Device Tree Guide

Configure the Linux device tree for your BMC platform.

Table of Contents

1. Overview
2. Device Tree Structure
3. GPIO Configuration
   1. LED GPIOs
   2. Button GPIOs
4. I2C Bus Configuration
5. SPI Flash Configuration
6. Network Configuration
   1. NCSI (Sideband)
   2. Dedicated Ethernet
7. PWM/Fan Configuration
8. Console/UART Configuration
9. Video Capture (KVM)
10. Adding Device Tree to Build
11. Debugging Device Tree
12. References

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Overview

The device tree describes your hardware to the Linux kernel, including GPIO pins, I2C buses, SPI flash, and peripherals.

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Device Tree Structure

Source Reference: OpenBMC Device Tree examples

• ASPEED DTS: linux/arch/arm/boot/dts/aspeed/
• Reference: aspeed-bmc-opp-romulus.dts

/dts-v1/;

#include "aspeed-g5.dtsi"
#include <dt-bindings/gpio/aspeed-gpio.h>

/ {
    model = "MyPlatform MyBoard BMC";
    compatible = "myplatform,myboard-bmc", "aspeed,ast2500";

    chosen {
        stdout-path = &uart5;
        bootargs = "console=ttyS4,115200 earlyprintk";
    };

    memory@80000000 {
        reg = <0x80000000 0x20000000>;  /* 512MB */
    };
};

---

GPIO Configuration

LED GPIOs

leds {
    compatible = "gpio-leds";

    identify {
        label = "identify";
        gpios = <&gpio ASPEED_GPIO(A, 0) GPIO_ACTIVE_HIGH>;
        default-state = "off";
    };

    power {
        label = "power";
        gpios = <&gpio ASPEED_GPIO(A, 1) GPIO_ACTIVE_HIGH>;
        default-state = "on";
    };

    fault {
        label = "fault";
        gpios = <&gpio ASPEED_GPIO(A, 2) GPIO_ACTIVE_HIGH>;
        default-state = "off";
    };
};

Button GPIOs

gpio-keys {
    compatible = "gpio-keys";

    power-button {
        label = "power-button";
        gpios = <&gpio ASPEED_GPIO(B, 0) GPIO_ACTIVE_LOW>;
        linux,code = <116>;  /* KEY_POWER */
    };

    reset-button {
        label = "reset-button";
        gpios = <&gpio ASPEED_GPIO(B, 1) GPIO_ACTIVE_LOW>;
        linux,code = <0x198>;  /* KEY_RESTART */
    };
};

---

I2C Bus Configuration

&i2c0 {
    status = "okay";

    /* Temperature sensor */
    tmp75@48 {
        compatible = "ti,tmp75";
        reg = <0x48>;
    };
};

&i2c1 {
    status = "okay";

    /* FRU EEPROM */
    eeprom@50 {
        compatible = "atmel,24c256";
        reg = <0x50>;
    };
};

&i2c2 {
    status = "okay";

    /* Power supply */
    psu@58 {
        compatible = "pmbus";
        reg = <0x58>;
    };
};

---

SPI Flash Configuration

&fmc {
    status = "okay";
    flash@0 {
        status = "okay";
        m25p,fast-read;
        label = "bmc";
        spi-max-frequency = <50000000>;
        #include "openbmc-flash-layout.dtsi"
    };
};

&spi1 {
    status = "okay";
    flash@0 {
        status = "okay";
        label = "pnor";
        spi-max-frequency = <100000000>;
    };
};

---

Network Configuration

NCSI (Sideband)

&mac0 {
    status = "okay";
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_rmii1_default>;
    use-ncsi;
};

Dedicated Ethernet

&mac1 {
    status = "okay";
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_rgmii2_default>;
    phy-mode = "rgmii";
    phy-handle = <&phy1>;
};

---

PWM/Fan Configuration

&pwm_tacho {
    status = "okay";
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_pwm0_default &pinctrl_pwm1_default>;

    fan@0 {
        reg = <0x00>;
        aspeed,fan-tach-ch = /bits/ 8 <0x00>;
    };

    fan@1 {
        reg = <0x01>;
        aspeed,fan-tach-ch = /bits/ 8 <0x01>;
    };
};

---

Console/UART Configuration

&uart5 {
    status = "okay";
};

&vuart {
    status = "okay";
};

---

Video Capture (KVM)

reserved-memory {
    video_memory: video {
        size = <0x04000000>;  /* 64MB */
        alignment = <0x01000000>;
        compatible = "shared-dma-pool";
        reusable;
    };
};

&video {
    status = "okay";
    memory-region = <&video_memory>;
};

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Adding Device Tree to Build

# recipes-kernel/linux/linux-aspeed_%.bbappend

FILESEXTRAPATHS:prepend := "${THISDIR}/linux-aspeed:"

SRC_URI += "file://aspeed-bmc-myplatform-myboard.dts"

do_configure:append() {
    cp ${WORKDIR}/aspeed-bmc-myplatform-myboard.dts \
        ${S}/arch/arm/boot/dts/aspeed/
}

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Debugging Device Tree

# View compiled device tree
dtc -I dtb -O dts /sys/firmware/fdt

# Check GPIO assignments
cat /sys/kernel/debug/gpio

# Check I2C buses
i2cdetect -l
i2cdetect -y 0

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References

• Linux Device Tree Documentation
• ASPEED Device Tree Bindings
• Device Tree Specification

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Prerequisites: Linux kernel and device tree knowledge required