linux-kernel - Re: [PATCH v6 00/12] Unregister critical branch clocks + some RPM

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Date: Wed, 24 Jan 2024 13:31:06 +0100
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Abel Vesa <abel.vesa@...aro.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,
 Johan Hovold <johan+linaro@...nel.org>,
 Bryan O'Donoghue <bryan.odonoghue@...aro.org>
Subject: Re: [PATCH v6 00/12] Unregister critical branch clocks + some RPM

On 1/24/24 08:41, Abel Vesa wrote:
> On 24-01-13 15:50:49, Konrad Dybcio wrote:
>> On Qualcomm SoCs, certain branch clocks either need to be always-on, or
>> should be if you're interested in touching some part of the hardware.
>>
>> Using CLK_IS_CRITICAL for this purpose sounds like a genius idea,
>> however that messes with the runtime pm handling - if a clock is
>> marked as such, the clock controller device will never enter the
>> "suspended" state, leaving the associated resources online, which in
>> turn breaks SoC-wide suspend.
> 
> Generally speaking, HW-wise, if the power domain of a clock controller
> is being disabled, all clocks that it provides are being disabled.

Generally speaking, if that's the case, that's true.

> 
> Are you saying that is not the case ?

Dragons however, are peculiar creatures and it seems like the clock
controllers are not *really* disabled when we think they are,
e.g. due to RPM(h) owning a share of GCC clocks, or due to the
MX rail being always-on. It would indeed be an issue with
hibernation where the registers would need to be reprogrammed
after battery power is removed.

As we spoke off-list, I'll split this series into two: adding
common helpers and then taking care of 2290/6375/6115.

I'm not yet sure how far we can go with converting existing clock
drivers to use pm_clk_add so that the _AHB, _XO, and _SLEEP clocks
for a given subsystem are only enabled when necessary - if we
require an entry in clock-names, backwards compatibility goes away,
and if we don't - we potentially miss out on a devlink between X_CC
and GCC, plus the name needs to be hardcoded for global parent lookup.

For new drivers, we'll likely just require a clean solution (runtime
PM enabled + subsys clocks gotten as pm_clk through a dt entry on
the consumer).

Konrad