Understanding the Device Tree

Source https://forum.xda-developers.com/showthread.php?t=2620389

Note - This is by no means complete, and there will be omissions as have explained all this top of my head and copied pasted certain bits that I have here on my own device tree.

Android.mk - this will tell the build system to include and to build sources specifically for your device. I have explained a bit of it above as well. 

AndroidBoard.mk - this is for the kernel, the build system uses that to drop the kernel image in place.

AndroidProducts.mk - specifies the appropriate device's make file, to use for building. i.e. device/htc/pico/pico.mk(In my case), This is device-specific as well.

device_codename.mk - Every device has a codename, and there is a file named as the codename of device. It specifies the properties and extras to copy over into the final output, in this case, it could be for example, pico.mk

BoardConfig.mk - This is the meat of it all, this is where compiler conditional flags are set, partition layouts, boot addresses, ramdisk size, and so on.

Android.mk

Code:

LOCAL_PATH := $(my-dir)

ifeq ($(TARGET_DEVICE),pico)
include $(call all-makefiles-under,$(LOCAL_PATH))
endif

^This is how the build will use that to build recovery, sensors, lights and camera (of course there will be more), its saying 'Yo Builder, go into each of the directories specified, and build the respective sources' 

AndroidBoard.mk:

Code:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

ALL_PREBUILT += $(INSTALLED_KERNEL_TARGET)

# include the non-open-source counterpart to this file
-include vendor/htc/pico/AndroidBoardVendor.mk

^ This one is pretty simple and tells the build system to go to the vendor tree of the device and include AndroidBoardVendor.mk for building.

AndroidProducts.mk

Code:

PRODUCT_MAKEFILES := \
    $(LOCAL_DIR)/cm.mk \
    $(LOCAL_DIR)/pico.mk

^Just specifying the makefiles of the device. 

BoardConfig.mk
Now in this one, I'm taking a BoardConfig.mk file that I found over the internet with a suitable explanation of it. Credits to the post over stackoverflow for the valuable efforts.

Code:

LOCAL_PATH:= $(call my-dir)

TARGET_NO_BOOTLOADER := true
TARGET_PREBUILT_KERNEL := device/lg/gt540/kernel
TARGET_PREBUILT_RECOVERY_KERNEL := device/lg/gt540/recovery_kernel

# This will vary from device!
TARGET_BOARD_PLATFORM := msm7k
TARGET_ARCH_VARIANT := armv6-vfp
TARGET_CPU_ABI := armeabi
TARGET_CPU_ABI := armeabi-v6l
TARGET_CPU_ABI2 := armeabi

# OpenGL drivers config file path
BOARD_EGL_CFG := device/lg/gt540/egl.cfg

# Dependant, not to be taken literally!
BOARD_GLOBAL_CFLAGS += -DHAVE_FM_RADIO

# Dependant, not to be taken literally!
BOARD_KERNEL_BASE := 0x02600000

# this will be device specific, and by doing cat /proc/mtd will give you the correct sizes
BOARD_BOOTIMAGE_PARTITION_SIZE     := 0x00480000
BOARD_RECOVERYIMAGE_PARTITION_SIZE := 0x00480000
BOARD_SYSTEMIMAGE_PARTITION_SIZE   := 0x0cf80000
BOARD_USERDATAIMAGE_PARTITION_SIZE := 0x0d020000
BOARD_FLASH_BLOCK_SIZE := 131072

^That is an excerpt, notice how we specify kernel's base address, this is how the boot.img gets generated after compilation is done and yet again, gets dropped into out/target/product/lg/gt540/boot.img. Also, more importantly, we're telling the build system to use the target platform for cross-compiling the sources (*TARGET_BOARD_PLATFORM*/*TARGET_CPU_ABI*) There will be more information in there such as conditional flags to pass to the compiler, for an example. we specified the directive HAVE_FM_RADIO to tell it, when it comes to handling the source for the FM radio system, to conditionally compile parts of the source. Again, this is hardware specific and mileage will vary, also this applies to the address for boot. In a nutshell, this is saying 'Yo Builder, read the damn variables and remember them and apply them when cross-compiling those source files!'

Now that the internals of each of those Android build make-files are shown.

Now, onto the vendor/ part of it, in AOSP, simply, once again, correlation and corresponds with the device/ tree, as in continuing with this example, vendor/lg/gt540/ which gets picked up by the lunch. There's more make files in there but the general consensus is there's a directory called proprietary which contains the proprietary libs (due to close-source etc) that gets copied over. The copying over of the libraries gets specified in the file device-vendor-blobs.mk, in this case, gt540-vendor-blobs.mk.