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

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Message-ID: <CAHCN7x++XGXu6DsPaOc2EpAbxzy+RZbDq5A5-j5a4roGDhv4fQ@mail.gmail.com>
Date: Fri, 27 Jun 2025 13:39:48 -0500
From: Adam Ford <aford173@...il.com>
To: Tim Harvey <tharvey@...eworks.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 27, 2025 at 12:56 PM Tim Harvey <tharvey@...eworks.com> wrote:
>
> 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).

My 8MP board already appears to be running at 400MHz, but I did check.
The reference I found was in the 8MM TRM, under 10.3.3.5 Clock
generator, there is a note:

CLK is different for the SDR and DDR modes.
- In the SDR mode, CLK is equal to the internal working clock (card_clk).
- In the DDR mode, CLK is equal to card_clk/2.

>
> 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

One of my colleagues reported that the eMMC was running slower than he
expected, and I looked at the reference clock and noticed the 200MHz.
He asked if it needed to run 2x that since HS400 clocks on both edges.
I looked it up and found the note from above.  When I increased the
rate to 400MHz from 200MHz, the throughput doubled. I also noticed
some other boards, including the reference from NXP had the clock rate
set to 400MHz, so I don't think anything unreasonable.

adam

> 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