Linux provides a powerful and expansive API for applications, but sometimes that's not enough. Interacting with a piece of hardware or conducting operations that require accessed to privileged information in the system require a kernel module.
A Linux kernel module is a piece of compiled binary code that is inserted directly into the Linux kernel, running at ring 0, the lowest and least protected ring of execution in the x86–64 processor. Code here runs completely unchecked but operates at incredible speed and has access to everything in the system.
Before we get started, we need to make sure we have the correct tools for the job. Most importantly, you'll need a Linux machine. I know that comes as a complete surprise! While any Linux distribution will do, I am using Ubuntu 16.04 LTS in this example, so if you're using a different distribution you may need to slightly adjust your installation commands.
Secondly, you'll need either a separate physical machine or a virtual machine. I prefer to do my work in a virtual machine, but this is entirely up to you. I don't suggest using your primary machine because data loss can occur when you make a mistake. I say when, not if, because you undoubtedly will lock up your machine at least a few times during the process. Your latest code changes may still be in the write buffer when the kernel panics, so it's possible that your source files can become corrupted. Testing in a virtual machine eliminates this risk.
And finally, you'll need to know at least some C. The C++ runtime is far too large for the kernel, so writing bare metal C is essential. For interaction with hardware, knowing some assembly might be helpful.