Date:   Wed, 7 Oct 2020 12:43:55 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>, rafael@...nel.org,
        srinivas.pandruvada@...ux.intel.com
Cc:     lukasz.luba@....com, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, rui.zhang@...el.com
Subject: Re: [PATCH 0/4] powercap/dtpm: Add the DTPM framework

Hi,

On 10/6/20 2:20 PM, Daniel Lezcano wrote:
> 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.
> 
> The skin temperature is managed as a whole by an user space daemon,
> which is catching the current application profile, to allocate a power
> budget to the different components where the resulting heating effect
> will comply with the skin temperature constraint.
> 
> This technique is called the Dynamic Thermal Power Management.
> 
> The Linux kernel does not provide any unified interface to act on the
> power of the different devices. Currently, the thermal framework is
> changed to export artificially the performance states of different
> devices via the cooling device software component with opaque values.
> This change is done regardless of the in-kernel logic to mitigate the
> temperature. The user space daemon uses all the available knobs to act
> on the power limit and those differ from one platform to another.
> 
> This series provides a Dynamic Thermal Power Management framework to
> provide an unified way to act on the power of the devices.

Interesting, we have a discussion going on about a related
(while at the same time almost orthogonal) discussion for
setting policies for if the code managing the restraints
(which on x86 is often hidden in firmware or ACPI DPTF tables)
should have a bias towards trying to have as long a battery life
as possible, vs maximum performance. I know those 2 aren't
always opposite ends of a spectrum with race-to-idle, yet most
modern x86 hardware has some notion of what I call performance-profiles
where we can tell the firmware managing this to go for a bias towards
low-power / balanced / performance.

I've send a RFC / sysfs API proposal for this here:
https://lore.kernel.org/linux-pm/20201003131938.9426-1-hdegoede@redhat.com/

I've read the patches in this thread and as said already I think
the 2 APIs are mostly orthogonal. The API in this thread is giving
userspace direct access to detailed power-limits allowing userspace
to configure things directly (and for things to work optimal userspace
must do this). Where as in the x86 case with which I'm dealing everything
is mostly handled in a black-box and userspace can merely configure
the low-power / balanced / performance bias (*) of that black-box.

Still I think it is good if we are aware of each-others efforts here.

So Daniel, if you can take a quick look at my proposal:
https://lore.kernel.org/linux-pm/20201003131938.9426-1-hdegoede@redhat.com/

That would be great. I think we definitely want to avoid having 2
APIs for the same thing here. Again I don't think that is actually
the case, but maybe you see this differently ?

Regards,

Hans


*) bias might not always give the correct impression at least
on some Thinkpads switching from balanced (which they call medium)
to performance boosts the long time power-limit from aprox.
15W to 25W which as expected results in a significant performance
boost.

Note usually we have no idea what the black-box knob which we are
tweaking actually does, all we know is that it is there and
Windows 10 often has a slider to configure it and users want
the same functionality under Linux.

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