linux-kernel - Re: [PATCH 03/15] clk: qcom: gcc-sm6375: Unregister critical clocks

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Message-ID: <cc9e6464-5e47-4044-9785-c57167f0e1c5@linaro.org>
Date:   Wed, 9 Aug 2023 18:52:17 +0200
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Johan Hovold <johan@...nel.org>,
        Bjorn Andersson <andersson@...nel.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>,
        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 03/15] clk: qcom: gcc-sm6375: Unregister critical clocks

On 19.07.2023 10:43, Johan Hovold wrote:
> On Tue, Jul 18, 2023 at 09:23:52AM -0700, Bjorn Andersson wrote:
>> On Tue, Jul 18, 2023 at 03:26:51PM +0200, Konrad Dybcio wrote:
>>> On 18.07.2023 15:20, Johan Hovold wrote:
>>>> On Mon, Jul 17, 2023 at 05:19:10PM +0200, Konrad Dybcio wrote:
>>>>> Some clocks need to be always-on, but we don't really do anything
>>>>> with them, other than calling enable() once and telling Linux they're
>>>>> enabled.
>>>>>
>>>>> Unregister them to save a couple of bytes and, perhaps more
>>>>> importantly, allow for runtime suspend of the clock controller device,
>>>>> as CLK_IS_CRITICAL prevents the latter.
>>>>
>>>> But this doesn't sound right. How can you disable a controller which
>>>> still has clocks enabled?
>>>>
>>>> Shouldn't instead these clocks be modelled properly so that they are
>>>> only enabled when actually needed?
>>> Hm.. We do have clk_branch2_aon_ops, but something still needs to
>>> toggle these clocks.
>>>
>>
>> Before we started replacing these clocks with static votes, I handled
>> exactly this problem in the turingcc-qcs404 driver by registering the
>> ahb clock with a pm_clk_add(). The clock framework will then
>> automagically keep the clock enabled around operations, but it will also
>> keep the runtime state active as long as the clock is prepared.
>>
>> As mentioned in an earlier reply today, there's no similarity to this in
>> the reset or gdsc code, so we'd need to add that in order to rely on
>> such mechanism.
> 
> This reminds me of:
> 
> 	4cc47e8add63 ("clk: qcom: gdsc: Remove direct runtime PM calls")
> 
> which recently removed a broken attempt to implement this for gdscs.
> 
> Just stumbled over GENPD_FLAG_PM_CLK which may provide a way forward at
> least for genpd (but see below).
> 
>>> I *think* we could leave a permanent vote in probe() without breaking
>>> runtime pm! I'll give it a spin bit later..
>>>
>>
>> Modelling the AHB clock in DT and putting a devm_clk_get_enabled() would
>> properly connect the two, and thereby handle probe order between the two
>> clock controllers.
> 
> Yeah, this dependency really should be described eventually.
> 
>> But it would prevent the power-domain of the parent provider to ever
>> suspending. Using pm_clk_add() this would at least depend on client
>> votes.
> 
> IIUC using pm_clk_add() would also prevent the parent from suspending
> due to that resume call in clk_prepare().
> 
> And this mechanism is also used for GENPD_FLAG_PM_CLK...
So.. how do we go about solving the issue that this patch tried to
address?

Konrad