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Date:   Thu, 17 Feb 2022 09:14:17 -0800
From:   Matthias Kaehlcke <mka@chromium.org>
To:     Doug Anderson <dianders@chromium.org>
Cc:     Lukasz Luba <lukasz.luba@arm.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        amit daniel kachhap <amit.kachhap@gmail.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Amit Kucheria <amitk@kernel.org>,
        Zhang Rui <rui.zhang@intel.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Pierre.Gondois@arm.com, Stephen Boyd <swboyd@chromium.org>,
        Rajendra Nayak <rnayak@codeaurora.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        jorcrous@amazon.com, Rob Clark <robdclark@chromium.org>
Subject: Re: [PATCH 1/2] thermal: cooling: Check Energy Model type in
 cpufreq_cooling and devfreq_cooling
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On Thu, Feb 17, 2022 at 08:37:39AM -0800, Doug Anderson wrote:
> Hi,
> 
> On Thu, Feb 17, 2022 at 2:47 AM Lukasz Luba <lukasz.luba@arm.com> wrote:
> >
> > Hi Daniel,
> >
> > On 2/17/22 10:10 AM, Daniel Lezcano wrote:
> > > On 16/02/2022 18:33, Doug Anderson wrote:
> > >> Hi,
> > >>
> > >> On Wed, Feb 16, 2022 at 7:35 AM Lukasz Luba <lukasz.luba@arm.com> wrote:
> > >>>
> > >>> Hi Matthias,
> > >>>
> > >>> On 2/9/22 10:17 PM, Matthias Kaehlcke wrote:
> > >>>> On Wed, Feb 09, 2022 at 11:16:36AM +0000, Lukasz Luba wrote:
> > >>>>>
> > >>>>>
> > >>>>> On 2/8/22 5:25 PM, Matthias Kaehlcke wrote:
> > >>>>>> On Tue, Feb 08, 2022 at 09:32:28AM +0000, Lukasz Luba wrote:
> > >>>>>>>
> > >>>>>>>
> > >>>
> > >>> [snip]
> > >>>
> > >>>>>>> Could you point me to those devices please?
> > >>>>>>
> > >>>>>> arch/arm64/boot/dts/qcom/sc7180-trogdor-*
> > >>>>>>
> > >>>>>> Though as per above they shouldn't be impacted by your change,
> > >>>>>> since the
> > >>>>>> CPUs always pretend to use milli-Watts.
> > >>>>>>
> > >>>>>> [skipped some questions/answers since sc7180 isn't actually
> > >>>>>> impacted by
> > >>>>>>     the change]
> > >>>>>
> > >>>>> Thank you Matthias. I will investigate your setup to get better
> > >>>>> understanding.
> > >>>>
> > >>>> Thanks!
> > >>>>
> > >>>
> > >>> I've checked those DT files and related code.
> > >>> As you already said, this patch is safe for them.
> > >>> So we can apply it IMO.
> > >>>
> > >>>
> > >>> -------------Off-topic------------------
> > >>> Not in $subject comments:
> > >>>
> > >>> AFAICS based on two files which define thermal zones:
> > >>> sc7180-trogdor-homestar.dtsi
> > >>> sc7180-trogdor-coachz.dtsi
> > >>>
> > >>> only the 'big' cores are used as cooling devices in the
> > >>> 'skin_temp_thermal' - the CPU6 and CPU7.
> > >>>
> > >>> I assume you don't want to model at all the power usage
> > >>> from the Little cluster (which is quite big: 6 CPUs), do you?
> > >>> I can see that the Little CPUs have small dyn-power-coeff
> > >>> ~30% of the big and lower max freq, but still might be worth
> > >>> to add them to IPA. You might give them more 'weight', to
> > >>> make sure they receive more power during power split.
> > >>>
> > >>> You also don't have GPU cooling device in that thermal zone.
