It varies a lot by vendor. The older SiFive P550 is pretty good, see these forum threads: https://forums.sifive.com/t/where-is-the-source-code-of-the-bootloade

It varies a lot by vendor. The older SiFive P550 is pretty good, see these forum threads: https://forums.sifive.com/t/where-is-the-source-code-of-the-bootloader/6679 https://forums.sifive.com/t/where-is-the-source-code-of-the-... https://forums.sifive.com/t/building-opensbi-and-uboot/7681/2 https://forums.sifive.com/t/building-opensbi-and-uboot/7681/...