#988876 grub-efi-amd64: outb command no longer works

Package:
grub-efi-amd64
Source:
grub2
Description:
GRand Unified Bootloader, version 2 (EFI-AMD64 version)
Submitter:
Marco Kühnel
Date:
2021-05-21 15:51:03 UTC
Severity:
important
Tags:
#988876#5
Date:
2021-05-20 16:13:16 UTC
From:
To:
After a fresh install of debian buster a grub.cfg containing an outb command throws the error "outb command not found" (and, in my case,
cannot start lightdm thereafter). This problem does not appear when downgrading all installed grub packages to version
2.02+dfsg1-20+deb10u3.
#988876#10
Date:
2021-05-20 22:02:16 UTC
From:
To:
Can you clarify whether you have UEFI Secure Boot enabled?  The outb
command had to be disabled in Secure Boot mode, because it could be used
to subvert boot security mechanisms.  Systems that use the outb command
will have to disable Secure Boot, or find some other way to achieve the
same effect.

I'm just noting explicitly here that these are local customizations to
grub.cfg (presumably by way of editing /etc/grub.d/10_linux or similar),
and not in the default setup.
#988876#15
Date:
2021-05-21 15:40:43 UTC
From:
To:
UEFI Secure Boot is disabled. The machine is a Macbook8,3 converted to a pure
Linux box, however. So, the EFI firmware might be damaged. Thank you for the
clarification.

In order to disable the (not properly supported) discrete graphics card, I
need to execute the mentioned outb commands. Indeed, I edited /etc/grub.d/
10_linux in order to do this.