> > >>> Based on my experience if your GPU is a power hungry one,
> > >>> e.g. 2-4Watts, you might get better results when you model
> > >>> this 'hot' device (which impacts your temp sensor reported value).
> > >>
> > >> I think the two boards you point at (homestar and coachz) are just the
> > >> two that override the default defined in the SoC dtsi file. If you
> > >> look in sc7180.dtsi you'll see 'gpuss1-thermal' which has a cooling
> > >> map. You can also see the cooling maps for the littles.
> > >>
> > >> I guess we don't have a `dynamic-power-coefficient` for the GPU,
> > >> though? Seems like we should, but I haven't dug through all the code
> > >> here...
> > >
> > > The dynamic-power-coefficient is available for OPPs which includes
> > > CPUfreq and devfreq. As the GPU is managed by devfreq, setting the
> > > dynamic-power-coefficient makes the energy model available for it.
> > >
> > > However, the OPPs must define the frequency and the voltage. That is the
> > > case for most platforms except on QCom platform.
> > >
> > > That may not be specified as it uses a frequency index and the hardware
> > > does the voltage change in our back. The QCom cpufreq backend get the
> > > voltage table from a register (or whatever) and completes the voltage
> > > values for the OPPs, thus adding the information which is missing in the
> > > device tree. The energy model can then initializes itself and allows the
> > > usage of the Energy Aware Scheduler.
> > >
> > > However this piece of code is missing for the GPU part.
> > >
> >
> > Thank you for joining the discussion. I don't know about that Qcom
> > GPU voltage information is missing.
> >
> > If the voltage is not available (only the frequencies), there is
> > another way. There is an 'advanced' EM which uses registration function:
> > em_dev_register_perf_domain(). It uses a local driver callback to get
> > power for each found frequency. It has benefit because there is no
> > restriction to 'fit' into the math formula, instead just avg power
> > values can be feed into EM. It's called 'advanced' EM [1].
> 
> It seems like there _should_ be a way to get the voltage out for GPU
> operating points, like is done with cpufreq in
> qcom_cpufreq_hw_read_lut(), but it might need someone with Qualcomm
> documentation to help with it. Maybe Rajendra would be able to help?
> Adding Jordon and Rob to this conversation in case they're aware of
> anything.
> 
> As you said, we could just list a power for each frequency, though.
> 
> I'm actually not sure which one would be more accurate across a range
> of devices with different "corners": specifying a dynamic power
> coefficient used for all "corners" and then using the actual voltage
> and doing the math, or specifying a power number for each frequency
> and ignoring the actual voltage used. In any case we're trying to get
> ballpark numbers and not every device will be exactly the same, so
> probably it doesn't matter that much.
> 
> 
> > Now we hit (again) the DT & EM issue (it's an old one, IIRC Morten
> > was proposing from ~2014 this upstream, but EAS wasn't merged back
> > then):
> > where to store these power-freq values, which are then used by the
> > callback. We have the 'dynamic-power-coefficient' in DT, but
> > it has limitations. It would be good to have this simple array
> > attached to the GPU/CPU node. IMHO it meet the requirement of DT,
> > it describes the HW (it would have HZ and Watts values).
> >
> > Doug, Matthias could you have a look at that function and its
> > usage, please [1]?
> > If you guys would support me in this, I would start, with an RFC
> > proposal, a discussion on LKML.
> >
> > [1]
> > https://elixir.bootlin.com/linux/v5.17-rc4/source/Documentation/power/energy-model.rst#L87
> 
> Matthias: I think you've spent more time on the thermal stuff than me
> so I'll assume you'll follow-up here. If not then please yell!
> 
> Ideally, though, someone from Qualcomm would jump in an own this.
> Basically it allows more intelligently throttling the GPU and CPU
> together in tandem instead of treating them separately IIUC, right?

Yes, I think for the em_dev_register_perf_domain() route support from
Qualcomm would be needed since "Drivers must provide a callback
function returning <frequency, power> tuples for each performance
state. ".