===== Which OS to run? =====
  * **Raspbian**
    * is easiest to setup, and most widely used
    * [[https://www.raspberrypi.com/software/operating-systems/|Raspberry Pi OS (64 bit)]]
    * https://dietpi.com/ -- DietPi, streamlines Raspberry Pi OS, over/underclock options, logging to ram-fs by default and hourly sync
  * **Fedora**
    * [[/software/fedora/fedora_on_raspi4|Howto setup Fedora36 or rawhide]]
    * [[https://fedoraproject.org/wiki/Architectures/ARM/Raspberry_Pi#Raspberry_Pi_4|official Fedora/pi4 status]]
  * **Centos** images exist
    * [[https://people.centos.org/pgreco/CentOS-Userland-8-stream-aarch64-RaspberryPI-Minimal-4/|Centos 8 userland 4raspi4 image]]
  * **Debian**
    * [[https://david.wragg.org/blog/2020/01/installing-64-bit-debian-on-rpi.html|Installing 64-bit Debian/Raspbian dualboot]] on a Raspberry Pi 4.
    * Images can boot with kernel 5.5 and 5.6
    * https://wiki.debian.org/RaspberryPi4 -- Debian status
  * **Android**
    * [[https://www.raspberrypi.org/forums/viewtopic.php?f=73&t=261228|Android9]] - it's flagged "for devs"
    * [[https://forum.xda-developers.com/raspberry-pi/orig-development/omnirom-android-r-11-pi-4-t4183121?__cf_chl_jschl_tk__=fa65a5e876cd5aeb2ec1d994e795daee19027c26-1604566549-0-AQhGP6JhyEg5bjczrn6XuTshxUTr8jsaLb_A3yK6WVUa4Abzvoo5L20rr7w5ReVneUlvgNxCnTJtSQdW0amzIhVd7gxGs5vyVrCD3pSwkqDcbsV_1d-G7F4E5RWROLxlfIOWo_WoBB2jOp0SCWaNDFoJ0X6qR_0Sdr3ZAisxNNHy9YB7FNrfBrXlD-aCrj50MZ0mjyaLkl2k2orwmifMB7RM80aBT3UWepqmZ2EA7yXTnPjVDOpibr6e1786r8KykceASIgrq24vaAOiYWcqEN3nw8OaH5YamcSo5U3CHR3cMuESfCc5mdkhlWDRJxdedoRshzVypSa_xeMdQVX58ZYmIs2gh4WdMi5yDquhoE7clpJIE111W3IFoOjN54BkB2s9BmSrYOtraFsD7Wc87XWR5vKb46aWR2foDogrScki|Android11]] image

You can also put multiple OS's on one card, for example RaspiOS/Fedora32/Debian all together.
===== Links =====
  * [[/software/benchmark/raspberry_pi_4|Some pi4 benchmarks]]
  * Building a HDMI signal grabber with the raspi would be nice
    * Toshiba TC358743 based addon boards for the raspi are available, there is Linux support. 
    * [[https://mzyy94.com/blog/2020/04/10/raspberrypi-hdmi-input/|Project idea (Japanese)]]
  * How to [[https://www.hardill.me.uk/wordpress/2019/11/02/pi4-usb-c-gadget/#|run power/ethernet over a sincle usb-c cable to the raspi4]]

===== It feels slow! =====
How to distinguish which resource is under pressure? Measure. 'iostat' can show I/O load, vmstat/htop load on CPU.

  * **CPU:**
    * Buy good case/cooler
    * consider overclocking
    * 32bit or 64bit kernel? Depends on your application. Some results are [[https://github.com/ThomasKaiser/sbc-bench/blob/master/Results.md|better on the 32bit kernel]].
  * **Network:**
    * Verify gbit mode is properly set, with ethtool
  * **I/O:** My first 'its slow' impression was from 'dnf -y update' updating 120 packages in ~40 minutes. CPUs not loaded, but storage was the issue: sdcards are optimized for big sequential load, not small random access. [[/software/benchmark/sdcards|Here]] are details. Just using a different sdcard, this went to 15minutes. Approaches to fix:
    * Move the partition to USB3 connected devices (but USB flash can be as slow as sdcards. SSD are an option.)
    * Move the part where I/O is done to NFS via ethernet, for example if kernel compilation is causing the load these directories can be moved.
    * Buy sdcards doing well with random 4k access.

