Vivien,

Thanks for the report!  Sorry it took me a while to get back to 
you.

As discussed in #guix, an immediate work-around is:

  (initrd-modules
    ;; This cannot be built as a module for linux-libre-lts.
    ((@ (srfi srfi-1) delete) "simplefb" %base-initrd-modules))

I'm not sure what to do going forward.  I see two reasonable 
solutions.  Perhaps there are more, and I welcome suggestions.

One is to provide a separate %base-initrd-modules-lts list that's 
equivalent to the snippet above.  It's… not great, but then 
linux-libre-lts itself isn't documented and people still seem to 
find it.  If someone were to document it, they could also add a 
note about modules.

A more straightforward route is to simply build in simplefb even 
for kernels 5.15 and beyond.  Apart from an (unmeasured) size 
increase of the base image, the main drawback here is that we lose 
our long-standing CONFIG_DRM_SIMPLEDRM=m due to a conflict:

- CONFIG_DRM_GEM_SHMEM_HELPER=y
- CONFIG_DRM_SIMPLEDRM=m
+ # CONFIG_DRM_SIMPLEDRM unset tristate: Simple framebuffer driver
- CONFIG_FB_SIMPLE=m
+ CONFIG_FB_SIMPLE=y tristate: Simple framebuffer support

When preparing <https://issues.guix.gnu.org/51619> I considered 
this a regression and hence bad, but maybe nobody cares…?

Kind regards,

T G-R