SD cards speak SPI. In fact SPI is all SiFive supported on their first 2018 HiFive Unleashed. As you say, it's good enough to bootstrap. That old board has gigE

SD cards speak SPI. In fact SPI is all SiFive supported on their first 2018 HiFive Unleashed. As you say, it's good enough to bootstrap. That old board has gigE. Once you're up enough to TFTP you're away. I've never actually bothered to set that up on mine -- I boot a full kernel with NFS support and then switch to that.