manu found in the noted PR that overlays seemed to be clobbering the kenv
and killing the boot. Further inspection revealed that one can `fdt ls` at
the loader prompt for a successful boot, but autoboot breaks it.
In the autoboot case, first setup of FDT is happening in the middle of
bi_load, which triggers loading of the DTBO from /boot.
This is bad, bad, bad. Files in the loader are loaded somewhere in the
middle of the address space one after another. bi_load starts building the
needed kernel bootinfo immediately after the highest-addr loaded file. File
loads in the middle of bi_load suddenly clobber bootinfo and everything goes
off the rails.
The solution to this is to use take advantage of arch_autoload to setup FDT
in efiloader compiled with LOADER_FDT_SUPPORT. This matches how it works in
ubldr land, and is how it should have worked when overlay support was added
to efiloader since fdt_setup_fdtp now has the potential to load files
(courtesy of fdt_platform_load_dtb).
PR: 230804
Discussed with: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D16858
35437b1f16