linux-kernel - Re: [PATCH] arm64: dts: arm: juno: Fix UART frequency

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Date:   Tue, 19 Nov 2019 12:16:00 +0000
From:   Liviu Dudau <liviu.dudau@....com>
To:     Andre Przywara <andre.przywara@....com>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: arm: juno: Fix UART frequency

On Tue, Nov 19, 2019 at 12:03:31PM +0000, Andre Przywara wrote:
> Older versions of the Juno *SoC* TRM [1] recommended that the UART clock
> source should be 7.2738 MHz, whereas the *system* TRM [2] stated a more
> correct value of 7.3728 MHz. Somehow the wrong value managed to end up in
> our DT.
> Doing a prime factorisation, a modulo divide by 115200 and trying
> to buy a 7.2738 MHz crystal at your favourite electronics dealer suggest
> that the old value was actually a typo. The actual UART clock is driven
> by a PLL, configured via a parameter in some board.txt file in the
> firmware, which reads 7.37 MHz (sic!).
> 
> Fix this to correct the baud rate divisor calculation on the Juno board.
> 
> Signed-off-by: Andre Przywara <andre.przywara@....com>
> 
> [1] http://infocenter.arm.com/help/topic/com.arm.doc.ddi0515b.b/DDI0515B_b_juno_arm_development_platform_soc_trm.pdf
> [2] http://infocenter.arm.com/help/topic/com.arm.doc.100113_0000_07_en/arm_versatile_express_juno_development_platform_(v2m_juno)_technical_reference_manual_100113_0000_07_en.pdf

Acked-by: Liviu Dudau <liviu.dudau@....com>

Best regards,
Liviu

> ---
>  arch/arm64/boot/dts/arm/juno-clocks.dtsi | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/boot/dts/arm/juno-clocks.dtsi b/arch/arm64/boot/dts/arm/juno-clocks.dtsi
> index e5e265dfa902..2870b5eeb198 100644
> --- a/arch/arm64/boot/dts/arm/juno-clocks.dtsi
> +++ b/arch/arm64/boot/dts/arm/juno-clocks.dtsi
> @@ -8,10 +8,10 @@
>   */
>  / {
>  	/* SoC fixed clocks */
> -	soc_uartclk: refclk7273800hz {
> +	soc_uartclk: refclk7372800hz {
>  		compatible = "fixed-clock";
>  		#clock-cells = <0>;
> -		clock-frequency = <7273800>;
> +		clock-frequency = <7372800>;
>  		clock-output-names = "juno:uartclk";
>  	};
>  
> -- 
> 2.17.1
> 

-- 
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