On 2021-03-27 23:40, Joshua Kinard wrote:

> The MIPS machine has functioning local disk drives, and currently, it
> boots fine by just pulling a kernel off my TFTP server and booting
> from the local drive, no initramfs needed because I compiled
> everything into it.

Out of curiosity, if your kernel images already come from the TFTP 
servers why not simply put separate initramfs files there too?

> I wonder if there's a small C program out there that can call
> whatever the kernel functions are to mount a disk partition that
> could be embedded into a tiny initramfs, then pivot_root to
> $REAL_ROOT and run /bin/init?

You might be interested in this FOSDEM 2020 talk:

https://archive.fosdem.org/2020/schedule/event/ema_boot_linux_only/

Not exactly what you have asked for but the problem they are trying to 
solve is the same as yours - boot Linux on a system whose first-stage 
bootloader impose considerable size constraints. And since it uses kexec 
at its core, it's essentially what Rich has suggested - except it's 
already been (at least partially) done :-)

-- 
MS