linux-kernel - Re: [PATCH RFT v2 01/14] dt-bindings: clock: qcom,rpmcc: Add a way to enable unused clock cleanup

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Message-ID: <20230322032302.objqdzryhq2mbljf@ripper>
Date:   Tue, 21 Mar 2023 20:23:02 -0700
From:   Bjorn Andersson <andersson@...nel.org>
To:     Konrad Dybcio <konrad.dybcio@...aro.org>
Cc:     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>,
        linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        devicetree@...r.kernel.org
Subject: Re: [PATCH RFT v2 01/14] dt-bindings: clock: qcom,rpmcc: Add a way
 to enable unused clock cleanup

On Wed, Mar 08, 2023 at 10:35:17PM +0100, Konrad Dybcio wrote:
> Disabling RPMCC clocks can be a bit touchy. If we can't guarantee all
> (or at least most) of the oneline peripherals ask the interconnect
> framework to keep their buses online and guarantee enough bandwidth,
> we're relying on bootloader defaults to keep the said buses alive through
> RPM requests and rate setting on RPM clocks.
> 
> Without that in place, the RPM clocks are never enabled in the CCF, which
> qualifies them to be cleaned up, since - as far as Linux is concerned -
> nobody's using them and they're just wasting power. Doing so will end
> tragically, as within miliseconds we'll get *some* access attempt on an
> unlocked bus which will cause a platform crash.
> 
> On the other hand, if we want to save power and put well-supported
> platforms to sleep, we should be shutting off at least some of these
> clocks (this time with a clear distinction of which ones are *actually*
> not in use, coming from the interconnect driver).
> 
> To differentiate between these two cases while not breaking older DTs,
> introduce an opt-in property to correctly mark RPM clocks as enabled
> after handoff (the initial max freq vote) and hence qualify them for the
> common unused clock cleanup.
> 
> Signed-off-by: Konrad Dybcio <konrad.dybcio@...aro.org>
> ---
>  Documentation/devicetree/bindings/clock/qcom,rpmcc.yaml | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/clock/qcom,rpmcc.yaml b/Documentation/devicetree/bindings/clock/qcom,rpmcc.yaml
> index 2a95bf8664f9..386153f61971 100644
> --- a/Documentation/devicetree/bindings/clock/qcom,rpmcc.yaml
> +++ b/Documentation/devicetree/bindings/clock/qcom,rpmcc.yaml
> @@ -58,6 +58,12 @@ properties:
>      minItems: 1
>      maxItems: 2
>  
> +  qcom,clk-disable-unused:

This is a Linux implementation detail, not a description of the
hardware. So it unfortunately doesn't belong here.

> +    type: boolean
> +    description:
> +      Indicates whether unused RPM clocks can be shut down with the common
> +      unused clock cleanup. Requires a functional interconnect driver.

This is an interesting aspect though, there's a lot of things that will
break if any one of these building blocks are missing, for any reason.

Regards,
Bjorn

> +
>  required:
>    - compatible
>    - '#clock-cells'
> 
> -- 
> 2.39.2
>