RISC-V is a free, open, extensible instruction set architecture (ISA), the specification is now maintained by the nonprofit RISC-V Foundation
U540 is the first (and currently only) available Linux-capable RISC-V SoC.
There are three options you can use to run RISC-V:
External package repository until the target is merged:
# cat /etc/opkg/customfeeds.conf src/gz riscv64_kmod http://openwrt.uid0.hu/riscv64/generic/generic-glibc/packages src/gz riscv64_base http://openwrt.uid0.hu/packages/riscv64_riscv64/base src/gz riscv64_packages http://openwrt.uid0.hu/packages/riscv64_riscv64/packages src/gz riscv64_luci http://openwrt.uid0.hu/packages/riscv64_riscv64/luci src/gz riscv64_telephony http://openwrt.uid0.hu/packages/riscv64_riscv64/telephony src/gz riscv64_routing http://openwrt.uid0.hu/packages/riscv64_riscv64/routing
To download from this repo, you'll need to remove this line from /etc/opkg.conf:
option check_signature 1
(Obviously, this have to be at your discretion.)
There were some controversies around the full openness of the ZSBL/FSBL (around the DDR init code), but that has been fixed/released in July. There are also efforts to replace the FSBL with a U-boot port, but that will still require the BBL to be built.