How to boot to Grub instead of Bianbu directly?

Jupiter
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Aloha everyone!

I recently received my Jupiter in the mail, and have gotten it set up with Bianbu on an SD card. It’s my first time using a RISC-V computer, and it’s been a long of fun tinkering with something that isn’t x86 or ARM.

I’ve been looking to expand into installing Gentoo on here as well though, so I was wondering if anyone might know off the top of their heads how to boot to Grub instead of Bianbu’s kernel directly?

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You don’t.

You may have seen in the docs that it uses U-Boot instead of GRUB and requires specially made firmware images.

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I have indeed! My understanding is that u-boot provides a stripped down UEFI environment to boot Linux from - I was wondering if there’s a way to instead boot GRUB into it, like how you do on an x86 computer (UEFI → GRUB → Linux).

I found this post from the SiFive forums where they seem to have accomplished exactly this, but it involved patching the source code for u-boot and grub. I was curious if it the three years since, things had gotten any smoother, but it sounds like not?

If so, I was wondering if there might be any other ways to boot multiple operating systems on the Jupiter - is there a way to get a command line in u-boot for example? I tried spamming the usual function keys that you would use for an x86 uefi, but it didn’t seem to have any effect…

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First, yes, if your bootloader has capabilities to boot on a specific computer, then yes.
Second, yes, das U-Boot can pass control to anything after it.
Third, yes, you can have some convenient CLI in U-Boot by default (unless you disable it when compiling, IIRC); I don’t have a Jupiter board, but I bet you can gain access to the CLI of U-Boot via serial debug interface on the board.

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A little late, but what you could do is the following:

  • U-Boot has support for booting any EFI
  • Compile U-Boot with that feature enabled
  • Install grub
  • In the U-Boot environment, overwrite the bootcmd (iirc) variable to the command that loads and runs an EFI
  • Point that command to the grub EFI

I have only ever done that on x86 (where u-boot replaces the BIOS), but I would assume that would work the same in RISC-V. Does grub have RISC-V support?

References:

  • The U-Boot command to run an EFI
  • On my x86 machine, grub’s EFI file is at /efi/EFI/<distro name>/grubx64.efi. You could try installing grub on the Jupiter and check that location (or a similar one) and configure U-Boot’s bootefi command accordingly.
  • In the documentation on the bootefi command (linked above), they have an example that looks as follows:
load mmc 0:2 $fdt_addr_r dtb
load mmc 0:1 $kernel_addr_r /EFI/grub/grubaa64.efi
bootefi $kernel_addr_r $fdt_addr_r
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