Measuring size of your I/O requests:
<code>
# https://github.com/iovisor/bcc
dnf -y install bcc
/usr/share/bcc/tools/bitesize'

# Then perform your I/O intensive operation, for example:
dnf -y update

# Evaluation of my I/O for 'dnf update', computed over all processes:
         0 -> 1          : 2487         (7%)
         2 -> 3          : 2327         (6%)
         4 -> 7          : 19501        (54%)
         8 -> 15         : 4443         (12%)
        16 -> 31         : 3687         (10%)
        32 -> 63         : 1979         (5%)
        64 -> 127        : 371          (1%)
       128 -> 255        : 223          (1%)
       256 -> 511        : 514          (1%)
       512 -> 1023       : 327          (1%)
      1024 -> 2047       : 497          (1%)
=> So when looking at benchmarks, 4k random access is most interesting
</code>

===== Raspberry Pi OS cheatsheet =====
<code>
### Installation, with sdcard in /dev/mmcblk0,
### or usb3 connected nvme-enclosure
# https://www.raspberrypi.com/software/operating-systems/
xzcat 2023-05-03-raspios-bullseye-arm64-lite.img.xz | \
  dd bs=4M of=/dev/mmcblk0 conv=fsync
mount /dev/mmcblk0p1 /mnt/tmp

# if the raspi should activate sshd
touch /mnt/tmp/SSH
vi /mnt/tmp/userconf.txt
# add here user + hashed password
# pattern: user:hashedpassword
# password hashing:
# echo 'mypassword' | openssl passwd -6 -stdin

# if resizing of the partition should be disabled
vi /mnt/tmp/cmdline.txt
# remove init=/usr/lib/raspi-config/init_resize.sh

umount /mnt/tmp

# Put the card into the raspi, boot it
nmap -sP 192.168.0.0/24 # find out IP
# ssh <ip> / user /password from userconf.txt

cat >>~/.bashrc<<EOT
# extend prompt to have temperature
export PS1='[\u@\h \$(cut -b 1,2 /sys/devices/virtual/thermal/thermal_zone0/temp)°C \W]\$ '
# colored iostat output
export S_COLORS=always
alias ll='ls -al'
export LC_ALL=en_GB.UTF-8
EOT

### check cpu temperature
$ vcgencmd measure_temp
temp=64.0'C

### current cpu details
$ lscpu|grep MHz
CPU max MHz:         1500.0000
CPU min MHz:         600.0000

$ vcgencmd measure_clock arm
frequency(48)=600117184
$ vcgencmd measure_volts
volt=0.8367V

### Are we throttled? Anything else than 0 means "yes"
$ vcgencmd get_throttled

### temperature in prompt
$ export PS1='\u@\h:'`vcgencmd measure_temp|cut -b 6,7`'°C:\w\$ '

### firmware update
apt update 
apt upgrade
apt full-upgrade # update to new major versions
apt install rpi-eeprom 
rpi-eeprom-update
rpi-eeprom-update -a

# If you want to try newer kernels for testing
BRANCH=master rpi-update
# BRANCH=next rpi-update
</code>

===== Overheating issues =====
  * cpu temp: 'vcgencmd measure_temp' output
  * cpu temp with load: 4x 'md5sum /dev/urandom'
  * [[https://www.amazon.co.jp/gp/product/B07X47L6DD/|Geekworm Raspberry pi 4]] is recommended
  * [[https://www.raspberrypi.org/documentation/configuration/config-txt/overclocking.md|boot option details]]
  * Underclocking could be an option:
<code>
vi /boot/config.txt

