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Device Tree Configuration

Device Tree describes CPUs, buses, peripherals, registers, interrupts, GPIOs, clocks, and resets. The bootloader passes the DTB to Linux, which creates platform, I2C, SPI, and other devices from it.

T527 Device Tree Layers​

SoC common DTSI
    ↓
platform/controller DTSI
    ↓
board.dts
    ↓
DTC produces DTB
    ↓
bootloader passes DTB to Linux

T527 commonly uses the SoC identifier sun55iw3p1. A common SoC-level path is:

longan/bsp/configs/linux-5.15/sun55iw3p1.dtsi

Board files commonly use one of these layouts:

longan/device/config/chips/t527/configs/<BOARD>/board.dts
longan/device/config/chips/t527/configs/<BOARD>/linux-5.15/board.dts

Find the actual files first:

cd <ANDROID_TOP>/longan

find bsp/configs/linux-5.15 device/config/chips/t527 \
    -type f \( -name '*.dts' -o -name '*.dtsi' \) \
    -print | sort

Editing Rules​

  • Put configuration shared by every board in the SoC-level DTSI.
  • Put board-specific pinctrl, GPIO, supply, reset, enable, and panel parameters in board.dts.
  • Prefer overriding a labelled node with &label instead of copying an entire SoC node.
&uart7 {
        pinctrl-names = "default", "sleep";
        pinctrl-0 = <&uart7_pins_a>;
        pinctrl-1 = <&uart7_pins_sleep>;
        status = "okay";
};

Common properties include compatible, reg, interrupts, clocks, resets, pinctrl-0, supply references, and status.

Find Nodes and Drivers​

grep -Rni "uart7" bsp/configs/linux-5.15 device/config/chips/t527
grep -Rni "uart7:" bsp/configs/linux-5.15 device/config/chips/t527
grep -Rni 'compatible.*sunxi.*uart' bsp/drivers kernel/linux-5.15

Build​

cd <ANDROID_TOP>/longan
./build.sh kernel
find out -type f -name '*.dtb' -print

Decompile the Actual DTB​

dtc -I dtb -O dts \
    -o /tmp/t527-final.dts \
    <actual-generated-DTB>

grep -n -A20 -B5 "uart7" /tmp/t527-final.dts

Do not stop at checking the source file. Verify the DTB that is actually packaged.

Runtime Device Tree​

adb root
adb shell 'cat /proc/device-tree/model 2>/dev/null'
adb shell 'find /proc/device-tree -maxdepth 4 -type f 2>/dev/null | head'

Read a string property without its trailing NUL byte:

adb shell 'tr -d "\000" < /proc/device-tree/<node-path>/status'

Only after the source, generated DTB, and runtime Device Tree agree should driver probe debugging continue.