linux-kernel - Re: [PATCH 1/2] arm64: dts: imx8mm-beacon: Fix HS400 USDHC clock speed

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Message-ID: <CAJ+vNU0caeeC6in5dO_jkkbYNAnTL7drBZcmNBsstbrPWqUkHw@mail.gmail.com>
Date: Fri, 27 Jun 2025 10:56:46 -0700
From: Tim Harvey <tharvey@...eworks.com>
To: Adam Ford <aford173@...il.com>
Cc: linux-arm-kernel@...ts.infradead.org, aford@...conembedded.com, 
	Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Shawn Guo <shawnguo@...nel.org>, Sascha Hauer <s.hauer@...gutronix.de>, 
	Pengutronix Kernel Team <kernel@...gutronix.de>, Fabio Estevam <festevam@...il.com>, devicetree@...r.kernel.org, 
	imx@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64: dts: imx8mm-beacon: Fix HS400 USDHC clock speed

On Fri, Jun 20, 2025 at 2:52 PM Adam Ford <aford173@...il.com> wrote:
>
> The reference manual for the i.MX8MM states the clock rate in
> MMC mode is 1/2 of the input clock, therefore to properly run
> at HS400 rates, the input clock must be 400MHz to operate at
> 200MHz.  Currently the clock is set to 200MHz which is half the
> rate it should be, so the throughput is half of what it should be
> for HS400 operation.
>
> Fixes: 593816fa2f35 ("arm64: dts: imx: Add Beacon i.MX8m-Mini development kit")
> Signed-off-by: Adam Ford <aford173@...il.com>
>
> diff --git a/arch/arm64/boot/dts/freescale/imx8mm-beacon-som.dtsi b/arch/arm64/boot/dts/freescale/imx8mm-beacon-som.dtsi
> index 21bcd82fd092..8287a7f66ed3 100644
> --- a/arch/arm64/boot/dts/freescale/imx8mm-beacon-som.dtsi
> +++ b/arch/arm64/boot/dts/freescale/imx8mm-beacon-som.dtsi
> @@ -294,6 +294,8 @@ &usdhc3 {
>         pinctrl-0 = <&pinctrl_usdhc3>;
>         pinctrl-1 = <&pinctrl_usdhc3_100mhz>;
>         pinctrl-2 = <&pinctrl_usdhc3_200mhz>;
> +       assigned-clocks = <&clk IMX8MM_CLK_USDHC3>;
> +       assigned-clock-rates = <400000000>;
>         bus-width = <8>;
>         non-removable;
>         status = "okay";
> --
> 2.48.1
>
>

Hi Adam,

This caught my interest. Where in the IMX8MMRM do you see this and
would it also apply to the IMX8MP? (You've patched your IMX8MM and
IMX8MN boards).

Have you encountered any issues when running eMMC at HS400 due to this
or is it just something you noticed in the RM more recently like with
my recent patch that lowers SPI clock due to an obscure RM note [1]

Best Regards,

Tim
[1] https://patchwork.kernel.org/project/linux-arm-kernel/list/?series=965976&archive=both