Android image/kernel building/flashing - A *VERY* short guide :-)

This week, I had to go through the process of Android OS/Kernel building/installation. And it was a lot much better and 6 months ago (maybe, because I built it for a device and not for the emulator?). I compiled the images in Ubuntu 12.04 and I used a Samsung Galaxy Nexus device (maguro with tuna as kernel). Therefore, I decided to summarize the steps that I took. This mini-tutorial is a lot shorter and simpler (and really works!!).

Update (13/07.2015): info for arm64 => http://seandroid.bitbucket.org/BuildingKernels.html

1. Android OS

1.0 Setting up the building environment

Check this instructions (here and here) to set up the basic environment and download the code. I used the branch [android-4.3_r1.1].

1.1 Compiling the Android OS

a. Download and unpack the manufacturer drivers from this link. They have to be unpacked into the directory [android_source_code]/vendors -- but don't worry, as the .zip files contain a script that does all the work for you.

b. Once the drivers are in the proper place, run the following commands:

  @desktop:$ cd [android_source_code]
  @desktop:$ make clobber
  @desktop:$ lunch full_maguro-userdebug
  @desktop:$ make -j4

It takes a long time to compile the image.

After these steps, the Android OS is ready.

1.2 Flashing the device with the new Android OS

Now, you need two tools from the Android SDK: adb and fastboot. These tools are located in the folder [androis_sdk]/platform-tools.

a. Reboot the device in the bootloader mode -- hold VolumeDown and VolumeUp and then press the PowerUp button.

b. Connect the USB cable.

c. Run the following commands:

  @desktop:$ export PATH=$PATH:[android_sdk]/platform-tools
  @desktop:$ cd [android_source_code]
  @desktop:$ sudo fastboot format cache
  @desktop:$ sudo fastboot format userdata
  @desktop:$ sudo ANDROID_PRODUCT_OUT=[android_source_code]/out/target/product/maguro/ fastboot -w flashall

After these steps, reboot the device. A clean installation will take place. To check the new version of you device, go to "Settings" - - > "About Phone" and check "Model number": now, it should be "AOSP on Maguro" (check attached image)

2. Android Kernel

Ok. Now, we have the AOSP in place and we need to compile a new kernel. But why do you need to compile and install a new kernel? Oh, well, let's say that you want to apply some patches or that you need to change the kernel to enable Linux module support (the default Android Linux Kernel does not support modules).

2.0 Setting up the building environment

If you have built the Android OS before, you don't need anything special for the kernel building. I used the official code from https://android.googlesource.com/kernel/omap.git, branch android-omap-tuna-3.0-jb-mr2.

2.1 Compiling the Kernel

First, you need to set some variables that are important for the building process (ARCH and CROSS_COMPILE):

  @desktop:$ export ARCH=arm
  @desktop:$ export CROSS_COMPILE=[android_source_code]/prebuilts/gcc/linux-x86/arm/arm-eabi-4.7/bin/arm-eabi-

Now, you have to generate a .config which contains all the options for the kernel building. By running the following command you generate a basic .config file for Android.

  @desktop:$ cd [android_kernel_code]
  @desktio:$ make tuna_defconfig

Sometimes, you need to set some specific entries of the .config to enable/disable certain features of the kernel. For this specific example, let's set the option CONFIG_MODULES to y (the entry in the .config file should be CONFIG_MODULES=y). With CONFIG_MODULES set to y, it is possible to insert/remove kernel modules. Now, let's build the kernel

  @desktop:$ cd [android_kernel_code]
  @desktop:$ make

(it takes some time to compile the kernel)

2.2 Preparing the kernel for installation

The kernel image is almost ready: it's still necessary to wrap it up properly to flash it into the device. The Android source code contains scripts that do the work for us. Consider that the image was generated at [android_kernel_code]/arch/arm/boot/zImage.

  @desktop:$ cd [android_source_code]
  @desktop:$ export TARGET_PREBUILT_KERNEL= [android_kernel_code]/arch/arm/boot/zImage
  @desktop:$ make bootimage

At the end, a custom image is ready for installation at [android_source_code]/out/target/product/maguro/boot.img

2.3 Flashing the device with the new Kernel
 
Now, everything is in place and we can finally flash our kernel image. To do so:

a. You need to boot the device in bootloader mode (hold VolumeDown and VolumeUp and then press the PowerUp button)

b. Connect the USB cable

c. Run the following commands

  @desktop:$ cd [android_source_code]
  @desktop:$ sudo ANDROID_PRODUCT_OUT=[android_source_code]/out/target/product/maguro/ fastboot flash boot [android_source_code]/out/target/product/maguro/boot.img

After these steps, reboot the device. A clean installation will take place. To check the new version of you kernel, go to "Settings" - - > "About Phone" and check "Kernel version": you will see a different name for you kernel image (as for the previuos image).