RISC-V 32-bit Linux From Scratch

RISC-V has a growing following with an increasing number of commercial vendors of RISC-V compliant cores, as well as many open-source implementations.
RISC-V has had a Linux port for a while, however only the 64-bit variant of RISC-V is well supported.

Whilst the 64-bit variant has distro support, the 32-bit target is somewhat unloved and it can be harder to build a working kernel for it.
There are build systems such as Yocto which enable complete Linux distributions for embedded systems to built.

This isn’t that - this is a minimal GCC + Linux Kernel + Busybox build flow targetting RISC-V 32-bit (RV32IMA).

Getting Started

0. Build system deps

# Ubuntu
sudo apt-get install autoconf automake autotools-dev curl python3 libmpc-dev libmpfr-dev libgmp-dev gawk build-essential bison flex texinfo gperf libtool patchutils bc zlib1g-dev libexpat-dev git libncurses-dev openssl libssl-dev dkms libelf-dev libudev-dev libpci-dev libiberty-dev

1. Cloning

To clone this repo;

git clone https://github.com/ultraembedded/riscv32_linux_from_scratch.git

2. Acquire external source

# Clone Busybox
make clone_busybox
# Clone Linux Kernel and checkout known working release for RV32
make clone_kernel

3. Clone and build toolchains

Skip this if you already have riscv32-unknown-elf-gcc and riscv32-unknown-linux-gnu-gcc already built;

# Clone toolchain repositories
make clone_toolchain
# Build riscv32-unknown-elf-gcc for RV32IM
make build_gcc_newlib
# Build riscv32-unknown-linux-gnu-gcc for RV32IMA
make build_gcc_linux
# This results in ./toolchain/riscv32im and ./toolchain/riscv32ima-linux being created after a long wait...