[PATCH] efi_loader: improve detection of ESP for storing UEFI variables

[Mark Kettenis]

> From: Paulo Alcantara <pc at cjr.nz>
> Date: Mon, 09 Nov 2020 10:24:08 -0300
> 
> Heinrich Schuchardt <xypron.glpk at gmx.de> writes:
> 
> > On 09.11.20 00:58, Paulo Alcantara wrote:
> >> The UEFI specification does not restrict on the number and location of
> >> ESPs in a system.  They are discovered as required by looking at the
> >> partition type, but firmware implementations are allowed to support
> >> ESPs which do not contain a valid partition type.
> >
> > I guess you refer to chapter "13.3.3 Number and Location of System
> > Partitions" of the UEFI spec saying: "Further, UEFI implementations
> > may allow the use of conforming FAT partitions which do not use the
> > ESP GUID."
> 
> Yep, sorry.  Thanks for pointing it out!
> 
> > Why should U-Boot support FAT partitions that are not of type FAT 0xef
> > and GPT partition that do not use the ESP GUID?
> 
> In most UEFI (EDK2-based) systems I used, I could boot my OSes and
> diagnostic tools by simply having a supported filesystem (FAT12/16/32)
> and /EFI/BOOT/BOOT{ARCH}.EFI file and never cared about setting the
> partition type at all.  It took me a while for figuring out why I
> couldn't get my UEFI variables loaded from my FAT partition that
> contained /ubootefi.var and /EFI/BOOT/BOOTAA64.efi files.

The OpenBSD installation media for armv7 and arm64 use a FAT partition
of type 0x0c because the Raspberry Pi firmware doesn't support 0xef.
This allows us to have a single FAT partition with the Raspberry Pi
firmware, U-Boot and /EFI/BOOT/BOOT{ARCH}.EFI.

So far this works on all UEFI firmware I've tried (EDK2, U-Boot and
AMI AptioV UEFI).