DIY Fully working Alpine Linux for Allwinner and Other ARM SOCs

This is working progress - Till it completes - This is taken from multiple sources and is copyright of the respective authors albiet copied shamelessly.

Comprehensive Introduction

Why do we need alpine linux when there are so many xyz distros available? Well it is one the most lightweight platforms with hot swap support for SD-cards and USB devices. There are options for allocation of SD-card free space for application storage.

There are possibilities to completely upgrade a device running remotely with minimum downtime. Console logins are possible with USB null modems for field servicing. Most importantly you can be rest assured that it can survive against power cuts, restarts. For devices without onboard mmc, especially working on sd-card its way more important.

The philosophy of commit only when required, keeps the entire OS read-only and in-memory without touching the storage at all. Thus devices can survive for longer times without crash.

Arm devices unlike x86 dont come with bios in general. BIOS in x86 PCs is generally, a firmware configuring and connecting the hardware to the operating system and it offers support for a variety of OS and supports new OS versions.

ARM use a different approach involving a boot loader for hardware configuration and operating system start-up. The boot loader is developed specifically for the application, adapted to one well-defined hardware SOC configuration, one operating system and only one version of it, which means you cannot probably use it for other SOCs without significant changes.

We here generally talk about Allwinner especially H3 SOCs but you could probably apply this philosophy to other SOCs. The nanopi m1, Orange pi pc and Orange pi lite are only around $10 making them great home servers, firewalls for lowest possible cost.

A linux os cannot be started just like that on an ARM Device, without a small amount of machine specific code to initialize that system.

There are typically 4 stages involved in the ARM Device boot-up process

• (stage 1) ROM - Reads from initialized persistent storage (selected by boot mode), loads SPL into internal Ram
• (stage 2) SPL Loader - SPL once loaded does additional setup and loads from persistent storage bootloader (u-boot) into DDR RAM
• (stage 3) U-boot - U-boot once loaded continues the processor setup and reads the Linux Kernel into DDR RAM
• (stage 4) Kernel - Once Kernel is loaded, it boots Linux and initializes the user run time environment

Each stage adds functionality. It is important to note that SPL, U-Boot and the Linux Kernel are all statically linked to start running at specific locations from a memory map defined by the CPU. That memory map is nothing but a layout that defines where both internal memory and DDR are mapped to.

In here we are concerned with SD-Cards, and depending on how the SD card is connected, the location to where to write the data to can be different.

OKAY WAIT!!! WRITING DATA???

Dont be confused, yes you will need to compile from source the SPL Loader and write it to SD-Card in a particular way and then compile u-boot from source and write it in a particular and same goes for the rest. Otherwise how will you boot our favorite alpine linux?

But where do you get the sources? Generally if the SOC is supported in Linux (its called mainline) you can directly use the latest kernel, otherwise you will need to get the sources from the SOC manufacturer.

Below is a SD-CARD layout for typical Allwinner which generally combine U-boot with the SPL loader from block 8. This then loads initramfs etc.

"start" is a 1k-block number here. (Multiply it by two to get the corresponding sector number - assuming 512 byte sectors). This layout works generally for Linux Mainline Kernels 4.x and above

start	size	usage
0	8KB	Unused, available for partition table
8	1024KB	Initial SPL loader + u-boot (raw format preferred/vfat)
1024	till end	vmlinuz + initramfs + modloop + dtb (format fat + enable boot flag)

Notice I have combined SPL with U-boot, it is now possible to generate a single file say "u-boot-sunxi-with-spl.bin"