Chapter 5. Customizing Kernel and Boot Sequence

In the previous chapter, we created and deployed our first custom version of Android. We created a version for a commercial smartphone, the Google Nexus 6, and a more hard-core version for a development board, the Udoo Quad. We learned about more development tools, such as ADB and Fastboot. We focused on the debugging tools, mastering the serial connection, and the boot sequence.

In this chapter, we will dive into the system—from the kernel customization to the boot sequence. You will learn how to retrieve the proper source code for Google devices, how to set up the build environment, how to build your first custom version of the Linux kernel, and deploy it to your device. You will learn about:

• Toolchain overview

• How to configure the host system to compile your own Linux kernel

• How to configure the Linux kernel

• Linux kernel overview

• Android boot sequence

• The Init process

An overview of the Linux kernel

In Chapter 1, Understanding the Architecture, we learned how Android has been designed and built around the Linux kernel. One of the reasons to choose the Linux kernel was its unquestioned flexibility and the infinite possibilities to adjust it to any specific scenario and requirement. These are the features that have made Linux the most popular kernel in the embedded industry.

Linux kernel comes with a GPL license. This particular license allowed Google to contribute to the project since the early stages of Android. Google provided bug fixing and new features, helping Linux to overcome a few obstacles and limitations of the 2.6 version. In the beginning, Linux 2.6.32 was the most popular version for the most part of the Android device market. Nowadays, we see more and more devices shipping with the new 3.x versions.

The following screenshot shows the current build for the official Google Motorola Nexus 6, with kernel 3.10.40:

The Android version we created in...

Setting up the toolchain

The toolchain is the set of all the tools needed to effectively compile a specific software to a binary version, enabling the user to run it. In our specific domain, the toolchain allows us to create a system image ready to be flashed to our Android device. The interesting part is that the toolchain allows us to create a system image for an architecture that is different from our current one: odds are that we are using an x86 system and we want to create a system image targeting an ARM (Advanced RISC Machine) device. Compiling software targeting an architecture different from the one on our host system is called cross-compilation.

The Internet offers a couple of handy solutions for this task—we can use the standard toolchain, available with the AOSP (Android Open Source Project) or we can use an alternative, very popular toolchain, the Linaro toolchain. Both toolchains will do the job—compile every single C/C++ file for the ARM architecture.

As usual, even the toolchain...

Preparing the host system

To successfully compile our custom kernel, we need a properly configured host system. The requirements are similar to those we satisfied to build the whole Android system in the previous chapter:

• Ubuntu

• Linux kernel source code

• Toolchain

• Fastboot

Ubuntu needs a bit of love to accomplish this task: we need to install the ncurses-dev package:

$ sudo apt-get install ncurses-dev

Once we have all the required tools installed, we can start configuring the environment variables we need. These variables are used during the cross-compilation and can be set via the console. Fire up your trusted Terminal and launch the following commands:

$ export PATH=<toolchain-path>/arm-eabi-4.8/bin:$PATH
$ export ARCH=arm
$ export SUBARCH=arm
$ export CROSS_COMPILE=arm-eabi-

Configuring the kernel

Before being able to compile the kernel, we need to properly configure it. Every device in the Android repository has a specific branch with a specific kernel with a specific configuration to be applied.

The table on page 2 has a column with the exact information we need—Build configuration. This information represents the parameter we need to properly configure the kernel build system. Let's configure everything for our Google Nexus 6. In your terminal, launch the following command:

$ make shamu_defconfig

This command will create a kernel configuration specific for your device. The following screenshot shows the command running and the final success message:

Once the .config file is in place, you could already build the kernel, using the default configuration. As advanced users, we want more and that's why we will take full control of the system, digging into the kernel configuration. Editing the configuration could enable missing features or disable unneeded hardware...

Compiling the kernel

Once you have a properly configured environment and a brand new configuration file, you just need one single command to start the building process. On your terminal emulator, in the kernel source folder, launch:

$ make

The make command will wrap up the necessary configuration and will launch the compiling and assembling process. The duration of the process heavily depends on the performance of your system: it could be one minute or one hour. As a reference, an i5 2.40 GHz CPU with 8 GB of RAM takes 5-10 minutes to complete a clean build. This is incredibly quicker than compiling the whole AOSP image, as you can see, due to the different complexity and size of the code base.

