linux-kernel - Re: [PATCH RFT v2 01/14] dt-bindings: clock: qcom,rpmcc: Add a way to enable unused clock cleanup

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Date:   Fri, 17 Mar 2023 01:31:34 +0100
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        devicetree@...r.kernel.org
Subject: Re: [PATCH RFT v2 01/14] dt-bindings: clock: qcom,rpmcc: Add a way to
 enable unused clock cleanup



On 16.03.2023 23:58, Rob Herring wrote:
> On Wed, Mar 08, 2023 at 10:35:17PM +0100, Konrad Dybcio wrote:
>> Disabling RPMCC clocks can be a bit touchy. If we can't guarantee all
>> (or at least most) of the oneline peripherals ask the interconnect
>> framework to keep their buses online and guarantee enough bandwidth,
>> we're relying on bootloader defaults to keep the said buses alive through
>> RPM requests and rate setting on RPM clocks.
>>
>> Without that in place, the RPM clocks are never enabled in the CCF, which
>> qualifies them to be cleaned up, since - as far as Linux is concerned -
>> nobody's using them and they're just wasting power. Doing so will end
>> tragically, as within miliseconds we'll get *some* access attempt on an
>> unlocked bus which will cause a platform crash.
>>
>> On the other hand, if we want to save power and put well-supported
>> platforms to sleep, we should be shutting off at least some of these
>> clocks (this time with a clear distinction of which ones are *actually*
>> not in use, coming from the interconnect driver).
>>
>> To differentiate between these two cases while not breaking older DTs,
>> introduce an opt-in property to correctly mark RPM clocks as enabled
>> after handoff (the initial max freq vote) and hence qualify them for the
>> common unused clock cleanup.
>>
>> Signed-off-by: Konrad Dybcio <konrad.dybcio@...aro.org>
>> ---
>>  Documentation/devicetree/bindings/clock/qcom,rpmcc.yaml | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/clock/qcom,rpmcc.yaml b/Documentation/devicetree/bindings/clock/qcom,rpmcc.yaml
>> index 2a95bf8664f9..386153f61971 100644
>> --- a/Documentation/devicetree/bindings/clock/qcom,rpmcc.yaml
>> +++ b/Documentation/devicetree/bindings/clock/qcom,rpmcc.yaml
>> @@ -58,6 +58,12 @@ properties:
>>      minItems: 1
>>      maxItems: 2
>>  
>> +  qcom,clk-disable-unused:
>> +    type: boolean
>> +    description:
>> +      Indicates whether unused RPM clocks can be shut down with the common
>> +      unused clock cleanup. Requires a functional interconnect driver.
> 
> I don't think this should be QCom specific. Come up with something 
> common (which will probably have some debate). 
Generally the opposite (ignoring unused clocks during the cleanup) is
the thing you need to opt into.

I can however see how (especially with the focus on not breaking things
for older DTs) somebody else may also decide to only allow them to be
cleaned up conditionally (by marking the clocks that were enabled earlier
as enabled in Linux OR not addding clk.flags |= CLK_IGNORE_UNUSED) as we
do here.

Stephen, Rob, would `clk-disable-unused` be a fitting generic property
name for that? Should we also think about `clk-ignore-unused` as a
clock-controller-specific alternative to the CCF-wide clk_ignore_unused
cmdline?

Konrad


> 
> Rob