Linux is a very powerful and complex operating system kernel that can be married to various types of user space ranging from a simple embedded device, to increasingly complex mobile devices using Android, to a full server operating system. One of its strengths is the degree of configurability. The definitive place to get the source code is www.kerenl.org, but you will probably need to get the source for a particular SoC or board from the vendor of that device or a third-party that supports that device. The customization of the kernel for a particular target may consist of changes to the core kernel code, additional drivers for devices that are not in mainline Linux, a default kernel configuration file and, a device tree source file.

Normally you start with the default configuration for your target board, and then tweak it by running one of the configuration tools such as menuconfig. One of the things you should consider at this point is whether kernel features and drivers should be compiled as modules or built-in. Kernel modules are usually no great advantage for embedded systems, where the feature set and hardware are usually well defined. However, modules are often used as a way to import proprietary code into the kernel, and also to reduce boot time by loading non-essential drivers after boot. Building the kernel produces a compressed kernel image file, named zImage, bzImage, or uImage depending on the bootloader you will be using and the target architecture. A kernel build will also generate any kernel modules (as .ko files) that you have configured, and device tree binaries (as .dtb files) if your target requires them.

Porting Linux to a new target board can be quite simple or very difficult depending on how different the hardware is from that in the mainline or vendor supplied kernel. If your hardware is based on a well-known reference design, then it may be just a question of making changes to the device tree or to the platform data. You may well need to add device drivers, which I discuss in Chapter 8, Introducing Device Drivers. However, if the hardware is radically different to a reference design, you may need additional core support, which is outside the scope of this book.

The kernel is the core of a Linux based system, but it cannot work by itself. It requires a root filesystem that contains user space. The root filesystem can be a ramdisk or a filesystem accessed via a block device, which will be the subject of the next chapter. As we have seen, booting a kernel without a root filesystem results in a kernel panic.