During the last two years, I have collected ARM and RISC-V gadgets and development boards. What I realized is that many of them run systems far off mainline Linux, and quite often do the vendors not publish the sources. However, when able to get serial output, I can fix that. And that is where many engineers start with regular development, so we are in the same boat.
Now how do the original drivers work? Applications in existing products often have userspace or hybrid drivers, which opens up a door. We can emulate the app, e.g., in QEMU, and monitor I/O accesses. Then we can write our own driver. But that is a tedious and time consuming process, the most inconvenient step being