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

[Paulo Alcantara <pc@cjr.nz>]

Heinrich Schuchardt <xypron.glpk@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.

I am not trying to support ESPs that lack the appropriate partition type
everywhere in U-Boot, but at least for the UEFI variables (ubootefi.var)
feature seemed a nice thing to have.

Perhaps we could make it a separate config option like "Allow searching
for ESPs without partition type" and default it to N, don't know...