# in section [pi4] add
over_voltage=-1  # or "-2"
arm_freq=1250    # default is 1500Mhz
arm_freq_min=150 # also worth trying, default is 600Mhz

# then reboot
</code>

===== Using 2.9“ 296×128 ePaper Display Modue/SPI Interface =====
My initial issue: I did not get any output from the device. After recompiling the demo-app with DEBUG, I got this:
<code>
chris@raspberrypi:~/rpi/RaspberryPi/bcm2835 $ sudo ./epd 
2.9inch e-Paper demo
bcm2835 init success !!! 
Debug: e-Paper busy
^C
Handler:Goto Sleep mode
chris@raspberrypi:~/rpi/RaspberryPi/bcm2835 $ 
</code>
Tried out:
  * Ensure that spi devices are activated in /boot/config.txt: "dtparam=spi=on"
  * running rpi-update
  * updating kernel and system with "apt-get update; apt-get dist-upgrade"
  * doublechecking the connected pins
  * running raspi-config, to activate SPI
  * https://www.waveshare.com/wiki/2.9inch_e-Paper_Module https://www.waveshare.com/w/upload/9/98/2.9inch-e-paper-module-user-manual-en.pdf
  * The code from the upstream git repo works for me:
<code>
git clone https://github.com/waveshare/e-Paper
cd e-Paper/RaspberryPi&JetsonNano/c
vi examples/main.c
#   // EPD_2in9_test();
#   // EPD_2in9bc_test();
#   EPD_2in9d_test();

# and then run 'epd'
sudo ./epd
# this finally works for me
</code>

===== Sensors/addon list =====
List of the hardware/sensors I got.
  * Raspberry Pi USB-C power supply 5,1V / 3,0A, EU
  * Micro HDMI Adaptercable D-Stecker -A-Buchse 15cm schwarz
  * 4 teiliges Kühlkörper Set für Raspberry Pi 4, silber ([[https://www.amazon.de/dp/B07VFYHCJZ|amazon]])
  * Raspberry Pi 4 B, 2GB RAM
  * RFID Lesegerät mit SPI Schnittstelle inkl. Karte & Dongle ([[https://www.berrybase.de/raspberry-pi-co/raspberry-pi/module-sensoren/rfid-leseger-228-t-mit-spi-schnittstelle-inkl.-karte-dongle|link]])
  * PCF8591 AD/ DA Konverter Modul [[https://www.berrybase.de/raspberry-pi-co/raspberry-pi/module-sensoren/pcf8591-ad/da-konverter-modul|link]]
  * 2.9" 296×128 ePaper Display Modul mit SPI Interface, dreifarbig (rot, schwarz, weiß) [[https://www.berrybase.de/raspberry-pi-co/raspberry-pi/module-sensoren/2.9-296-215-128-epaper-display-modul-mit-spi-interface-dreifarbig-40-rot-schwarz-wei-223-4|link]] [[https://www.waveshare.com/wiki/2.9inch_e-Paper_Module|docs]]

===== Configuring network over usb-c =====
<code>
# https://www.raspberrypi.org/forums/viewtopic.php?t=245810
vi /boot/config.txt
# add: dtoverlay=dwc2
vi /boot/cmdline.txt
# add: modules-load=dwc2,g_ether
reboot
ifconfig -a # should show interface usb0
ip addr add 192.168.1.4/24 dev usb0
ip link set up dev usb0

# on the opposite system
ip addr add 192.168.1.4/24 dev enp0s20f0u1i1
ip link set up dev enp0s20f0u1i1
</code>

^                 ^latency: ICMP RTT^bandwidth                    ^
|usb-c network    |0.2-0.3ms        |down 140mbit/s / up 187mbit/s|
|pi4 gbit ethernet|0.24-0.3ms       |down 764mbit/s / up 914mbit/s|

  * latency, ICMP RTT: 0.2-0.3ms (gbit ethernet: 0.24-0.3ms)
  * bandwidth: 16mbyte/sec down / 22mbyte/sec up (gbit ethernet: 90mbyte/sec down / 50mbyte/sec up)