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1. Introduction

I have two Raspberry Pi 4 which do not have any purpose currently. I bought them a while ago to learn Kubernetes with `K3s`, and that mission is now complete. As I think that minimalism and simplicity will save us all (hopefully!), I want to explore how minimal of a Linux I am able to install on the RPi.

This will be a more or less accurate log of my progress and may be a bit chaotic here and there as I jump forth and back. So please read first before you try, to see if it makes sense to you :)

Project goals

use an SD card friendly file system for the partition holding the OS (the boot partition has to be `vfat` though), disable `atime` and `nodiratime` (access time stamps) for all partitions to preserve life time of SD card
make the boot partition as small as possible and include indispensable files only, it still needs enough free space to hold a second kernel whenever I want to upgrade
do not use any Linux distribution, compile everything from scratch
static builds only, no dynamic linking at all (I think `glibc` doesn't support that entirely, so need to look into `musl` or `dietlibc`)
configure the Linux kernel to be as minimal as possible (disable support for modules, no initramfs, ...) and tightly adjust the settings to exclude everything that's not indispensable
optimise boot times to be as fast as possible (kernel configuration, EEPROM settings, `config.txt`, ...)
in spite of all the minimalism I still want to end up with a completely usable system, compile and run Xorg and Blender to prove that point
overclock the RPi without risking damage or stability (I've already attached additional heat sinks and a small fan, so I should be able to overclock somewhat)
although there's initial cross-compilation needed using my existing x86 Debian Linux PC, I want to end up with a system which can completely compile itself
make use of hardware H.264 decoding capabilities
use `vcgencmd` for temperature monitoring (lm-sensors is inaccurate) [1]
do not use swap at all, as this can reduce the lifespan of the SD card [2]

Ideas & notes:

try `F2FS` as file system for OS partition
for initial Linux setup it's probably helpful to check the LFS (Linux from scratch) documentation
try to compile kernel with musl instead of glibc, although this should have minimal effect
set USB enumeration delay time as short as possible [3]
use a smaller init system
`/dev` should be as minimal as possible
replace `coreutils` with something smaller, see [4] and [5]
remove everything in `/etc/group`, `/etc/passwd`, and `/etc/shadow` which is not immediately needed, maybe do a search on the filesystem to find all users and groups in use, understand their purpose

Questions:

is permanent "performance" CPU governor sustainable?
do I want RTOS (real time OS) patches for the Linux kernel?
if we can't use anything else than the `ext` family of file systems for the OS partition, should we switch off journalling to preserve life time of SD card?
can we get rid of most, if not all overlay files on the boot partition? I've seen on a previously installed Debian that it doesn't come with those files, so we need to check if they're optional. If not, only keep indispensable files
the H.264 capability on the RPi 4 is only decoding, right (no encoding)?
does the kernel compile with `tcc`?
in general, should we keep using `gcc` or switch to a smaller compiler? (`tcc`, ...?)
there's a copy of the kernel located in `/boot` and another one in `/boot/firmware`, can we remove the former one?
can we use uncompressed kernel image?
can we speed up `IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready` step at boot?
if we don't use any package manager, how can we track installed files per component for easy uninstall?
what IO schedulers do we want to use for which media? [6]

[1] Measuring Temperatures

[2] Debian Wiki

[3] Using ‘delay_use’ to speed up USB Enumeration

[4] Core utilities alternatives

[5] suckless.org 9base

[6] Selecting a Linux I/O Scheduler

➡ 2. Making a plan

⬆ The minimal Raspberry 4 project

🏠 callistix Gemini capsule

Created: 6/Jan/2024

Modified: 9/Feb/2024