Notes taken at build of Sbox20220116 (compatibility PI4 and upgrade Python 2.7 => Python 3 required a build from scratch).
It's possibly not 100% accurate (nightly builds...) and may have had some corrections afterwards.
Also: it's for Buster, which is outdated by now..


Some info first:
================

https://www.raspberrypi.org/documentation/computers/config_txt.html
https://community.blokas.io/t/pisound-with-raspberry-pi-4/1238/12
https://www.hifiberry.com/docs/software/configuring-linux-3-18-x/

find /etc/systemd -name \*.service -type l -a -xtype f -printf %f\\n
journalctl -axe

dnsmasq config (add !): dhcp-leasefile=/var/lib/misc/dnsmasq.leases -> solved with ln below

$ sudo apt-get install usbmount
The following NEW packages will be installed:
  liblockfile-bin liblockfile1 lockfile-progs pmount usbmount
0 upgraded, 5 newly installed, 0 to remove and 0 not upgraded.
Need to get 142 kB of archives.
After this operation, 666 kB of additional disk space will be used.
sudo nano /etc/usbmount/usbmount.conf (filesystems)

/lib/systemd/system/systemd-udevd.service
Before: PrivateMounts=yes
After: PrivateMounts=no

The actions:
============

Install buster on 2Gb SD
Start (buster autoresizes)
Copy to >4Gb SD
fdisk delete+define P2 with +4Gb, don't erase ext4 signature !
reboot, resize2fs
$ sudo raspi-config: hostname samplerbox
set password to sbox via sudo passwd pi
$ sudo dpkg-reconfigure keyboard-configuration
  and then "other" -> US (generic) and reboot
https://www.element14.com/community/community/raspberry-pi/blog/2019/09/14/asla-audio-glitch-issue-pi-4-fix
https://linuxconfig.org/how-to-change-default-python-version-on-debian-9-stretch-linux
https://setuptools.readthedocs.io/en/latest/python3.html

Dependencies:
sudo apt-get install python-dev libasound2-dev python-setuptools swig libportaudio2 libffi-dev python3-pip libatlas-base-dev ntfs-3g libjack-dev libjpeg-dev libopenjp2-7 libtiff5 midisport-firmware dialog
sudo pip3 install cython psutil RPi.GPIO smbus sounddevice pyalsaaudio st7789 numpy 2to3
===> ##rtmidi2
===> # manual install rtmidi2 requires libjack-dev
OLED:
$ sudo -H pip3 install --upgrade luma.oled

NTFS:
fdisk: change type to "7" (HPFS/NTFS/exFAT)
Format with:
mkfs.ntfs -f /dev/sdx1
umount /samples
mount /dev/mmcblk0p3 /samples


<---------->
Make it readonly: (https://medium.com/swlh/make-your-raspberry-pi-file-system-read-only-raspbian-buster-c558694de79)

- Housekeeping

$ sudo apt-get update && apt-get upgrade
$ sudo apt-get remove --purge triggerhappy logrotate dphys-swapfile
$ sudo apt-get autoremove --purge
$ update /boot/cmdline.txt: add " fastboot noswap ro"

- Replace your log manager

$ sudo apt-get install busybox-syslogd
$ sudo apt-get remove --purge rsyslog
  ... From now on use sudo logread to check your system logs

- Make the file-systems read-only and add the temporary storage

Update the file /etc/fstab and add the ,ro flag to all block devices (/boot and /)
and add:
tmpfs        /tmp            tmpfs   nosuid,nodev         0       0
tmpfs        /var/log        tmpfs   nosuid,nodev         0       0
tmpfs        /var/tmp        tmpfs   nosuid,nodev         0       

