linux-kernel - Re: [PATCH 6/9] arm64: dts: imx8mn-bsh-smm-s2-common: Set minimum value for VDD_DRAM_VPU

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Message-ID: <7b23de30-06e3-4f02-a8a5-90791628ceed@gmail.com>
Date: Thu, 15 May 2025 12:19:35 +0300
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Dario Binacchi <dario.binacchi@...rulasolutions.com>,
 linux-kernel@...r.kernel.org
Cc: Simon Holesch <simon.holesch@...g.com>,
 Karthikdatt Anantharamrao <karthikdatt.anantharamrao@...bosch.com>,
 michael@...rulasolutions.com, linux-amarula@...rulasolutions.com,
 Wolfgang Birkner <wolfgang.birkner@...g.com>,
 Conor Dooley <conor+dt@...nel.org>, Fabio Estevam <festevam@...il.com>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Pengutronix Kernel Team <kernel@...gutronix.de>,
 Rob Herring <robh@...nel.org>, Sascha Hauer <s.hauer@...gutronix.de>,
 Shawn Guo <shawnguo@...nel.org>, devicetree@...r.kernel.org,
 imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 6/9] arm64: dts: imx8mn-bsh-smm-s2-common: Set minimum
 value for VDD_DRAM_VPU_GPU

On 14/05/2025 11:25, Dario Binacchi wrote:
> From: Wolfgang Birkner <wolfgang.birkner@...g.com>
> 
> Buck3 is called Buck5 in the BD71847 datasheet. This buck supports
> 0.55...1.35V. Set the minimum allowed value.

Setting this to PMIC's minimum value has little benefits because the 
voltage can't be set lower than it anyways.

AFICS, the idea of the regulator-min-microvolt and the 
regulator-max-microvolt is to protect a system which can't tolerate 
lower/higher than NNN voltage. So, basically, these properties are 
meaningful when PMIC can go higher / lower than the system design 
tolerates. In these cases the constrains set in device tree should be 
strictier than the PMIC's range.

The existing regulator-min-microvolt = <700000>; could have resulted 
from the knowledge that the minimum voltage devices connected to the 
BUCK3 (5 in data-sheet) can survive is 700 mV. Dropping this to 500mV 
just because PMIC can go there might be plain wrong, and in some case 
get the device(s) connected to BUCK3/5 upset..

So, please provide better (more) rationale for this change.

> 
> Signed-off-by: Wolfgang Birkner <wolfgang.birkner@...g.com>
> Signed-off-by: Dario Binacchi <dario.binacchi@...rulasolutions.com>
> ---
> 
>   arch/arm64/boot/dts/freescale/imx8mn-bsh-smm-s2-common.dtsi | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/boot/dts/freescale/imx8mn-bsh-smm-s2-common.dtsi b/arch/arm64/boot/dts/freescale/imx8mn-bsh-smm-s2-common.dtsi
> index fd12b97525d1..81fa0a8767e2 100644
> --- a/arch/arm64/boot/dts/freescale/imx8mn-bsh-smm-s2-common.dtsi
> +++ b/arch/arm64/boot/dts/freescale/imx8mn-bsh-smm-s2-common.dtsi
> @@ -135,7 +135,7 @@ buck2_reg: BUCK2 {
>   			buck3_reg: BUCK3 {
>   				/* PMIC_BUCK5 - VDD_DRAM_VPU_GPU */
>   				regulator-name = "buck3";
> -				regulator-min-microvolt = <700000>;
> +				regulator-min-microvolt = <550000>;
>   				regulator-max-microvolt = <1350000>;
>   				regulator-boot-on;
>   				regulator-always-on;

Yours,
	-- Matti