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Date:   Fri, 25 Dec 2020 12:54:15 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Pavel Machek <pavel@....cz>
Cc:     rjw@...ysocki.net, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Lina Iyer <ilina@...eaurora.org>,
        Ram Chandrasekar <rkumbako@...eaurora.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Lukasz Luba <lukasz.luba@....com>
Subject: Re: [PATCH v5 0/4] powercap/dtpm: Add the DTPM framework

On 24/12/2020 19:46, Pavel Machek wrote:
> Hi!
> 
>> The density of components greatly increased the last decade bringing a
>> numerous number of heating sources which are monitored by more than 20
>> sensors on recent SoC. The skin temperature, which is the case
>> temperature of the device, must stay below approximately 45°C in order
>> to comply with the legal requirements.
> 
> What kind of device is that?

Any complex embedded devices like a phone, a laptop or a tablet with
components like NPU, CPU, GPU, GPS, DSPs, Camera, ...

> Does that mean that running fsck is now "illegal" because temperature
> will not be managed during that time?

The heating effect of the different devices will be conducted through a
common dissipation device.

The 'skin' temperature or 'case' temperature has a dedicated sensor in
the path of this dissipation device. So the temperature will increase
slower at this sensor level because of a higher thermal capacity.

The 'skin' temperature will be the result of the different components
running at the same time (eg. GPS + CPU + GPU + DSPs).

In the case of fsck, the system is in degraded mode, thus the
application using these components are not supposed to run and the
'skin' temperature should stay below.

If you are interested, here you can find some background to explain the
'skin' temperature [1] and the spreading of the heat [2].

Hope that helps

  -- Daniel

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4188373/

[2]
https://nanoheat.stanford.edu/sites/default/files/publications/Electronics%20Cooling%20Article.pdf


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