Device Tree Structure

In AOSP, the "device tree" (not to be confused with the kernel's Device Tree Source or DTS) refers to the directory structure containing the configuration and build files needed to compile Android for a specific hardware target. This directory acts as the blueprint for assembling the OS.

The device/<vendor>/<codename>/ Directory

The standard location for a device tree is device/<vendor>/<codename>/. For example, the Pixel 7 (codename panther) by Google has its device tree at device/google/pantah/ (sharing a common base with the Pixel 7 Pro, cheetah).

A typical device tree contains several crucial Makefiles and configuration directories:

• BoardConfig.mk: Hardware-specific build variables.
• device.mk: Software packages and overlays.
• AndroidProducts.mk: Registration of the build target.
• overlay/: XML overlays to override framework resources.
• rootdir/: Scripts and init.rc files placed in the root filesystem.

BoardConfig.mk: Hardware Blueprint

BoardConfig.mk is arguably the most critical file in the device tree. It defines the low-level hardware characteristics and dictates how the build system should construct the partition images.

Key variables defined here include:

• Architecture: TARGET_ARCH := arm64, TARGET_CPU_VARIANT := cortex-a75.
• Partition Sizes and Types: Defines the size limits for system.img, vendor.img, etc., and the filesystem type (ext4, f2fs, EROFS).

  BOARD_SYSTEMIMAGE_PARTITION_SIZE := 3221225472
  BOARD_SYSTEMIMAGE_FILE_SYSTEM_TYPE := erofs
  

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• Kernel Configuration: Specifies the prebuilt kernel image to use, or the defconfig if building the kernel from source.

  TARGET_PREBUILT_KERNEL := device/google/panther-kernel/Image.lz4
  

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• Bootloader Flags: Arguments passed to the kernel at boot (e.g., BOARD_KERNEL_CMDLINE).
• SELinux: Paths to the device-specific SELinux policies (BOARD_VENDOR_SEPOLICY_DIRS).

device.mk: Software Packages and Overlays

While BoardConfig.mk handles the hardware, device.mk (or a similarly named file like device_panther.mk) handles the software. It specifies which APKs, HALs, and configuration files should be installed on the device.

• PRODUCT_PACKAGES: A list of modules (defined in Android.bp or Android.mk) to include in the build.

  PRODUCT_PACKAGES += \
      android.hardware.camera.provider@2.4-impl \
      Camera2 \
      libGLES_mali
  

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• PRODUCT_COPY_FILES: Instructions to copy static files (like audio configs, permissions XMLs, or prebuilt binaries) directly to the output image.

  PRODUCT_COPY_FILES += \
      frameworks/native/data/etc/android.hardware.wifi.xml:$(TARGET_COPY_OUT_VENDOR)/etc/permissions/android.hardware.wifi.xml
  

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• DEVICE_PACKAGE_OVERLAYS: Specifies directories containing resource overlays to modify the look and feel of framework apps without changing their source code.

AndroidProducts.mk: The Lunch Menu

When you run lunch in the AOSP root, the build system scans for AndroidProducts.mk files to build the list of available targets.

This file maps a "lunch choice" (e.g., aosp_panther-userdebug) to the specific Makefile that defines that product.

PRODUCT_MAKEFILES := \
    $(LOCAL_DIR)/aosp_panther.mk

COMMON_LUNCH_CHOICES := \
    aosp_panther-userdebug \
    aosp_panther-eng

Init Files: init.<device>.rc

Android's init process uses .rc files to configure the system at boot. The device tree provides hardware-specific init scripts, usually named init.<codename>.rc.

These scripts handle tasks that must occur early in the boot process:

• Mounting specific partitions (if not handled by fstab).
• Setting file permissions and ownership for hardware nodes (e.g., /dev/video0).
• Starting vendor-specific daemons.

# Example from init.panther.rc
on boot
    # Camera
    chown media camera /dev/video0
    chmod 0660 /dev/video0

service vendor.camera-provider-2-4 /vendor/bin/hw/android.hardware.camera.provider@2.4-service
    class hal
    user cameraserver
    group audio camera input drmrpc
    ioprio rt 4
    capabilities SYS_NICE