linux-kernel - Re: [PATCH v3 4/7] thermal/drivers/tegra: Add driver for Tegra30 thermal sensor

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Message-ID: <595f5e53-b872-bcc6-e886-ed225e26e9fe@gmail.com>
Date:   Tue, 15 Jun 2021 16:01:48 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Zhang Rui <rui.zhang@...el.com>,
        Amit Kucheria <amitk@...nel.org>,
        Andreas Westman Dorcsak <hedmoo@...oo.com>,
        Maxim Schwalm <maxim.schwalm@...il.com>,
        Svyatoslav Ryhel <clamor95@...il.com>,
        Ihor Didenko <tailormoon@...bler.ru>,
        Ion Agorria <ion@...rria.com>,
        Matt Merhar <mattmerhar@...tonmail.com>,
        Peter Geis <pgwipeout@...il.com>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 4/7] thermal/drivers/tegra: Add driver for Tegra30
 thermal sensor

15.06.2021 13:26, Viresh Kumar пишет:
> On 15-06-21, 12:03, Daniel Lezcano wrote:
>>
>> [Cc Viresh]
>>
>> On 29/05/2021 19:09, Dmitry Osipenko wrote:
>>> All NVIDIA Tegra30 SoCs have a two-channel on-chip sensor unit which
>>> monitors temperature and voltage of the SoC. Sensors control CPU frequency
>>> throttling, which is activated by hardware once preprogrammed temperature
>>> level is breached, they also send signal to Power Management controller to
>>> perform emergency shutdown on a critical overheat of the SoC die. Add
>>> driver for the Tegra30 TSENSOR module, exposing it as a thermal sensor
>>> and a cooling device.
>>
>> IMO it does not make sense to expose the hardware throttling mechanism
>> as a cooling device because it is not usable anywhere from the thermal
>> framework.
>>
>> Moreover, that will collide with the thermal / cpufreq framework
>> mitigation (hardware sets the frequency but the software thinks the freq
>> is different), right ?

H/w mitigation is additional and should be transparent to the software
mitigation. The software mitigation is much more flexible, but it has
latency. Software also could crash and hang.

Hardware mitigation doesn't have latency and it will continue to work
regardless of the software state.

The CCF driver is aware about the h/w cooling status [1], hence software
sees the actual frequency.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit?id=344d5df34f5abd468267daa98f041abf90b2f660

> I am not even sure what the cooling device is doing here:
> 
> tegra_tsensor_set_cur_state() is not implemented and it says hardware
> changed it by itself. What is the benefit you are getting out of the
> cooling device here ?

It allows userspace to check whether hardware cooling is active via the
cooling_device sysfs. Otherwise we don't have ability to check whether
h/w cooling is active, I think it's a useful information. It's also
interesting to see the cooling_device stats, showing how many times h/w
mitigation was active.

>> The hardware limiter should let know the cpufreq framework about the
>> frequency change.
>>
>> 	https://lkml.org/lkml/2021/6/8/1792
>>
>> May be post the sensor without the hw limiter for now and address that
>> in a separate series ?
> 

I wasn't aware about existence of the thermal pressure, thank you for
pointing at it. At a quick glance it should be possible to benefit from
the information about the additional pressure.

Seems the current thermal pressure API assumes that there is only one
user of the API. So it's impossible to aggregate the pressure from
different sources, like software cpufreq pressure + h/w freq pressure.
Correct? If yes, then please let me know yours thoughts about the best
approach of supporting the aggregation.

I'll factor out the h/w limiter from this patchset and prepare the v4.
Thank you all for taking a look at the patches.