Working with non-Google devices

So far, we have worked with Google devices, enjoying the Google open-source mindset. As advanced users, we frequently deal with devices that are not from Google or that are not even a smartphone. As a real-world example, we are going to use again a UDOO board: a single-board computer that supports Ubuntu or Android. For the time being, the most popular version of UDOO is the UDOO Quad and that's the version we are targeting.

As for every other device, the standard approach is to trust the manufacturer's website to obtain kernel source code and any useful documentation for the process: most of all, how to properly flash the new kernel to the system. When working with a custom kernel, the procedure is quite consolidated. You need the source code, the toolchain, a few configuration steps, and, maybe, some specific software package to be installed on to your host system. When it comes to flashing the kernel, every device can have a different procedure. This depends...

Driver management

Since version 2.6.x, Linux gives the developer the opportunity to compile parts of the kernel as separated modules that can be injected into the core, to add more features at runtime. This approach gives flexibility and freedom: there is no need to reboot the system to enjoy new features and there is no need to rebuild the whole kernel if you only need to update a specific module. This approach is widely use in the PC world, by embedded devices such as routers, smart TVs, and even by our familiar UDOO board.

To code a new kernel module is no easy task and it's far from the purpose of this book: there are plenty of books on the topic and most of the skill set comes from experience. In these pages, you are going to learn about the big picture, the key points, and the possibilities.

Unfortunately, Android doesn't use this modular approach: every required feature is built in a single binary kernel file, for practical and simplicity reasons. In the last few years there has been...

Altering the CPU frequency

Overclocking a CPU is one of the most loved topics among advanced users. The idea of getting the maximum amount of power from your device is exciting. Forums and blogs are filled with discussions about overclocking and in this section we are going to have an overview and clarify a few tricky aspects that you could deal with on your journey.

Every CPU is designed to work with a specific clock frequency or within a specific frequency range. Any modern CPU has the possibility to scale its clock frequency to maximize performance when needed and power consumption when performance is not needed, saving precious battery in case of our beloved mobile devices. Overclocking, then, denotes the possibility to alter this working clock frequency via software, increasing it to achieve performance higher than the one the CPU was designed for.

Contrary to what we often read on unscrupulous forum threads or blogs, overclocking a CPU can be a very dangerous operation: we are forcing...

Customizing the boot image

So far, you learned how to obtain the kernel source code, how to set up the system, how to configure the kernel, and how to create your first custom kernel image. The next step is about equipping your device with your new kernel. To achieve this, we are going to analyze the internal structure of the boot.img file used by every Android device.

Creating the boot image

A custom ROM comes with four .img files, necessary to create a working Android system. Two of them (system.img and data.img) are compressed images of a Linux compatible filesystem.

The remaining two files (boot.img and recovery.img) don't contain a standard filesystem. Instead, they are custom image files, specific to Android. These images contain a 2KB header sector, the kernel core, compressed with gzip, a ramdisk, and an optional second stated loader.

Android provides further info about the internal structure of the image file in the boot.img.h file contained in the mkbootimg package in the AOSP source...

Android boot sequence

To fully understand all Android internals, we are going to learn how the whole boot sequence works: from the power-on to the actual Android system boot. The Android boot sequence is similar to any other embedded system based on Linux: in a very abstract way, after the power-on, the system initializes the hardware, loads the kernel, and finally the Android framework. Any Linux-based system undergoes a similar process during its boot sequence: your Ubuntu computer or even your home DSL router.

In the next sections, we are going to dive deeper in to these steps to fully comprehend the operating system we love so much.

Internal ROM – bios

When you press the power button on your device, the system loads a tiny amount of code, stored inside a ROM memory. You can think about this as an equivalent of the BIOS software you have in your PC. This software is in charge of setting up all the parameters for CPU clock and running the RAM memory check. After this, the system loads the...

Summary

In this chapter, you learned how to obtain the Linux kernel for your device, how to set up your host PC to properly build your custom kernel, how to add new features to the kernel, build it, package it, and flash it to your device.

You learned how the Android boot sequence works and how to manipulate the init scripts to customize the boot sequence.

In the next chapter, you will learn how to cook your first custom ROM, how to root your device, and replace the recovery partition.