linux-kernel - Re: [PATCH] arm64: dts: sdm845: Add CPU topology

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Date:   Wed, 5 Jun 2019 18:20:52 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Amit Kucheria <amit.kucheria@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        David Brown <david.brown@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "open list:ARM/QUALCOMM SUPPORT" <linux-soc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Rajendra Nayak <rnayak@...eaurora.org>
Subject: Re: [PATCH] arm64: dts: sdm845: Add CPU topology

On Friday 17 May 2019 at 14:55:19 (-0700), Stephen Boyd wrote:
> Quoting Amit Kucheria (2019-05-16 04:54:45)
> > (cc'ing Andy's correct email address)
> > 
> > On Wed, May 15, 2019 at 2:46 AM Stephen Boyd <swboyd@...omium.org> wrote:
> > >
> > > Quoting Amit Kucheria (2019-05-13 04:54:12)
> > > > On Mon, May 13, 2019 at 4:31 PM Amit Kucheria <amit.kucheria@...aro.org> wrote:
> > > > >
> > > > > On Tue, Jan 15, 2019 at 12:13 AM Matthias Kaehlcke <mka@...omium.org> wrote:
> > > > > >
> > > > > > The 8 CPU cores of the SDM845 are organized in two clusters of 4 big
> > > > > > ("gold") and 4 little ("silver") cores. Add a cpu-map node to the DT
> > > > > > that describes this topology.
> > > > >
> > > > > This is partly true. There are two groups of gold and silver cores,
> > > > > but AFAICT they are in a single cluster, not two separate ones. SDM845
> > > > > is one of the early examples of ARM's Dynamiq architecture.
> > > > >
> > > > > > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> > > > >
> > > > > I noticed that this patch sneaked through for this merge window but
> > > > > perhaps we can whip up a quick fix for -rc2?
> > > > >
> > > >
> > > > And please find attached a patch to fix this up. Andy, since this
> > > > hasn't landed yet (can we still squash this into the original patch?),
> > > > I couldn't add a Fixes tag.
> > > >
> > >
> > > I had the same concern. Thanks for catching this. I suspect this must
> > > cause some problem for IPA given that it can't discern between the big
> > > and little "power clusters"?
> > 
> > Both EAS and IPA, I believe. It influences the scheduler's view of the
> > the topology.
> 
> And EAS and IPA are OK with the real topology? I'm just curious if
> changing the topology to reflect reality will be a problem for those
> two.

FWIW, neither EAS nor IPA depends on this. Not the upstream version of
EAS at least (which is used in recent Android kernels -- 4.19+).

But doing this is still required for other things in the scheduler (the
so-called 'capacity-awareness' code). So until we have a better
solution, this patch is doing the right thing.

I hope that helps.

Thanks,
Quentin