This is my initramfs generator. I use this on my desktop Gentoo system. For my
laptop Arch system, I use mkinitcpio.

Basically, it's a suckless initramfs generator: it does exactly what I need.
You need something else? Patch it, it's only 117 lines.

Features:

- password LUKS unlock
- SSH remote unlock
- firmware loading
- extremely fast initramfs creation: compression off: ~70ms cold cache, ~30ms
  warm; lz4 --best: 1.6s
- extremely fast initramfs runtime: ~zero overhead compared to direct kernel
  loading
- minimal code: 117 SLOC total (init + make + Makefile + unlock)
- pure POSIX shell + POSIX Makefile

Drawbacks:

- no modprobe support. custom kernel is required to use myinitramfs. if you
  want configurability, probably better to use mkinitcpio or dracut.
- pure POSIX shell + POSIX Makefile

Usage:

0. Read (skim) https://wiki.gentoo.org/wiki/Custom_Initramfs.

1. Install busybox, dropbear, e2fsprogs (if you use ext4).

2. Adjust ./init as required:
   a) Adjust networking as required (e.g. maybe static IP instead of DHCP, or
      you don't use eth0)
   b) Adjust UUID (probably yours is not the same as mine)
   c) Adjust fsck as needed (if you are not using ext4)
   d) Put whatever you want:

3. Adjust ./make as required:
   a) Put your needed commands in gen_cpio_list.
   b) Select your desired compressor at the end. Try make comp_bench for a comparison.
      summary: lz4 is usually best, xz if storage is important above all (saves
      a few MB but adds ~0.5s to boot), gzip is mediocre on both size and
      speed, lzo is almost always worse than lz4, never use bzip2 or lzma.

4. Customize cpio_list.txt with your required files. Remember that commands
   must go in ./make (for library detection), and device files except
   /dev/console and non-empty directories can be omitted.

5. Add your public keys to authorized_keys. Note that dropbear only supports
   RSA and ECDSA keys.

6. make

7. sudo make install

8. Configure your boot loader/boot manager to use initramfs.img.