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Subject: Re: [PATCH v1 03/10] ARM: tegra: acer-a500: Bump thermal trips by 10C
To:     Daniel Lezcano <daniel.lezcano@linaro.org>,
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Cc:     Thierry Reding <thierry.reding@gmail.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
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        Svyatoslav Ryhel <clamor95@gmail.com>,
        linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org
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From:   Dmitry Osipenko <digetx@gmail.com>
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Date:   Sat, 12 Jun 2021 13:40:30 +0300
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11.06.2021 12:52, Daniel Lezcano пишет:
> On 14/05/2021 23:16, Michał Mirosław wrote:
>> On Mon, May 10, 2021 at 11:25:53PM +0300, Dmitry Osipenko wrote:
>>> It's possible to hit the temperature of the thermal zone in a very warm
>>> environment under a constant load, like watching a video using software
>>> decoding. It's even easier to hit the limit with a slightly overclocked
>>> CPU. Bump the temperature limit by 10C in order to improve user
>>> experience. Acer A500 has a large board and 10" display panel which are
>>> used for the heat dissipation, the SoC is placed far away from battery,
>>> hence we can safely bump the temperature limit.
>>
>> 60^C looks like a touch-safety limit (to avoid burns for users). Did you
>> verify the touchable parts' temperature somehow after the change?
> 
> The skin temperature and the CPU/GPU etc ... temperatures are different
> things.
> 
> For the embedded system there is the dissipation system and a
> temperature sensor on it which is the skin temp. This temperature is the
> result of the heat of all the thermal zones on the board and must be
> below 45°C. The temperature slowly changes.
> 
> On the CPU, the temperature changes can be very fast and you have to
> take care of keeping it below the max temperature specified in the TRM
> by using different techniques (freq changes, idle injection, ...) but
> the temperature can be 75°C, 85°C or whatever the manual says.
> 
> 50°C and 60°C are low temperature for a CPU and that will inevitably
> impact the performances, so setting the temperature close the max
> temperature is what will allow max performances.
> 
> What matters is the skin temperature.
> 
> The skin temperature must be monitored by other techniques, eg. using
> the TDP of the system and throttle the different devices to keep them in
> this power budget. That is the role of an thermal daemon.

Thank you for the clarification. Indeed, I wasn't sure how to make use
of the skin temperature properly.

The skin temperature varies a lot depending on the thermal capabilities
of a particular device. It's about 15C below CPU core at a full load on
A500, while it's 2C below CPU core on Nexus 7. But this is expected
since Nexus 7 can't dissipate heat efficiently.

I will revisit the DT thermal zones again for the next kernel release.