How to reverse engineer a PCI-Express driver under Linux ?

Hello, I have a PCI-Express video capture card that has a proprietary driver for Linux. I have some experience with programming in C, and so I would like to start a hobby project to develop a free/libre driver for this device for Linux. Of course I don’t have access to any documentation about how to communicate with this device, so I think I will need to reverse-engineer the way the existing driver communicates with the hardware. How could I achieve this ? Also, the long term goal of this project would be to have this driver merged into mainline, so what is allowed or not while doing this to avoid problematic legal ramifications ? Thank you for your help. Best regards. Tomek Lecocq