Initramfs
Initramfs consists of busybox, a shell script and very few other command-line programs.
postmarketos-mkinitfs generates the initramfs.
After then, postmarketos-mkinitfs would run boot-deploy and boot-deploy would pack kernel and initramfs into a suitable format and generate config for the bootloader if necessary. Some devices with small boot partitions may need to generate an initramfs-extra by setting deviceinfo_create_initfs_extra=true.
The initramfs takes care of many things during the early booting phase, such as setting up USB networking, resizing the root partition on first boot, unlocking the root filesystem if full disk encryption is enabled, and more. Some of the tasks are listed in detail here, for others refer to the source code for now and consider expanding this article. Furthermore, the initramfs supports hooks, which may be used to work around device specific quirks, for example.
Here is a list of the boot process related to initramfs:
- Load device config from the deviceinfo file
- Initialize busybox
- Mount /dev, /proc, and /sys
- Configure logging
- Enable USB networking support for debug in the initramfs, if applicable
If the device is using an initramfs-extra, then the initramfs will:
- Attempt to mount any subpartitions that exist
- Detect and mount the postmarketOS boot partition and extract the archive
- Run mdev to detect/initialize devices
- Execute
jump_init_2ndto continue booting with the steps below as if it were using a single unified initramfs
If the device is using a single unified initramfs archive, then:
- Run hooks
- Launch debug shell if key combo is used
- Find, unlock(if necessary), and mount the postmarketOS root partition. This may also run fsck on filesystems
- Run cleanup hooks
- Switch to the root filesystem, exiting the initramfs, and continue booting the OS
Find the boot and root partitions
Due to the various possible partition layouts, the initramfs script has to look in various places for the postmarketOS boot and root partitions.
boot partition:
pmos_bootkernel parameter (from the device specific bootloader)- Any partition or subpartition with the
pmOS_bootlabel
root partition:
pmos_rootkernel parameter (from the device specific bootloader)- Any partition or subpartition with the
pmOS_rootlabel - Any partition or subpartition that is encrypted
Load initramfs-extra
Some Android and ChromeOS devices require a very small initramfs, because otherwise the resulting boot.img file is too big for their boot partition. Therefore, it's possible to split the initramfs into two parts, the tiny initramfs that is potentially placed in such a boot.img file (depending on the bootloader). And the initramfs-extra, which can hold bigger binaries, such as the UI to unlock an encrypted root partition (Unl0kr). mkinitfs can be instructed to split the archive by setting the deviceinfo_create_initfs_extra
Switch root
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It will look like this, once pmaports#664 is resolved. |
At the end of the initramfs script, the root partition is mounted at /sysroot, and the boot partition is mounted at /sysroot/boot. The latter is done, so Init does not need to figure out the correct boot partition later on. Afterwards, the busybox command switch_root is called to switch over and pass control to Init.
Debugging
The initramfs includes a local debug shell that can be launched on boot by holding Volume-Up on a touchscreen-only device or Left Shift on a device with a keyboard.
The debug shell is also run automatically if the initramfs fails boot.