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

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Date:   Wed, 19 Jul 2023 10:43:31 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Bjorn Andersson <andersson@...nel.org>
Cc:     Konrad Dybcio <konrad.dybcio@...aro.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 03/15] clk: qcom: gcc-sm6375: Unregister critical clocks

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...

Johan