userspace and has no kernel side management of temperatures and fan 
>> speeds whatsoever.
>
> IIUC, you request was about having the userspace to deal with a fan and the 
> kernel to be a safe guard, so taking over the thermal management when the 
> temperature is too high.
Not completely taking over, just enforcing a min speed.
>
> Obviously the monitored temperature must be for a device with a "slow" 
> temperature motion, userspace temperature management is not suitable for fast 
> temperature transitions.
>
> The thermal engine can for example configure different temperatures, let's 
> say: 43°C, 44°C, 46°C, 49°C and 54°C.
>
> Then the DT describes additional trip points for mitigation, one trip point 
> for mitigation could be enough (eg 80°C). One for "hot" to send to the thermal 
> engine a notification about getting really high so it can do some userspace 
> action like killing an application, and finally a "critical" trip point to 
> shutdown the system.
I guess with DT you mean device table. I deal with a driver for x86 notebooks so 
there is not DT.
>
> The fan would be a cooling device with 0-100 values representing the speed in 
> percentage. The trip point at 80°C would be associated with the fan with the 
> <0, 100> cooling states.
>
> The dynamic of the thermal management could be the following:
>
> The temperature is changing and stays in the [35°C - 60°C] boundaries. The 
> thermal engine receives the events at the different aforementioned 
> temperatures and manage to act on the pwm fan via hwmon.
>
> For any reason the temperature goes above 80°C, at this moment the kernel 
> takes over the management and will increase/decrease the fan speed between the 
> 0% - 100% limits until the temperature goes below the 80°C.
>
> If it continues to increase and reaches the "hot" trip point, then an events 
> is sent to the userspace which should take an action to reduce the temperature 
> (kill the application, reduce the battery charge, drop the frame rates, etc ...).
>
> If it continues to increase and reaches the "critical" trip point, then the 
> system shuts down.
>
> If the temperature decreases and goes below 80°C, then it returns to the 
> normal state and the thermal engine can continue its work.
>
> Does it make sense ?
>
>

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