linux-kernel - Re: [PATCH 0/8] cpufreq: Auto-register with energy model

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Date:   Wed, 11 Aug 2021 10:34:09 +0100
From:   Quentin Perret <qperret@...gle.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Vincent Donnefort <vincent.donnefort@....com>,
        lukasz.luba@....com, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Cristian Marussi <cristian.marussi@....com>,
        Fabio Estevam <festevam@...il.com>,
        Kevin Hilman <khilman@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, linux-omap@...r.kernel.org
Subject: Re: [PATCH 0/8] cpufreq: Auto-register with energy model

On Wednesday 11 Aug 2021 at 14:43:21 (+0530), Viresh Kumar wrote:
> On 11-08-21, 09:37, Quentin Perret wrote:
> > On Wednesday 11 Aug 2021 at 10:48:59 (+0530), Viresh Kumar wrote:
> > > I had to use the pm-opp version, since almost everyone was using that.
> > > 
> > > On the other hand, there isn't a lot of OPP specific stuff in
> > > dev_pm_opp_of_register_em(). It just uses dev_pm_opp_get_opp_count(),
> > > that's all. This ended up in the OPP core, nothing else. Maybe we can
> > > now move it back to the EM core and name it differently ?
> > 
> > Well it also uses dev_pm_opp_find_freq_ceil() and
> > dev_pm_opp_get_voltage(), so not sure how easy it will be to move, but
> > if it is possible no objection from me.
> 
> What uses these routines ? dev_pm_opp_of_register_em() ? I am not able
> to see that at least :(

Yep, it's not immediately obvious, but see how it sets the struct
em_data_callback to point at _get_power() where the actual energy
calculation is done. So strictly speaking _get_power() is what uses
these routines, but it goes in hand with dev_pm_opp_of_register_em() so
I guess the same reasoning applies.

> > Right but the EM is a description of the hardware, so it seemed fair
> > to assume this wouldn't change across the lifetime of the OS, similar
> > to the DT which we can't reload at run-time. Yes it can be a little odd
> > if you load/unload your driver module, but note that you generally can't
> > load two completely different drivers on a single system. You'll just
> > load the same one again and the hardware hasn't changed in the meantime,
> > so the previously loaded EM will still be correct.
> 
> Yeah, it will be the same driver but a different version of it, which
> may have updated the freq table. For me the EM is attached to the
> freq-table, and the freq-table is not available anymore after the
> driver is gone.
> 
> Anyway, I will leave that for you guys to decide :)

IIUC Lukasz is working on something that should allow changing the EM at
run-time, so hopefully it'll enable this use-case as well, but we'll see :)