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

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Date:   Tue, 10 Aug 2021 13:35:39 +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 Tuesday 10 Aug 2021 at 13:06:47 (+0530), Viresh Kumar wrote:
> Provide a cpufreq driver flag so drivers can ask the cpufreq core to register
> with the EM core on their behalf.

Hmm, that's not quite what this does. This asks the cpufreq core to
use *PM_OPP* to register an EM, which I think is kinda wrong to do from
there IMO. The decision to use PM_OPP or another mechanism to register
an EM belongs to platform specific code (drivers), so it is odd for the
PM_OPP registration to have its own cpufreq flag but not the other ways.

As mentioned in another thread, the very reason to have PM_EM is to not
depend on PM_OPP, so I'm worried about the direction of travel with this
series TBH.

> This allows us to get rid of duplicated code
> in the drivers and fix the unregistration part as well, which none of the
> drivers have done until now.

This series adds more code than it removes, and the unregistration is
not a fix as we don't ever remove the EM tables by design, so not sure
either of these points are valid arguments.

> This would also make the registration with EM core to happen only after policy
> is fully initialized, and the EM core can do other stuff from in there, like
> marking frequencies as inefficient (WIP). Though this patchset is useful without
> that work being done and should be merged nevertheless.
> 
> This doesn't update scmi cpufreq driver for now as it is a special case and need
> to be handled differently. Though we can make it work with this if required.

Note that we'll have more 'special cases' if other architectures start
using PM_EM, which is what we have been trying to allow since the
beginning, so that's worth keeping in mind.

Thanks,
Quentin