As I found out after I posted my lat entry , the correct way to find the Root pointer for the ACPI tables is to get it from the EFI System table. Where does that get set? Here’s the general flow: again, we start at init/main.c. start_kernel.   However, the call is not in the ACPI code, but rather in setup_arch. The call chain goes

start-Kernel->setup_arch->efi_init->efi_get_fdt_params and that seems to pull it our of initial_boot_params. I can’t quite see where that is initialized. Yet. From context it looks like it is constructed out of the kernel command line parameters. Still learning….