Well, the one good example I have from a few years ago was a relatively simple ADC PCI card. I did have documentation from the company. They had a Windows drive

Well, the one good example I have from a few years ago was a relatively simple ADC PCI card. I did have documentation from the company. They had a Windows driver, but for Linux, their advice was direct memory writes (which requires root privileges). I didn't want the application to run as root, so I wrote a Linux diver for the card.