Device drivers have the job of handling devices, usually physical hardware but sometimes virtual interfaces, and presenting it to higher levels in a consistent and useful way. Linux device drivers fall into three broad categories: the character, the block, and the network. Of the three, the character driver interface is the most flexible and therefore, the most common. Linux drivers fit into a framework known as the driver model, which is exposed through sysfs. Pretty much the entire state of the devices and drivers is visible in /sys.

Each embedded system has its own unique set of hardware interfaces and requirements. Linux provides drivers for most standard interfaces, and by selecting the right kernel configuration, you can get the device operational very quickly. That leaves you with the non-standard components for which you will have to add your own device support.

In some cases, you can sidestep the issue by using generic drivers for GPIO, I2C, and so on, and write user space code to do the work. I recommend this as a starting point as it gives you the chance to get familiar with the hardware without writing kernel code. Writing kernel drivers is not particularly difficult but, if you do, you need to code carefully so as to not compromise the stability of the system.

I have talked about writing kernel driver code: if you go down that route, you will inevitably want to know how to check whether or not it is working correctly and detect any bugs. I will cover that topic in Chapter 12, Debugging with GDB.

The next chapter is all about user space initialization and the different options you have for the init program, from the simple BusyBox to the complex systemd.