Rob Clark has a new 20-part RFC patch to U-Boot to significantly improve U-Boot’s UEFI support. I’ve included most of Rob’s comments below, see the patch for the code.

[PATCH v0 00/20] enough UEFI for standard distro boot

This patchset fleshes out EFI_LOADER enough to support booting an upstream \EFI\BOOT\bootaa64.efi (which then loads fallback.efi and then eventually the per-distro shim.efi which loads the per-distro grubaa64.efi) without resorting to hacks to hard-code u-boot to load a particular distro’s grub, or other hacks like setting up the distro installation as live-media. The first seven patches add dependencies that will be needed later in the series. Patches 8-15 make u-boot work with upstream grub, without relying on distro patches. Patches 16-19 add missing bits of the UEFI implementation needed to support shim/fallback. And finally patch 20 adds bootmanager support to avoid shim/fallback after first boot.

Background: with a normal UEFI implementation, the boot process is:

a) firmware (u-boot) looks at BootOrder and the BootXXXX variables to try to determine what to boot.
b) the firmware will look at the BootXXXX variables (which contain an EFI_LOAD_OPTION “struct” in order specified by BootOrder, and will boot the first bootable option.
c) The EFI_LOAD_OPTION specifies a device-path which identifies the device and file path of the .efi payload to exectute.

If there are no bootable options the firmware falls back to loading \EFI\BOOT\bootaa64.efi (exact name varies depending on arch), which then loads fallback.efi which uses the EFI_SIMPLE_FILE_SYSTEM_PROTCOL and EFI_FILE_PROTOCOL to search for \EFI\*\boot.csv, and will then set BootOrder and BootXXXX EFI variables accordingly so that on next boot fallback.efi is not necessary.

(I’m ignoring secure boot, it is out of scope here.)

For example, if you had both fedora and opensuse installed on the same disk in different partitions, you would have both:

+ \EFI\fedora\boot.csv
+ \EFI\opensuse\boot.csv

The former would contain the filename of \EFI\fedora\shim.efi and the latter to \EFI\opensuse\shim.efi (each of which would know to load \EFI\fedora\grubaa64.efi or \EFI\opensuse\grubaa64.efi). Based on this, fallback.efi would setup EFI_LOAD_OPTION’s Boot0000 and Boot0001 (and BootOrder would control the order the load-options are considered).

With a real UEFI fw there would also be some sort of boot-order menu (ie. hold down f9 at boot, and get a menu to pick which of the Boot* load-options to try first). That is not part of this patchset but would be a useful next step to allow installing multiple operating systems on the same disk.

This patchset provides EFI variable support during bootservices, so viewing or modifying EFI variables after linux ExitBootServices()’s is not possible. If the board supports saveenv() this will be called in efi_exit_boot_services() to persist variables that where set during the boot process. Making variables available after EBS is tricky on hardware that does not have dedicated storage, as after EBS u-boot no longer controls the devices. An approach that Alexander Graf suggested, is that since reboot/halt is controlled via UEFI, is that on boards that can ensure memory is persisted across reboot, to store modified EFI variables in a special memory location and turn halt into reboot uboot -> appropriate setenv() calls -> saveenv() -> halt in order to persist modified variables. Which is also not part of this patchset, and will likely require some board-specific help.

There will be some updates to this patchset depending on whether we move to c11 as Heinrich suggested (ie s/L”string”/u”string” and some changeups in the vsprintf patch). But rather than calling this an RFC (which I figured was more likely to get ignored for review) I am calling this a v0.

Thanks to Peter Jones for a couple of the patches, and a bunch of help understanding what the things above the UEFI fw expect (and fixing a few shim and grub bugs that we found along the way).

32 files changed, 2508 insertions(+), 329 deletions(-)

Full announcement from Rob:
https://lists.denx.de/pipermail/u-boot/2017-August/301203.html