linux-kernel - Re: [PATCH 1/6] arm64: dts: ti: k3-am65: Remove UART baud rate selection

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Date: Tue, 26 Mar 2024 15:21:05 -0500
From: Nishanth Menon <nm@...com>
To: Andrew Davis <afd@...com>
CC: Vignesh Raghavendra <vigneshr@...com>, Tero Kristo <kristo@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski
	<krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] arm64: dts: ti: k3-am65: Remove UART baud rate
 selection

On 13:54-20240326, Andrew Davis wrote:
> As described in the binding document for the "current-speed" property:
> 
> "This should only be present in case a driver has no chance to know the
> baud rate of the slave device."
> 
> This is not the case for the UART used in K3 devices, the current
> baud-rate can be calculated from the registers. Having this property

I do not understand the explanation above -> how does one do this?
If you are talking of the 8250 divider registers, someone has to program
those - how do you compute the baud rate from registers that aren't
programmed? Note: I am not commenting on the rationale of removing the
property, just trying to understand the assertion above.

> has the effect of actually skipping the baud-rate setup in some drivers
> as it assumes it will already be set to this rate, which may not always
> be the case.
> 
> It seems this property's purpose was mistaken as selecting the desired
> baud-rate, which it does not. It would have been wrong to select that
> here anyway as DT is not the place for configuration, especially when
> there are already more standard ways to set serial baud-rates.
> 
> Signed-off-by: Andrew Davis <afd@...com>
> ---
>  arch/arm64/boot/dts/ti/k3-am65-main.dtsi   | 1 -
>  arch/arm64/boot/dts/ti/k3-am65-mcu.dtsi    | 1 -
>  arch/arm64/boot/dts/ti/k3-am65-wakeup.dtsi | 1 -
>  3 files changed, 3 deletions(-)
> 
> diff --git a/arch/arm64/boot/dts/ti/k3-am65-main.dtsi b/arch/arm64/boot/dts/ti/k3-am65-main.dtsi
> index ff857117d7193..670557c89f756 100644
> --- a/arch/arm64/boot/dts/ti/k3-am65-main.dtsi
> +++ b/arch/arm64/boot/dts/ti/k3-am65-main.dtsi
> @@ -89,7 +89,6 @@ main_uart0: serial@...0000 {
>  		reg = <0x00 0x02800000 0x00 0x100>;
>  		interrupts = <GIC_SPI 192 IRQ_TYPE_LEVEL_HIGH>;
>  		clock-frequency = <48000000>;
> -		current-speed = <115200>;
>  		power-domains = <&k3_pds 146 TI_SCI_PD_EXCLUSIVE>;
>  		status = "disabled";
>  	};
> diff --git a/arch/arm64/boot/dts/ti/k3-am65-mcu.dtsi b/arch/arm64/boot/dts/ti/k3-am65-mcu.dtsi
> index 6ff3ccc39fb44..4f808e5089755 100644
> --- a/arch/arm64/boot/dts/ti/k3-am65-mcu.dtsi
> +++ b/arch/arm64/boot/dts/ti/k3-am65-mcu.dtsi
> @@ -43,7 +43,6 @@ mcu_uart0: serial@...00000 {
>  		reg = <0x00 0x40a00000 0x00 0x100>;
>  		interrupts = <GIC_SPI 565 IRQ_TYPE_LEVEL_HIGH>;
>  		clock-frequency = <96000000>;
> -		current-speed = <115200>;
>  		power-domains = <&k3_pds 149 TI_SCI_PD_EXCLUSIVE>;
>  		status = "disabled";
>  	};
> diff --git a/arch/arm64/boot/dts/ti/k3-am65-wakeup.dtsi b/arch/arm64/boot/dts/ti/k3-am65-wakeup.dtsi
> index 37527890ddeaf..eee072e44a42f 100644
> --- a/arch/arm64/boot/dts/ti/k3-am65-wakeup.dtsi
> +++ b/arch/arm64/boot/dts/ti/k3-am65-wakeup.dtsi
> @@ -59,7 +59,6 @@ wkup_uart0: serial@...00000 {
>  		reg = <0x42300000 0x100>;
>  		interrupts = <GIC_SPI 697 IRQ_TYPE_LEVEL_HIGH>;
>  		clock-frequency = <48000000>;
> -		current-speed = <115200>;
>  		power-domains = <&k3_pds 150 TI_SCI_PD_EXCLUSIVE>;
>  		status = "disabled";
>  	};
> -- 
> 2.39.2
> 

-- 
Regards,
Nishanth Menon
Key (0xDDB5849D1736249D) / Fingerprint: F8A2 8693 54EB 8232 17A3  1A34 DDB5 849D 1736 249D