Linux

Thank you very much all, for your inputs !

I've did

root: file /boot/broadcom/initrd.gz
initrd.gz: Zstandard compressed data ....

root: unmkinitramfs /boot/broadcom/initrd.gz Extracted/
# data where extracted to Extracted/
# but I go few error like:
# cpio: cannot link usr/sbin/vconfig to usr/sbin/watchdog: Operation not permitted

Extracted//
|-- conf/
|   |-- arch.conf*
|   |-- conf.d/
|   `-- initramfs.conf*
|-- etc/
|   |-- fstab*
|   |-- ld.so.cache*
|   |-- ld.so.conf*
|   |-- ld.so.conf.d/
|   |-- modprobe.d/
|   `-- udev/
|-- init*
|-- run/
|-- scripts/
|   |-- functions*
|   |-- init-bottom/
|   |-- init-top/
|   |-- local*
|   |-- local-bottom/
|   |-- local-premount/
|   `-- nfs*
`-- usr/
    |-- bin/
    |-- lib/
    `-- sbin/

So it tend to confirm that even if the file is named initrd.gz it's actually an initramfs method.... ( damn this is so misleading )

So I guess I can follow preparing-linux from the guide and overwrite the initrd.gz ?

So, firstly, about the nomenclature: initrd (initial ram disk) and initramfs (initial ram file system) are usually used interchangeably as far as I know. For example, even though my Debian uses initramfs-tools, the generated images are called /boot/initrd.img-*. (Edit: There is a technical difference but an initramfs may be referred to as an initrd (like in this case) due to how similar they are.)

For example, when installing a kernel, apt shows this output on my Debian machine:

linux-image-6.12.6-amd64 (6.12.6-1) wird eingerichtet ...
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-6.12.6-amd64

What you're talking about is probably the software used to generate this initial ramdisk, which on Debian is done using initramfs-tools (which contains the mkinitramfs command), while on other distros dracut (command: mkinitrd) might be used.

I will say it strikes me as weird that Devuan doesn't use initramfs-tools since it's a Debian derivative. Maybe you are mistaken about this? Possibly no initrd/initramfs is used at all on this specific Pi version of Devuan? IDK.

Edit: See my other comment. I'm wrong. There is an actual technical difference between initrd and initramfs, but I don't think that's actually relevant in this situation. Or rather, both are functionally the same, so it doesn't really matter from the perspective of the user or distro that there's a difference. I will keep the rest of the comment as is, since I do reckon OP's problem is unrelated to this difference, and that probably something else is tripping up OP.

AFAIK initramfs is the same thing as initrd. But do you have update-initramfs command available?