The Linux kernel driver API/ABI isn't stable so there's no permanent/versioned interface you can program against that would make out-of-tree drivers non-headache inducing: https://www.kernel.org/doc/Documentation/process/stable-api-...
> You think you want a stable kernel interface, but you really do not, and you don't even know it. What you want is a stable running driver, and you get that only if your driver is in the main kernel tree. You also get lots of other good benefits if your driver is in the main kernel tree, all of which has made Linux into such a strong, stable, and mature operating system which is the reason you are using it in the first place.