riscv - Rocket Chip on Non-Zynq FPGAs

Question

I want to port the rocket chip to a non-Zynq FPGA (an altera Stratix V), a board which doesn't contain an ARM core used to run the riscv-fesvr. How can I go about starting the port? Also, has anyone attempted to run the rocket chip on such a board? Can I pointed to some resources for that?

Answer 1

This is mainly an issue of interfacing, as Rocket Chip does not use anything Zynq specific internally. If this interfacing is done properly, you should not need to change pk/linux or Rocket Chip itself. You will need to both wrap Rocket Chip for the target FPGA and interface to it with a Frontend Server (fesvr).

For wrapping Rocket Chip, I would look at the top level IO it expects (RocketChip.scala), which most importantly includes HostIO (for HTIF) and MemIO (for DRAM). For additional information on those interfaces, I would consult the slides from the first workshop. Our current wrapper (rocketchip_wrapper.v) multiplexes those interfaces over AXI to the host ARM core that is running fesvr.

Your proposal sent to the mailing list of running fesvr on a NIOS II core and communicating to it via AXI could work. This will require modifying (fesvr-zedboard.cc) to match the AXI interface NIOS provides to software. This approach may require the least new development, but the soft-core will consume FPGA resources and may be slower.

Another way to accomplish this would be to run the front-end server on a PC attached to the board via ethernet. You would need to make a bridge between Rocket Chip's MemIO and the on-board DRAM. You will also need to run HTIF over ethernet, which will require another bridge between HostIO and the board's ethernet MAC. Years ago fesvr supported this (fesvr-eth.cc), but that code has not been maintained and will almost certainly need to be updated.

This all assumes you want to run the processor in a tethered fashion. To make it self-booting will require more work. There is work on a platform specification that will standardize this, but until then you will need to design your own or continue to use Rocket Chip tethered.