Re: MicroSD-based USB boot manager for Pi?

bp@www.zefox.net wrote:

Is there some sort of USB boot manager for Raspberry Pi versions
that don't have an EEPROM? I'm thinking of armv7 Pi2 and early Pi3.
 
Bootcode.bin on an otherwise blank msdos microsd partition works,
but seemingly only with a single USB drive connected. A second
USB disk left in place by mistake caused the machine to hang.
 
Something installed on a microSD that finds and reports bootable
USB drives, defaulting to the last one used, would be ideal.
 
This video from about 2016 suggests it's already been done:
https://www.youtube.com/watch?v=bNL1pd-rwCU
 
But, no implementations turned up in a web search. All the boot
manager references I found seemed to involve the Pi4. I'm interested
in using older machines already on hand. As it happens, most of
my machines run FreeBSD which uses u-boot as an intermediate step
but I don't think that matters.

There's no reason why you can't use u-boot as a boot manager for Linux too.
Many non-Pi Linux boards do this.  However it'll only handle the Linux
kernel - if you have different firmware, config.txt etc then u-boot is
unaware of that - it runs too late in the boot process to change those
things.  Really u-boot can only change the Device Tree, kernel,
initramfs and kernel environment (variables, command line).  U-boot is
running on the CPU, and so can't affect the GPU which is booted first on Pis
1-3.

EDK2, a UEFI bootloader (similar to a PC BIOS) is also a possibility, but
the same limitations will apply.

Theo