Project name

The Libre RISC-V SoC, Wishbone Streaming Proposal

Website / wiki

Please be short and to the point in your answers; focus primarily on the what and how, not so much on the why. Add longer descriptions as attachments (see below). If English isn't your first language, don't worry - our reviewers don't care about spelling errors, only about great ideas. We apologise for the inconvenience of having to submit in English. On the up side, you can be as technical as you need to be (but you don't have to). Do stay concrete. Use plain text in your reply only, if you need any HTML to make your point please include this as attachment.

Abstract: Can you explain the whole project and its expected outcome(s).

In projects such as the Libre RISC-V SoC, commercial grade System-on-Chip (SoC) bus infrastructure is needed. Nowadays this often means AMBA AXI4, AXI4-lite or AXI4-Stream, all published by ARM Limited. The AXI family is "royality-free" and it is not only patented but its patent holder has begun denying licenses due to the US Trade War.

The main alternative with large adoption is Wishbone, which is an Open Standard in contrast to AXI. However Wishbone does not have a "streaming" capability, which is typically needed for high-throughput data pathes and interfaces, e.g. for video applications and High-Performance Computing (HPC).

Therefore this project will write up an enhancement to the Wishbone B4 SoC Bus, provide Reference Implementations and Bus Function Models (BFM) which easily allows unit tests for all Wishbone BFM users. For demonstration we like to implement an example peripheral (here, an audio interface, for the Libre RISC-V SoC) also. This demonstrations proves our concept also.

A secondary objective will be to seek out Reference Implementations for Wishbone Master and Slave, provide formal correctness proofs, and add additional example peripherals - non-streaming ones - as resources permit.

Have you been involved with projects or organisations relevant to this project before? And if so, can you tell us a bit about your contributions?

Luke Leighton is an ethical technology specialist who has a consistent 24-year track record of developing code in a real-time transparent (fully libre) fashion, and in managing Software Libre teams. He is the lead developer on the Libre RISC-V SoC.

Hagen Sankowski is a Senior ASIC Design Engineer, with 20-year Experiences thru the whole Design Flow, from Digital Entry (Verilog HDL, VHDL) to DSM Backend and back. He has FPGA knowledge for Xilinx, Altera, Lattice and MicroSemi. Inventor and Patentee for a FPGA structure. Open Source Evangelist, member of the LibreSilicon project Team also.

Requested Amount

EUR 50,000.

Explain what the requested budget will be used for?

Improve the Wishbone B4 Specification to add streaming capability, comparable to AXI4-Stream, and feed the improvements back into the current stewardship for next Wishbone release.

Design Reference Implementations in nmigen and (System-)Verilog, Bus Function Models and other functionality in SystemVerilog for verification with full unit tests aiming best code coverage.

Use some of the Libre RISC-V SoC peripherals as a test platform and demonstrator (I2S Audio Streaming) for the proposed standard modifications.

Traveling expenses for presenting the Wishbone improvements to the RISC-V community once at the annual ORConf in 2020.

As a secondary objective: seek out existing (non-streaming) Wishbone Master and Slave Bus implementations (or implement them if necessary), provide formal proof unit tests of their correctness, and add additional example peripherals.

Does the project have other funding sources, both past and present?

The concept of extending Wishbone to have streaming capability is entirely new: it has no source of funding.

The Libre RISC-V SoC has funding from NLNet under a 2018 Grant: it was intending to use AXI4 prior to the U.S. Trade War.

Compare your own project with existing or historical efforts.

AXI4 has streaming (as AXI4-Stream) but it is proprietary and patented.

TileLink is an alternative protocol (with roots in the RISC-V academic community) but it is relatively new, quite complex, and does not have the same adoption as Wishbone.

There do exist a number of pre-existing Wishbone Bus Master and Slave implementations: Wishbone has been around for a significantly long time and has been the de-facto choice in the Libre/Open Hardware community. Formal correctness proofs for Wishbone have been written by Dan Gisselquist in SystemVerilog, but none are written in nmigen.

What are significant technical challenges you expect to solve during the project, if any?

This is a straightforward project. However the timing issues involved with Bus Negotiation can be awkward to get right and may need formal proofs to properly verify. Dan Gisselquist's work in his area shows how it can be done.

Describe the ecosystem of the project, and how you will engage with relevant actors and promote the outcomes

As mentioned in the 2018 submission, the Libre RISC-V SoC has a full set of resources for Libre Project Management and development: mailing list, bugtracker, git repository and wiki - all listed here:

In addition, we have a Crowdsupply page which provides a public gateway, and, reddit, phoronix, slashdot and other locations have all picked up the story. The list is updated and maintained here:

Extra info to be submitted