Hi Leo and Aiko,

This issue also affects powerpc64le-linux.

Aiko's patch would indeed fix the failing test.  Thank you, Aiko, for
taking the initiative to help investigate and solve the issue!  However,
even though that patch would fix the test, anyone who is using
set-xorg-configuration on a non-x86_64 system would still need to change
the way they call it, which is not ideal.

I've attached a different patch that attempts to fix the issue without
requiring callers of set-xorg-configuration to update their code.  I
believe this is more consistent with the intent of Ludo's original
change in commit 49599fab564f203b8e92d32e9b28c99e99849bfb.

You'll notice that this patch moves the (gnu services sddm) code into
(gnu services xorg) and deprecates (gnu services sddm).  I did this in
order to avoid a circular dependency between the two modules.  Perhaps
there's a better way.  However, many of the existing display managers
already live in (gnu services xorg), so it seemed reasonable to me.

I've verified that the tests/guix-system.sh test passes on
powerpc64le-linux after applying this patch to current master branch (on
top of commit 9309b488ca4ceef4fcc9283546e3b05c57b16ca8).  I've also
verified that the deprecation warnings are being emitted, and that the
existing bindings in (gnu services sddm) are still usable, at the REPL:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix repl
GNU Guile 3.0.7
Copyright (C) 1995-2021 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guix-user)> ,use (gnu services sddm)
scheme@(guix-user)> (sddm-configuration)
guix repl: warning: 'sddm-configuration' is deprecated, use '(@ (gnu services xorg) sddm-configuration)' instead
$1 = #<<sddm-configuration> sddm: [... omitted for brevity ...]
scheme@(guix-user)> (sddm-configuration? $1)
guix repl: warning: 'sddm-configuration?' is deprecated, use '(@ (gnu services xorg) sddm-configuration?)' instead
$2 = #t
scheme@(guix-user)> sddm-service-type
guix repl: warning: 'sddm-service-type' is deprecated, use '(@ (gnu services xorg) sddm-service-type)' instead
$3 = #<service-type sddm 4b3812c0>
scheme@(guix-user)> (sddm-service)
guix repl: warning: 'sddm-service' is deprecated, use '(@ (gnu services xorg) sddm-service)' instead
guix repl: warning: 'sddm-service' is deprecated, use 'sddm-service-type' instead
$4 = #<<service> type: #<service-type sddm 4b3812c0> value:
#<<sddm-configuration> sddm: [... omitted for brevity ...]
scheme@(guix-user)>
--8<---------------cut here---------------end--------------->8---

What do you think?

-- 
Chris

PGP: https://savannah.gnu.org/people/viewgpg.php?user_id=106836