I read somewhere that it might be possible to boot non-EFI Linux from EFI Linux.
If my EFI Linux is booted by grub-efi, and my non-EFI Linux is booted by extlinux. How to chainload from grub-efi to extlinux?
AFAIK, the only boot managers that can chain load from EFI mode to BIOS/CSM/legacy mode are:
scanfor line in refind.conf and ensure that hdbios is among the options on that line. This option isn't 100% reliable, though; some motherboards don't support it, and even if your firmware does support it, the feature will probably boot only from the first hard disk.