Developer Ben Cox has written of his experience in developing userspace Linux drivers for a long-abandoned video capture device, acquired in bulk for pennies on the dollar, through reverse-engineering and a modicum of guesswork.
"I recently found some USB devices on eBay (Epiphan VGA2USB LR) that could take VGA as input and present the output as a webcam," Cox explains. "Given that I was keen on the idea of not needing to lug out a VGA monitor ever again and there was claimed Linux support I took the risk and bought the whole job lot for about £20 ($25)."
There was a reason the devices were so cheap, however. Since discontinued by their creators, the boxes required a proprietary driver to operate and while a Linux version had been provided at launch it was only compatible up to Linux 4.9 — leaving modern Linux distributions in the cold. Any hopes of tweaking the provided files were dashed, too, by the discovery that they were provided as pre-compiled binaries — ripe for reverse engineering.
"To observe the driver in its normal operating environment, I made a Debian 9 (the last supported release) VM, and did a KVM USB Passthrough to give it direct access to the device. I then installed the driver and confirmed that it worked," Cox explains. What followed was a range of reverse-engineering hacks starting with packet capture over USB via Wireshark and moving onto physical disassembly to verify that the hardware was, in fact, based on a Xilinx Spartan-6 field-programmable gate array (FPGA), extraction of the bitstream, timing analysis, and then the tricky process of figuring out how the data from the camera was encoded.
"After analysis it appeared that the data was not obviously encoded in any way," Cox explains. Using a variety of tools, Cox eventually had a rough idea of how things worked: "I replicated the USB packet sequence to obtain these uncompressed blobs, and looking at the byte counts, it matched with getting around 3 bytes (RGB) per pixel. Initial processing of these images (just taking the output and writing it as RGB pixels) resulted in something roughly inspired by the input I was giving to the device over VGA."
A little more tweaking resulted in an image which looked correct bar the colours. "At this point I had a flashback to some 3D rendering/shader work I did long ago," Cox explains. "This looked a lot like YUV colour. After using Go’s built in YCbCrToRGB the image suddenly looked much closer to the original. We did it! Despite the 'WIP' quality we were able to do 7 FPS. Honestly, for me that was good enough, since my use for these are as an emergency VGA screen rather than anything else."