linux-kernel - Re: [PATCH v2 01/31] arm64: dts: qcom: msm8916: correct sleep clock frequency

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Message-ID: <al3tckby2jg7imhieehqwemygf6y5csfg7xfjta7alawhwfqv7@6gha46tr3rhx>
Date: Sat, 30 Nov 2024 12:42:24 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Stephan Gerhold <stephan@...hold.net>
Cc: Bjorn Andersson <andersson@...nel.org>, 
	Konrad Dybcio <konradybcio@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, Leo Yan <leo.yan@...ux.dev>, 
	Joseph Gates <jgates@...areup.com>, Georgi Djakov <djakov@...nel.org>, 
	Shawn Guo <shawn.guo@...aro.org>, Zac Crosby <zac@...areup.com>, 
	Bastian Köcher <git@...r.de>, Andy Gross <andy.gross@...aro.org>, 
	Jeremy McNicoll <jeremymc@...hat.com>, Rohit Agarwal <quic_rohiagar@...cinc.com>, 
	Melody Olvera <quic_molvera@...cinc.com>, Bhupesh Sharma <bhupesh.sharma@...aro.org>, 
	cros-qcom-dts-watchers@...omium.org, Stephen Boyd <swboyd@...omium.org>, 
	Rajendra Nayak <quic_rjendra@...cinc.com>, Martin Botka <martin.botka@...ainline.org>, 
	Jonathan Marek <jonathan@...ek.ca>, Vinod Koul <vkoul@...nel.org>, 
	Tengfei Fan <quic_tengfan@...cinc.com>, Fenglin Wu <quic_fenglinw@...cinc.com>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Abel Vesa <abel.vesa@...aro.org>, 
	Alexandru Marc Serdeliuc <serdeliuk@...oo.com>, Vladimir Zapolskiy <vladimir.zapolskiy@...aro.org>, 
	Sibi Sankar <quic_sibis@...cinc.com>, Bryan O'Donoghue <bryan.odonoghue@...aro.org>, 
	Jun Nie <jun.nie@...aro.org>, James Willcox <jwillcox@...areup.com>, 
	Max Chen <mchen@...areup.com>, Vincent Knecht <vincent.knecht@...loo.org>, 
	Benjamin Li <benl@...areup.com>, Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>, 
	linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Stephan Gerhold <stephan.gerhold@...aro.org>
Subject: Re: [PATCH v2 01/31] arm64: dts: qcom: msm8916: correct sleep clock
 frequency

On Sat, Nov 30, 2024 at 11:21:56AM +0100, Stephan Gerhold wrote:
> On Sat, Nov 30, 2024 at 03:44:13AM +0200, Dmitry Baryshkov wrote:
> > The MSM8916 platform uses PM8916 to provide sleep clock. According to the
> > documentation, that clock has 32.7645 kHz frequency. Correct the sleep
> > clock definition.
> > 
> > Fixes: f4fb6aeafaaa ("arm64: dts: qcom: msm8916: Add fixed rate on-board oscillators")
> > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> 
> Thanks for spotting this! This fix looks good independent of the more
> controversial "arm64: dts: qcom: move board clocks to SoC DTSI files"
> changes. Maybe move these to a separate series?
> 
> > ---
> > arch/arm64/boot/dts/qcom/msm8916.dtsi | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/boot/dts/qcom/msm8916.dtsi b/arch/arm64/boot/dts/qcom/msm8916.dtsi
> > index 5e558bcc9d87893486352e5e211f131d4a1f67e5..8f35c9af18782aa1da7089988692e6588c4b7c5d 100644
> > --- a/arch/arm64/boot/dts/qcom/msm8916.dtsi
> > +++ b/arch/arm64/boot/dts/qcom/msm8916.dtsi
> > @@ -125,7 +125,7 @@ xo_board: xo-board {
> > 		sleep_clk: sleep-clk {
> > 			compatible = "fixed-clock";
> > 			#clock-cells = <0>;
> > -			clock-frequency = <32768>;
> > +			clock-frequency = <32764>;
> 
> To be precise the PM8916 specification says the sleep clock is "The 19.2
> MHz XO divided by 586". Maybe we can actually describe it that way with
> a fixed-factor-clock?
> 
> 		sleep_clk: sleep-clk {
> 			compatible = "fixed-factor-clock";
> 			clocks = <&xo_board>;
> 			#clock-cells = <0>;
> 			clock-div = <586>;
> 			clock-mult = <1>;
> 		};

I thought about it, but then it's also not complete truth (at least for
some of PMICs, don't remember if that's the case for PM8916): there is
an external XO and also there is an on-PMIC RC, which is further
divided with PMIC actually selecting which source to use as a source for
sleep_clk.

> 
> If we keep the fixed-clock with the hardcoded frequency I wonder if we
> should put 32765 instead of 32764. If you calculate it exactly it's
> slightly closer to 32765 than 32764. :-)
> 
> 	19200000/586 = 32764.505119453926 = ~32765

Well, I think according to the most typical rounding rules it is 32764.

> 
> Thanks,
> Stephan

-- 
With best wishes
Dmitry