Initramfs development

From postmarketOS Wiki

This article explains how to modify the postmarketOS initramfs scripts, for which the source can be found in the postmarketos-mkinitfs pmaport. There's another page about adding a device specific initramfs hook.

We have two initramfs files

After the installation, you will have two initramfs files in your device's /boot folder.

% ls ~/.local/var/pmbootstrap/chroot_rootfs_samsung-i9100/boot
initramfs  initramfs-extra  vmlinuz

Regular Linux distributions only have one initramfs file. We have split it into, historically this was to work around the size limitations that some devices have (see #126). However for most devices this is not needed and -extra will soon be made redundant Template:Mr. The regular initramfs loads the contents of the initramfs-extra from the pmOS boot partition during the boot process.

The pmOS boot partition is a subpartition of the system partition (SD cards: simply a real partition on the SD card), so we can make it almost as big as we want.

Recommended development workflow

  • Only do one small change at a time
  • Rebuild and test on device (pmbootstrap initfs, pmbootstrap flasher flash_kernel)
  • When it works, version your change with git
  • Repeat

When you change too much, it will simply not work and you most likely will have no idea why it does not work. Go back to the last working version and do smaller changes at a time.

Hint Familiarize with inspecting the initramfs to learn more about available debugging options.

Regenerating the two initramfs from the phone

You can regenerate both initramfs, even if the first is in a boot.img by running as root

 sudo mkinitfs

Tips and tricks

  • pmbootstrap initfs can do more than rebuilding the initramfs:
% pmbootstrap initfs --help
usage: pmbootstrap initfs [-h]
                          {hook_ls,hook_add,hook_del,ls,build,extract} ...

positional arguments:
    hook_ls             list available and installed hook packages
    hook_add            add a hook package
    hook_del            uninstall a hook package
    ls                  list initramfs contents
    build               (re)build the initramfs
    extract             extract the initramfs to a temporary folder

optional arguments:
  -h, --help            show this help message and exit
  • Arbitrary files can be included from an APKBUILD by installing them to /usr/share/mkinitfs/files{-extra}:


package() {
    # ...
    install -Dm644 "$srcdir/30-my-custom-files.files" "$pkgdir/usr/share/mkinitfs/files/30-my-custom-files.files"



Manual unpacking and repacking the initramfs file

$ zcat initfs.gz | sudo cpio -iv --make-directories # unpacks
$ find . | sudo cpio -ov --format=newc | gzip -c -1 > initfs-new.gz # packs


My device reboots with my customized init script

This is probably caused by a syntax error. Use shellcheck to check your scripts for syntax errors. You could also loop forever or spawn a shell (if you have a hardware keyboard or serial cable connected) at some point for debugging (while true; do sleep 1; done) and use the telnet shell to test the commands, that are failing, interactively.

No log output on serial!

By default, all output from the initramfs is logged to the kernel ringbuffer, ensure you have console= set correctly on the kernel cmdline and that loglevel is >= INFO (level 6), if in doubt add ignore_loglevel to the cmdline. If you want the output through the debug cable, make sure quiet is NOT on the kernel command line.

How do i show logs on the screen at boot?

In order to disable the postmarketOS splash add PMOS_NOSPLASH to your kernel command line, in addition if you want the kernel logs to print to the screen make sure that console=tty0 is set in the kernel command line.