bootstd: Support for distro specific EFI folders

[Heinrich Schuchardt]

On 11/16/23 17:52, Shantur Rathore wrote:
> Hi Simon,
> 
> Currently bootstd - bootmethod_efi only looks for the default
> bootxx64.efi in /EFI/boot folder only.
> Generally many distros end up putting their bootloaders in
> EFI/<distro> folders like EFI/ubuntu and EFI/debian etc.
> 
> In x86 worlds, the NVRAM is modified and new boot entries are added to
> support these but in the U-boot world the NVRAM variables are
> read-only.

I guess you are referring to UEFI boot options. These typically are not 
stored in non-volatile RAM but on a SPI flash device.

> 
> What would be the best way to implement this?
> 
> I was thinking of having a "efi_prefixes" environment variable which
> can be set to "ubuntu debian centos" etc and bootmethod_efi can try
> all of them. Will bootmethod_efi be able to support multiple entries (
> thinking of multiboot ) ?

On my laptop I have:

EFI/Microsoft/Boot/bootmgr.efi
EFI/Microsoft/Boot/memtest.efi
EFI/Boot/bootx64.efi
EFI/Boot/fbx64.efi
EFI/Boot/mmx64.efi
EFI/debian/shimx64.efi
EFI/debian/grubx64.efi
EFI/debian/mmx64.efi
EFI/debian/fbx64.efi
EFI/ubuntu/grubx64.efi
EFI/ubuntu/shimx64.efi
EFI/ubuntu/mmx64.efi

Obviously each installed operating system provides multiple EFI binaries 
and non uses the fallback file name BOOT<ARCH>.EFI. A value "ubuntu 
debian centos" would not be able to describe which file you are looking 
for.

We already have the U-Boot command line eficonfig and efidebug commands 
to set up UEFI boot options which can describe which EFI binary to load 
and which command line to pass to it. These are considered by the 
existing boot flows.

If you are installing the shim-signed package on Ubuntu, the EFI boot 
option for Ubuntu is set up by EFI/BOOT/BOOT<ARCH>.EFI using the content 
of EFI/ubuntu/BOOT<ARCH>.CSV. This is done before ExitBootServices() and 
therefore should work with current U-Boot.

Patches are pending upstream to make EFI variables writable from Linux 
if they are stored in the RPMB partition in the eMMC. See this series:

https://lore.kernel.org/linux-efi/20231107054057.1893-2-masahisa.kojima@linaro.org/

Best regards

Heinrich