linux-kernel - Re: [PATCH 0/2]cpufreq,topology,arm: disable FI for BL

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Date:   Thu, 8 Oct 2020 15:05:58 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Ionela Voinescu <ionela.voinescu@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2]cpufreq,topology,arm: disable FI for BL_SWITCHER

On Wed, Oct 07, 2020 at 04:34:44PM +0200, Rafael J. Wysocki wrote:
> On Thu, Sep 24, 2020 at 2:30 PM Ionela Voinescu <ionela.voinescu@....com> wrote:
> >
> > This series is the result of the discussions ([1], [2]) around the
> > complications that the BL_SWITCHER poses when it comes to Frequency
> > Invariance (FI) and it aims to restart the discussions.
> >
> > To properly scale its per-entity load-tracking signals, the task
> > scheduler needs to be given a frequency scale factor, i.e. some image of
> > the current frequency the CPU is running at, relative to its maximum
> > frequency.
> >
> > But (reiterating the message in the changelog of patch 2/2), big.LITTLE
> > switching complicates the setting of a correct cpufreq-based frequency
> > invariance scale factor due to (as observed in
> > drivers/cpufreq/vexpress-spc-cpufreq.c):
> >  - Incorrect current and maximum frequencies as a result of the
> >    exposure of a virtual frequency table to the cpufreq core,
> >  - Missed updates as a result of asynchronous frequency adjustments
> >    caused by frequency changes in other CPU pairs.
> > More information on this feature can be found at [3].
> >
> > Given that its functionality is atypical in regards to FI and that this
> > is an old technology, patch 2/2 disable FI for when big.LITTLE switching
> > is configured in to prevent incorrect scale setting.
> >
> > For this purpose patch 1/2 changes the way arch_set_freq_scale() is
> > defined in architecture code which brings it in line with the logic of
> > other architectural function definitions while allowing for less invasive
> > filtering of FI support.
> >
> > In the discussions at [2], three possible solutions were suggested:
> >  - (1) conditioning FI by !CONFIG_BL_SWITCHER
> >  - (2) leave as is with note in driver specifying this FI broken
> >    functionality
> >  - (3) removing full BL_SWITCHER support
> >
> > This series restructures the solution at (1). The reason for it is that
> > the new patch limits the ifdef filtering to the arm topology include file,
> > a location where frequency invariance functions are defined. Therefore,
> > this seems more appropriate given that the b.L switcher is an arm
> > technology and that the new FI filtering location seems more natural for
> > conditioned FI disabling.
> >
> > Solutions (2) and (3) were not implemented given that there might be some
> > remaining users of this technology (Samsung Chromebook 2 - Samsung Exynos
> > 5 Octa 5420, Samsung Exynos 5 Octa 5800) and therefore leaving this
> > broken (2) seems equally bad to removing support for it (3).
> >
> > [1] https://lore.kernel.org/lkml/20200701090751.7543-5-ionela.voinescu@arm.com/
> > [2] https://lore.kernel.org/lkml/20200722093732.14297-4-ionela.voinescu@arm.com/
> > [3] https://lwn.net/Articles/481055/
> 
> I can take this set with the ACKs from Viresh if that's fine by
> everyone.  Catalin?  Sudeep?

Acked-by: Sudeep Holla <sudeep.holla@....com> (BL_SWITCHER and topology parts)

-- 
Regards,
Sudeep