topic Re: questions about uart node in different imx8mp device trees in i.MX Processors

questions about uart node in different imx8mp device trees

machangbao — Fri, 07 Nov 2025 09:35:53 GMT

In https://github.com/nxp-imx/linux-imx/blob/lf-6.12.y/arch/arm64/boot/dts/freescale/imx8mp-evk.dts
&uart1 { /* BT */
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_uart1>;
assigned-clocks = <&clk IMX8MP_CLK_UART1>;
assigned-clock-parents = <&clk IMX8MP_SYS_PLL1_80M>;
uart-has-rtscts;
status = "okay";

bluetooth {
compatible = "nxp,88w8997-bt";
};
};

&uart3 {
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_uart3>;
assigned-clocks = <&clk IMX8MP_CLK_UART3>;
assigned-clock-parents = <&clk IMX8MP_SYS_PLL1_80M>;
uart-has-rtscts;
status = "okay";
};

In https://github.com/nxp-imx/linux-imx/blob/lf-6.12.y/arch/arm64/boot/dts/freescale/imx8mp-venice-gw74xx.dts

/* GPS / off-board header */
&uart1 {
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_uart1>;
status = "okay";
};

/* bluetooth HCI */
&uart3 {
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_uart3>, <&pinctrl_uart3_gpio>;
cts-gpios = <&gpio3 21 GPIO_ACTIVE_LOW>;
rts-gpios = <&gpio3 22 GPIO_ACTIVE_LOW>;
status = "okay";

bluetooth {
compatible = "brcm,bcm4330-bt";
shutdown-gpios = <&gpio3 8 GPIO_ACTIVE_HIGH>;
};
};

Two questions:

1. are propertys assigned-clocks & assigned-clock-parents necessary? What would happen if it's lacking?

2. What are the differences between the two hardware flow control methods uart-has-rtscts and uart-has-rtscts &cts-gpios&rts-gpios?

Re: questions about uart node in different imx8mp device trees

Rita_Wang — Mon, 17 Nov 2025 06:06:22 GMT

Q1:

1. are propertys assigned-clocks & assigned-clock-parents necessary? What would happen if it's lacking?

A:Yes, necessary. These properties are part of the Common Clock Framework in Linux device trees. They are used to configure the clock source and parent for a peripheral at boot time.

If lacking, Incorrect baud rate if the default clock frequency doesn’t match what the driver expects.Potential instability if the clock source is not optimal for low-power or performance modes.

2. What are the differences between the two hardware flow control methods uart-has-rtscts and uart-has-rtscts &cts-gpios&rts-gpios?

A:uart-has-rtscts 是一个 Device Tree 属性，用于告诉 Linux UART 驱动该串口支持硬件流控 (RTS/CTS)，并且应该启用它。RTS (Request To Send) 和 CTS (Clear To Send) 信号由 UART 控制器硬件自动处理，不需要软件干预

The driver will enable hardware flow control( (RTS/CTS)) function when initializing UART.

cts-gpios 和 rts-gpios 是 Device Tree 属性，用于在硬件没有原生 RTS/CTS 引脚，或者需要通过 GPIO 来模拟流控时使用。驱动会通过 GPIO 来实现流控，而不是使用 UART 控制器的硬件 RTS/CTS.

Re: questions about uart node in different imx8mp device trees

kawateb265 — Fri, 14 Nov 2025 19:34:56 GMT

@Rita_Wang

There's a very serious mistake in your answer.

https://github.com/nxp-imx/linux-imx/blob/lf-6.12.y/Documentation/devicetree/bindings/serial/serial.yaml

if: required: - uart-has-rtscts then: properties: cts-gpios: false rts-gpios: false

https://github.com/nxp-imx/linux-imx/blob/lf-6.12.y/Documentation/devicetree/bindings/serial/serial.yaml

cts-gpios: maxItems: 1 description: Must contain a GPIO specifier, referring to the GPIO pin to be used as the UART's CTS line.

rts-gpios: maxItems: 1 description: Must contain a GPIO specifier, referring to the GPIO pin to be used as the UART's RTS line.

uart-has-rtscts: $ref: /schemas/types.yaml#/definitions/flag description: The presence of this property indicates that the UART has dedicated lines for RTS/CTS hardware flow control, and that they are available for use (wired and enabled by pinmux configuration). This depends on both the UART hardware and the board wiring.

if: required: - uart-has-rtscts then: properties: cts-gpios: false rts-gpios: false

Re: questions about uart node in different imx8mp device trees

Rita_Wang — Mon, 17 Nov 2025 06:09:18 GMT

I got it, and thank you piont it and sharing, great thanks @kawateb265

Re: questions about uart node in different imx8mp device trees

kawateb265 — Mon, 17 Nov 2025 23:21:49 GMT

@Rita_Wang

We came here because we trust the professional expertise of NXP engineers.
We came here because we are seeking professional support.

The trust in NXP led to @machangbao marking an incorrect answer as the solution.

It’s obvious at a glance from your table that it was generated by AI. We are not here to get AI-rephrased answers.
If NXP engineers are also just asking AI, then why wouldn’t we just ask AI directly?