Claudio Carvalho of IBM submitted a 4-part patch to the Linux kernel to the linux(ppc-dev,-efi,-integrity,-kernel) lists.
This patch set is part of a series that implements secure boot on PowerNV systems.
See the comments for more info:
(I haven’t had time today to look at this code, but presume I think this means getting OPAL firmware to support Linux’s implementation of UEFI’s Secure Boot. There are two separate OpenPOWER UEFI implementations, neither of which are official, but I don’t think either of those are needed for this.)