linux-kernel - Re: [PATCH 04/15] clk: qcom: gcc-sm6375: Add runtime PM

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Date:   Mon, 17 Jul 2023 18:26:35 +0200
From:   Stephan Gerhold <stephan@...hold.net>
To:     Konrad Dybcio <konrad.dybcio@...aro.org>
Cc:     Bjorn Andersson <andersson@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 04/15] clk: qcom: gcc-sm6375: Add runtime PM

On Mon, Jul 17, 2023 at 05:19:11PM +0200, Konrad Dybcio wrote:
> The GCC block on SM6375 is powered by the VDD_CX rail. We need to ensure
> that it's enabled to prevent unwanted power collapse.
> 
> Enable runtime PM to keep the power flowing only when necessary.
> 

Are you sure this is necessary? If VDD_CX was really possible to fully
"power collapse" then I would expect that you lose all register
settings. This is not something we want or can even handle for GCC.
You would need to restore all frequency settings, branch bits etc etc.

Otherwise it's a retention state, but these are covered by the
corners/levels, not the enable/disable state.

I think most of these power domains are effectively always-on. The
important part is voting for minimal corners/levels so they can go to
minimal retention state with vlow/vmin.

Thanks,
Stephan