I’m not aware of any licensing or patent issues with using CAN-FD on Klipper when using an external chip such as the MCP2518. In previous conversations I indicated I was not interested in writing a software implemention of CAN-FD on the rp2040 in the can2040 library. Implementing the low-level CAN-FD in software seems to add unnecessary risk (given various patents), but this is unrelated to utilizing a hardware implementation such as the MCP2518.
I don’t see any particular gain in utilizing CAN-FD on Klipper as there are far fewer chips available. In particular, it’s harder to find common Linux USB adapters that support CAN-FD. The older CAN 2.0 protocol can be annoying, but it generally works fine for our usages. To wit, using CAN-FD instead of CAN 2.0 enables some additional bandwidth, at the cost of notably worse hardware availability.
I’m sure it is possible to support the MCP2518 chip in Klipper (using either CAN-FD or CAN 2.0), but I suspect it would take notable work. An interested developer would need to implement that support. It is not something that I personally plan to look at.
Cheers,
-Kevin