Update June 2020

Tock includes preliminary RISC-V support, and several boards that are RISC-V based. There are a few lingering TODOs to get reasonably comparable support with Cortex-M.

Tracking

• Add support for risc-v (at least riscv32imac) in nightly rust. (RISCV Arch Support rust-embedded/wg#36, Enable RISCV rust-lang/rust#52787)
• Simplify binutils for riscv32. Ideally the LLVM tools that ship with rust will work. But if not we need some way to distribute riscv32-ld and riscv32-objcopy that doesn't rely on users building from source.
  • Upstream lld support for riscv: Upstreaming riscvarchive/riscv-lld#1. First PR is in! llvm-mirror/lld@9e54d15
• Develop support for risc-v in Tock.
  • Add a RV32I arch crate. RISC-V: Add context switching #1323
  • Create a HiFive1 board in /boards. PR: Add initial RISC-V support + HiFive board #1317
  • Create an arty-e21 board for running the E21 core on the ARTY FPGA. RISC-V: Add context switching #1323
  • Implement a driver for the Physical Memory Protection (PMP) peripheral.
  • Implement a driver for the CLIC peripheral. RISC-V: Add context switching #1323
  • Implement UserlandKernelBoundary for risc-v. This would enable context switches to userland. RISC-V: Add context switching #1323
  • Implement a systick replacement for providing timeslices for apps.
  • Support the HiFive1(b) board. https://github.com/tock/tock/tree/hifive1b
  • Add better fault printing and debugging support.
• Add support to Tockloader for loading code on risc-v boards. It seems like they use openocd, but unfortunately it seems to be a custom fork. https://github.com/riscv/riscv-wiki/wiki/RISC-V-Software-Status#openocd Status: this exists for arty-e21, pending for HiFive1.
• Add links to external documentation. Make it easier for developers to find information about the ISA, peripherals, and sifive-specific peripherals.
• Support userland c apps
  • Update crt0.c and tock.c to support risc-v. On libtock-c: make RISCV=1.
  • Get PIC working. This seems like it might be a ways off: Support for -mno-pic-data-is-text-relative? riscvarchive/riscv-gcc#158
    • Alternative: use N x K linking Tracking: Implement N x K Application Linking libtock-c#63
  • Verify various more advanced apps work.

Intro

This RISC-V architecture continues to gain momentum, and having Tock support risc-v boards seems quite feasible. With the embedded version still in development, it seems like Tock could be a leading embedded OS for risc-v platforms. Also, supporting multiple architectures would demonstrate the Tock architecture flexibility.