linux-kernel - Re: [PATCH] arm64: dts: qcom: sc7280: Adjust zombie PWM frequency

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Message-ID: <Y+VNW3ikQVgXK4VV@google.com>
Date:   Thu, 9 Feb 2023 19:45:31 +0000
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Owen Yang <ecs.taipeikernel@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Gavin Lee <gavin.lee@....com.tw>,
        Doug Anderson <dianders@...omium.org>,
        Harvey <hunge@...gle.com>, Abner Yen <abner.yen@....com.tw>,
        Stephen Boyd <swboyd@...omium.org>,
        Bob Moragues <moragues@...gle.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: qcom: sc7280: Adjust zombie PWM frequency

On Thu, Feb 09, 2023 at 10:28:16AM +0800, Owen Yang wrote:
> Tune the PWM to solve screen flashing issue and high frequency noise.
> 
> Signed-off-by: Owen Yang <ecs.taipeikernel@...il.com>
> ---
> 
>  arch/arm64/boot/dts/qcom/sc7280-herobrine-zombie.dtsi | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/boot/dts/qcom/sc7280-herobrine-zombie.dtsi b/arch/arm64/boot/dts/qcom/sc7280-herobrine-zombie.dtsi
> index 64deaaabac0f..000f64d27ab9 100644
> --- a/arch/arm64/boot/dts/qcom/sc7280-herobrine-zombie.dtsi
> +++ b/arch/arm64/boot/dts/qcom/sc7280-herobrine-zombie.dtsi
> @@ -61,8 +61,8 @@ ap_tp_i2c: &i2c0 {
>  };
>  
>  &pm8350c_pwm_backlight {
> -	/* Set the PWM period to 200 microseconds (5kHz duty cycle) */
> -	pwms = <&pm8350c_pwm 3 200000>;
> +	/* Set the PWM period to 320 microseconds (8kHz duty cycle) */
> +	pwms = <&pm8350c_pwm 3 320000>;

Is 'duty cycle' really correct here? The unit of the duty cycle is
percent, not kHz and it shouldn't be impacted by this change. I guess
you mean frequency.

And how does this change result in a frequency of 8kHz? Wouldn't it be
3.125 kHz with a peridod of 320us?