linux-kernel - Re: [PATCH v3 04/15] clk: qcom: gcc-sm6375: Add runtime PM

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Message-ID: <0cbdb9fb-9ad6-46c4-9d20-b0a766ab2629@linaro.org>
Date: Wed, 20 Dec 2023 13:26:55 +0100
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Johan Hovold <johan@...nel.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 v3 04/15] clk: qcom: gcc-sm6375: Add runtime PM

On 20.12.2023 10:26, Johan Hovold wrote:
> On Wed, Dec 20, 2023 at 01:30:45AM +0100, Konrad Dybcio wrote:
>> The GCC block on SM6375 is powered by the VDD_CX rail. We need to ensure
>> that CX is enabled to prevent unwanted power collapse 
> 
> As I pointed out earlier, this bit of the commit message is incorrect
> and misleading as the power domain will never be disabled until you
> enable runtime PM as part of this very patch:
> 
> 	https://lore.kernel.org/all/ZLaSpFFBzP_Yz5yY@hovoldconsulting.com/
> 
> Specifically, genpd will not power off CX (at runtime) while the driver
> is bound when runtime PM is left disabled.
OK I only now see what you really meant.

What this bit says is true, but it may be confusing within the context
of this patch.

The CX domain must be turned on [for the SoC to function], however this
patch does not solve the issue of it being powered down [like you've said
just binding the PD will keep it always-active for RPM-disabled devices].
It complements this process, by allowing it to shut down when unnecessary.

> 
>> and that the
>> reference is dropped when unused so that the system can enter a
>> firmware-managed lower power state.
>>
>> Enable runtime PM to keep the power flowing only when necessary.
> 
> The rest is correct.
Let me try to reword this and see if you like it:

The GCC block on SM6375 is powered by the VDD_CX rail. The Device Tree
description of this dependency lets Linux keep the rail online to prevent
power outages. It is however undesirable to keep it enabled at all times,
as that consumes additional power.

Moreover, failing to drop the "enabled" vote prevents firmware-managed,
SoC-wide power collapse in suspend, which leads to even more wasted power.

Enable runtime PM to keep the power flowing only when necessary.

Konrad