Booting - User device selection in U-Boot?

[amiraeva]

Yes, either Grub with menu, or the simple U-Boot device selection would work.
People doing testing could then select which OS to boot. If it fails, no problem,
just reboot and select the working OS and make a change to the one under
test. Then try again.

At present, I don't see Grub getting both text menu, (on the LCD), and keyboard
support, (also the PBP keyboard, not serial console), any time soon. Grub seems
to support such for x86/x64, but that is likely through BIOS calls that don't exist
for any ARM computer.

Thinking about it, I've added ESC & Return keys to the original post, to do the
obvious thing.

I got the PBP keyboard to work with u-boot and grub (using the arm64-efi target). It's as simple as adding a few flags when compiling u-boot to instruct it to enable the necessary USB support, and having "usb start" as part of the boot script (note this does slow down the boot by a second or two). Unfortunately the menu only displays over a serial console since u-boot doesn't know how to initialize LCD.

If anyone has any pointers on how to port the LCD/eDP drivers that linux uses to u-boot I'd love to hear it. The code for both are sooo similar, and unfortunately I wasn't smart enough to figure out the right combination of edits to the u-boot code to make it work (can only make the screen briefly flicker).