I am currently working on selecting proper CPU and GPU soft cores to be synthesized on to the FPGA for performance and power efficiency.
I have looked into multiple open-source RISCV IPs including the Rocket-Core (a widely known UC Berkley project based on HLS(High Level Synthesis) languages), the VexRISCV project (frequently used in 18-525/725 tape-out, proven to work in multiple real chips) and the hazard-3 core designed by Luke Wren, principle engineer of Raspberry Pi, an is currently onboard multiple RPI products. I worked with all of the projects and decide to go forth and select the VexRISC-V core as the benchmark softcore because:
- It has a long history of success, the project is designed for FPGA softcore and has been verified on multiple FPGA fabrics, including ones that we might use later in the project. Unlike the Hazard3 core, which is designed to be used in silicon.
- The project is simple and has lots of example to draw from, while the Berkley Rocket-Core and the Chipyard framework has a huge dependency of more than 30G in total and ended up not working out of the box.
- VexRISC-V, also being an HLS project, offers great flexibility as well, and have vanilla options for multiple bus protocol options, which will facilitate communication when synthesized onto the FPGA. It also has support for directly booting linux for even greater ease of use.
Currently my progress is on schedule, the next steps are testing out the Vex soft core on 240 FPGA (we are planning on using Xilinx boards so the Vivado toolchain would match) and find and evaluate appropriate GPU soft IPs as well.