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Date:   Tue, 10 Oct 2023 23:21:39 +0200
From:   Stephan Gerhold <stephan@gerhold.net>
To:     Konrad Dybcio <konrad.dybcio@linaro.org>
Cc:     Stephan Gerhold <stephan.gerhold@kernkonzept.com>,
        Bjorn Andersson <andersson@kernel.org>,
        Andy Gross <agross@kernel.org>,
        Michael Turquette <mturquette@baylibre.com>,
        Stephen Boyd <sboyd@kernel.org>, linux-arm-msm@vger.kernel.org,
        linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] clk: qcom: smd: Disable unused clocks
Message-ID: <ZSXAY5mbXB7Gbz2x@gerhold.net>
References: <20231004-clk-qcom-smd-rpm-unused-v2-1-9a5281f324dc@kernkonzept.com>
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On Tue, Oct 10, 2023 at 10:45:15PM +0200, Konrad Dybcio wrote:
> On 10/9/23 22:15, Stephan Gerhold wrote:
> > On Fri, Oct 06, 2023 at 11:08:39PM +0200, Konrad Dybcio wrote:
> > > On 4.10.2023 14:10, Stephan Gerhold wrote:
> > > > At the moment, clk-smd-rpm forces all clocks on at probe time (for
> > > > "handoff"). However, it does not make the clk core aware of that.
> > > > 
> > > > This means that the clocks stay enabled forever if they are not used
> > > > by anything. We can easily disable them again after bootup has been
> > > > completed, by making the clk core aware of the state. This is
> > > > implemented by returning the current state of the clock in
> > > > is_prepared().
> > > > 
> > > > Checking the SPMI clock registers reveals that this allows the RPM to
> > > > disable unused BB/RF clocks. This reduces the power consumption quite
> > > > significantly and is also needed to allow entering low-power states.
> > > > 
> > > > As of commit d6edc31f3a68 ("clk: qcom: smd-rpm: Separate out
> > > > interconnect bus clocks") the interconnect-related clocks are no longer
> > > > managed/exposed by clk-smd-rpm. Also the BI_TCXO_AO clock is now
> > > > critical (and never disabled).
> > > > 
> > > > There is still a slight chance that this change will break boot on some
> > > > devices. However, this will be most likely caused by actual mistakes in
> > > > the device tree (where required clocks were not actually specified).
> > > Precisely this, and solely as a consequence of the interconnect driver
> > > not covering all the required clocks (usually named GCC_SOME_NOC_XYZ_CLK,
> > > but there's quite a lot more).
> > > 
> > > For platforms without an interconnect driver, breaking stuff this **MOST
> > > LIKELY** means that Linux uses some hw that isn't voted for (e.g. missing
> > > crypto clock under scm or something).
> > > 
> > > For those with an interconnect driver, this will uncover issues that were
> > > previously hidden because of the smd-rpm interconnect being essentially
> > > broken for most of its existence. I can smell 660 breaking from however
> > > many miles you are away from me, but it's "good", as we were relying on
> > > (board specific) magic..
> > > 
> > > I've been carrying an equivalent patch in my tree for over half a year now
> > > and IIRC 8996 was mostly fine. It's also a good idea to test suspend
> > > (echo mem > /sys/power/state) and wakeup.
> > > 
> > 
> > I didn't notice any problems on 8916 and 8909 either. :-)
> > 
> > > For reasons that I don't fully recall, I do have both .is_prepared and
> > > .is_enabled though..
> > > 
> > 
> > clk-smd-rpm doesn't have any .enable()/.disable() ops (only .prepare()
> > and .unprepare()) so I don't think is_enabled is needed. For the unused
> > clock cleanup in drivers/clk/clk.c (clk_disable_unused()) we just care
> > about the clk_unprepare_unused_subtree() part. That part is run when the
> > clock reports true in .is_prepared(). The equivalent for .is_enabled()
> > would just be a no-op because there are no .enable()/.disable() ops.
> Oh I found out why :D
> 
> """
> The RPM clock enabling state can be found with 'enabled' in struct
> clk_smd_rpm. Add .is_enabled hook so that clk_summary in debugfs
> can a correct enabling state for RPM clocks.
> """
> 

I see, thanks! I think you should see at least the "prepared" state with
this patch. I'm not entirely convinced we should implement .is_enabled()
if we don't actually do anything on .enable()/.disable().

Anyway, given that the debugfs state is not directly related to the main
objective of disabling unused clocks I think that would be better
discussed in a separate patch later. :)

Thanks,
Stephan