[Linux] Initramfs Guide

Introduction

To boot a Linux OS on an ARMv8 architecture CPU, at least three components must be present:

1. A bootloader to initiate the kernel (often U-Boot).
2. The Linux kernel itself.
3. A root file system (RootFS).

Initramfs (Initial RAM File System) acts as a bridge before the final file system is mounted. It is a RAM-based file system that identifies and mounts essential devices. Once the basic configuration is complete, it performs a pivot_root to transition to the main file system, such as Ubuntu.

For a method to boot without Initramfs, refer to: Booting Linux without Initramfs (Note: This is a legacy method used before Initramfs became standard.)

In contrast, the modern approach involves creating a file system tree, adding an init script, and packaging it using the mkimage command. Ensure that CONFIG_BLK_DEV_INITRD is enabled in your kernel configuration:

CONFIG_BLK_DEV_INITRD=y

mkimage

mkimage is part of the u-boot-tools package. It is used to wrap the file system image with a header that U-Boot understands.

Creating Initramfs

1. Prepare your root directory structure.
2. Create an init script.
3. Package it using cpio and gzip.
4. Wrap it with mkimage.

# Example command
find . | cpio -H newc -o | gzip -9 > ../initramfs.cpio.gz
mkimage -A arm64 -O linux -T ramdisk -d ../initramfs.cpio.gz ../initramfs.uimg