The STM32 simply bridges between BDM frames
The problem is not what you want to use to implement the BDM, but rather getting it working in a stable and reliable way, which means reverse engineering all the details that are NOT known from the public documentation. These details do the difference.
I need time to complete the reverse engineering of the DI module of the EVS board, and I need the documentation of CPU module of the EVS board to try a different configuration, specifically how to ignore the ROM0-1 and bootstrap from the ROM2-3
ROM0-1 is a PLCC UV_ROM (16bit ROM) chip soldered on the CPU module
ROM2-3 are a couple of DIP UV_ROM (8bit, ODD and EVEN) mounted on the motherboard, they can be removed, hence I can use my ROM-emulator
This information should be covered in the manual of the EVS board, unfortunately, I don't have. It's not a problem, simply this would make the activity more comfortable since currently I have to upload files via the serial, which is good, but it's slow at max 9600bps while the ROM emulator uploads at 1-2Mbps.
and USB HID reports
Talking about Linux, HIS is not neat for this. USB-bulk is better.