root@desktop:/sbin# grep '^#' mkinitrd 
...

# Note:
# ----
# This must be done after all options have been handled and
# not when handling case '-o' because -k can be passed after
# -o in which case, the tag %KVER% would be replaced with the 
# current kernel version instead of version passed with -k.
#
# If kernel modules are needed but the kernel version is absent, exit now:
# If clearing source tree was requested, do that first
# If there's no $SOURCE_TREE, make one now:
# If $ROOTDEV and $ROOTFS are not set, assume we want the
# values for the currently mounted /
# (unless we find that values are already set in the initrd-tree):
# If needed, write them in the initrd-tree:
# Put a copy of the command line used to create this initrd in
# initrd-tree/command_line:
# If /etc/mkinitrd.conf exists, put a copy of that in there as well.
# (May or may not have been used according to the -F option, but we'll
# save a copy anyway just in case):
# If $WAIT is not set, assume we need only one second
# to have all devices done
# (unless we find that value is already set in the initrd-tree):
# If no OUTPUT_IMAGE was specified, read it from the SOURCE_TREE if possible:
# If we still have no value, apply the default:
# Finally, write the image name into the SOURCE_TREE:
# Fill /resumedev with the swap partition holding the hibernation image
# Add custom keymap support if one was given
# If LUKSDEV was set in the config file, then we need to set CRYPT=1
# Check for LUKSTRIM
# If LUKSKEY was set in the config file, then give it a warm welcome:
# Include RAID support in initrd
# Include Btrfs support in initrd
# Include udev in initrd
#!/bin/ash
# This space is intentionally left blank
# Include LVM support in initrd
# Include cryptsetup (LUKS) support in initrd
#!/bin/sh
# Include device mapper in initrd (needed for LUKS and LVM)
# If there is additional filesystems to be mounted, copy their configuration
# from system's /etc/fstab:
# Make module directories:
# Copy kmod/modprobe stuff to initrd:
# Make sure modules.builtin and modules.order are there (for kmod):
# If an executable $SOURCE_TREE/load_kernel_modules already exists, then
# we assume you will want to load the kernel modules mentioned in there.
# This means, you do not have to explicitly add those on the commandline:
# If the module list is not empty, copy the modules into place:
# Copy needed libraries
# Make sure all libraries have symlinks:
# And finally, build the initrd:
root@desktop:/sbin#

mkinitrd -F

# mkinitrd.conf.sample
# See "man mkinitrd.conf" for details on the syntax of this file
#
SOURCE_TREE="/boot/initrd-tree"
CLEAR_TREE="1"
OUTPUT_IMAGE="/boot/initrd-5.15.38.gz"  <----- I change/override this
#KERNEL_VERSION="$(uname -r)"
KERNEL_VERSION="5.15.38"  <----- and this
KEYMAP="fr"
#MODULE_LIST="ext4"
This is to adapt to your needs, if you have usb input devices or disks you want to have avaible early. For instance, I want to use a fex when docked to type my LUKS passphrase
MODULE_LIST="xhci-pci:ohci-pci:ehci-pci:xhci-hcd:uhci-hcd:ehci-hcd:hid:usbhid:i2c-hid:hid_generic:hid-asus:hid-cherry:hid-logitech:hid-logitech-dj:hid-logitech-hidpp:hid-lenovo:hid-microsoft:hid_multitouch:jbd2:mbcache:crc32c_intel:crc32c_generic:ext4"
LUKSDEV="/dev/sdb3"
# verify support with 'hdparm -I $dev | grep TRIM'
LUKSTRIM="/dev/sdb3:/devsda2"
#LUKSKEY="LABEL=TRAVELSTICK:/keys/alienbob.luks"
ROOTDEV="/dev/cryptvg15/root"
ROOTFS="ext4"
RESUMEDEV="/dev/cryptvg15/swap"
#RAID="0"
LVM="0"
UDEV="1"
MODCONF="0"
MICROCODE_ARCH="/boot/intel-ucode.cpio"
#WAIT="1"