- Move some system files to temp filesystem

Note: This part is different from previous Raspbian versions (Stretch etc.). On Raspbian Buster do NOT move the /var/lock and /var/run directories as they are already symlinked to tmpfs directories. You can read more about these changes in the Debian Buster tmpfs documentation.
$ sudo rm -rf /var/lib/dhcp /var/lib/dhcpcd5 /var/lib/misc/dnsmasq.leases /var/spool /etc/resolv.conf
$ sudo ln -s /tmp /var/lib/dhcp
$ sudo ln -s /tmp /var/lib/dhcpcd5
$ sudo ln -s /tmp/dnsmasq.leases /var/lib/misc/dnsmasq.leases
$ sudo ln -s /tmp /var/spool
$ sudo touch /tmp/dhcpcd.resolv.conf
$ sudo ln -s /tmp/dhcpcd.resolv.conf /etc/resolv.conf

- Update the systemd random seed

Link the random-seed file to the tmpfs location:
$ sudo rm /var/lib/systemd/random-seed
$ sudo ln -s /tmp/random-seed /var/lib/systemd/random-seed
Edit the service configuration file /lib/systemd/system/systemd-random-seed.service to have the file created on boot. Add the line ExecStartPre=/bin/echo "" >/tmp/random-seed under the [Service] section.
The modified [Service] section should look like this:
[Service]
Type=oneshot
RemainAfterExit=yes
ExecStartPre=/bin/echo "" >/tmp/random-seed
ExecStart=/lib/systemd/systemd-random-seed load
ExecStop=/lib/systemd/systemd-random-seed save
TimeoutSec=sudo pip install psutil30s

- get rid of rfkill: https://unix.stackexchange.com/questions/597292/how-to-disable-rfkill-after-it-has-been-uninstalled-from-raspbian
sudo systemctl disable rfkill
sudo systemctl stop rfkill
echo "rfkill" | sudo tee -a /etc/modprobe.d/raspi-blacklist.conf
sudo mv /lib/systemd/{systemd-rfkill,system/systemd-rfkill.{service,socket}} ~
echo "0" | sudo tee /var/lib/systemd/rfkill/*
Inside /lib/systemd/system/raspberrypi-net-mods.service, I commented out:
#After=systemd-rfkill.service
And finally appended "systemd.restore_state=0 rfkill.default_state=1" to /boot/cmdline.txt



At this point after rebooting you should have a working Raspberry Pi in read-only mode

<---------->

- Useful commands:
Edit the file (/etc/)bash.bashrc and add the following lines at the end:
set_bash_prompt() {
    fs_mode=$(mount | sed -n -e "s/^\/dev\/.* on \/ .*(\(r[w|o]\).*/\1/p")
    PS1='\[\033[01;32m\]\u@\h${fs_mode:+($fs_mode)}\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '
}
alias ro='sudo mount -o remount,ro / ; sudo mount -o remount,ro /boot'
alias rw='sudo mount -o remount,rw / ; sudo mount -o remount,rw /boot'
PROMPT_COMMAND=set_bash_prompt

cd /home/samplerbox

- misc

set root password to sbox sudo passwd
sudo systemctl disable bluetooth doesn't work, see https://scribles.net/disabling-bluetooth-on-raspberry-pi/
  sudo apt-get purge bluez -y
  sudo apt-get autoremove -y
copy & "sudo python setup.py install" for: rtmidi2, python-midi # pyalsaaudio
mkdir /samples & chown to pi
sudo systemctl disable raspi-config (..disable system governor)
..don't do: systemctl disable avahi-daemon (..disable discovering network services), but:
sudo systemctl disable avahi-daemon.socket
sudo systemctl disable avahi-daemon.service
sudo systemctl disable systemd-timesyncd
sudo systemctl disable systemd-journald
sudo systemctl disable cron (..disable job scheduling)
sudo systemctl mask rpi-eeprom-update (..use unmask to undo)
sudo systemctl disable apt-daily.service
sudo systemctl disable apt-daily.timer
sudo systemctl disable apt-daily-upgrade.timer
sudo systemctl disable apt-daily-upgrade.service
sudo systemctl disable rsync
===> check path statement in smfplayer.py
sudo apt-get install zerofree  (for restricting the size of the downloadable image. See http://frippery.org/uml/index.html and http://manpages.ubuntu.com/manpages/xenial/man8/zerofree.8.html)