On the riscv64 port, the default boot method is UEFI, with U-Boot typically used as the firmware. This approach aligns more closely with other architectures, which avoid developping riscv64 specific code. For advanced users, booting using U-Boot and extlinux is possible, thanks to the kernel being built with CONFIG_EFI_STUB=y.

The same applies to the Debian Installer, which is provided as ISO images in various sizes and formats like on other architectures. These images can be put on a USB drive or an SD-card and booted directly from U-Boot in UEFI mode. Some users prefer to use the netboot "image", which in practice consists of a Linux kernel, an initrd, plus a set of Device Tree Blob (DTB) files.

However, booting this in UEFI mode is not straightforward, unless you use a TFTP server, which is also not trivial. Less known to users, there is also a corresponding mini.iso image, which contains all the above plus a bootloader. This offers a simpler alternative for installation, but depending on your (vendor) U-Boot version this still requires to go through a media.

Systemd version 257-rc2 comes with a great new feature, the ability to include multiple DTB files in a single UKI (Unified Kernel Image) file, with systemd-stub automatically loading the appropriate one for the current hardware. A UKI file combines a UEFI boot stub program, a Linux kernel image, an optional initrd, and further resources in a single UEFI PE file. This finally solves the DTB problem in the UEFI world for distributions, as a single image can work on multiple boards.

Building upon this, debian-installer on riscv64 now also creates a UEFI UKI mini.efi image, which contains systemd-stub, a Linux kernel, an initrd, plus a set of Device Tree Blob (DTB) files. Using this image also ensures that the system is booted in UEFI mode. Booting it with debian-installer is as simple as:

load mmc 0:1 mini.efo $kernel_addr_r # (can also be done using tftpboot, wget, etc.)
bootefi $kernel_addr_r

Additional parameters can be passed to the image using the U-Boot bootargs environment variable. For instance, to boot in rescue mode:

setenv bootargs "rescue/enable=true"