linux-kernel - Re: [PATCH 1/2] arm64: dts: qcom: sc7180-lite: Tweak DDR/L3 scaling on SC7180-lite

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Date:   Fri, 16 Oct 2020 15:59:50 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sibi Sankar <sibis@...eaurora.org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Andy Gross <agross@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Evan Green <evgreen@...omium.org>,
        Rob Herring <robh+dt@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>
Subject: Re: [PATCH 1/2] arm64: dts: qcom: sc7180-lite: Tweak DDR/L3 scaling
 on SC7180-lite

Hi,

On Thu, Oct 15, 2020 at 10:53 AM Sibi Sankar <sibis@...eaurora.org> wrote:
>
> Tweak the DDR/L3 bandwidth votes on the lite variant of the SC7180 SoC
> since the gold cores only support frequencies upto 2.1 GHz.
>
> Signed-off-by: Sibi Sankar <sibis@...eaurora.org>
> ---
>  arch/arm64/boot/dts/qcom/sc7180-lite.dtsi | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>  create mode 100644 arch/arm64/boot/dts/qcom/sc7180-lite.dtsi
>
> diff --git a/arch/arm64/boot/dts/qcom/sc7180-lite.dtsi b/arch/arm64/boot/dts/qcom/sc7180-lite.dtsi
> new file mode 100644
> index 000000000000..cff50275cfe1
> --- /dev/null
> +++ b/arch/arm64/boot/dts/qcom/sc7180-lite.dtsi
> @@ -0,0 +1,14 @@
> +// SPDX-License-Identifier: BSD-3-Clause
> +/*
> + * SC7180 lite device tree source
> + *
> + * Copyright (c) 2020, The Linux Foundation. All rights reserved.
> + */
> +
> +&cpu6_opp11 {
> +       opp-peak-kBps = <8532000 22425600>;
> +};
> +
> +&cpu6_opp12 {
> +       opp-peak-kBps = <8532000 23347200>;
> +};

I guess this is OK, but something about it smells just a little
strange...  I guess:

a) There's suddenly a big jump from opp10 to opp11.  You don't use
7216000 at all anymore.

b) The fact that we need to do this at all feels like a sign that
somehow this wasn't designed quite right.

Just brainstorming a bit: If the higher memory rate wasn't useful for
OPP11/12 on the non-lite version of the chip, why are they useful for
that OPP on the lite version?  I guess you're just trying to eek out
the last little bits of performance once the cpufreq is maxed out?  It
almost feels like a better way to do this (though it wouldn't be
monotonically increasing anymore so it wouldn't actually work) would
be to have a few "OPP" points at the top where the cpufreq stops
increasing and all you do is increase the memory frequency.

c) In theory we're supposed to be able to probe whether we're on the
normal, lite, or pro version, right?  Anyway we could tweak this in
code so we don't have to know to include the right dtsi file?

-Doug