Installing LinuxCNC 2.9 on Raspberry Pi 4B with Preempt-RT kernel

I've seen a lot of people, including myself, have trouble getting LinuxCNC running on the Raspberry Pi 4. I've just gotten it all working, so I thought I'd share my recipe.

We will be overclocking and disabling power management in order to decrease latency. Make sure your Raspberry Pi has a *very large* heat sink, the kind that basically covers the entire board, before attempting this. A fan wouldn't hurt anything either.

Make sure you are using the fastest SD card you can find. Slow reads and writes will cause latency spikes.

1. On Linux desktop computer

Everything in this section should be run on your Linux desktop computer. These commands were tested on Linux Mint 21, but any reasonably recent Debian distro should work.

1.1 Install Raspberry Pi Imager

If that doesn't work, you can manually download and install from www.raspberrypi.com/software/

1.2 Build Rasberry Pi OS image with Realtime-RT kernel

The script will spit out the name of the OS image file at the end of its run.

    adding: 2022-09-22-raspios-bullseye-arm64-full.img (deflated 65%)

1.3 Burn image to SD card with Rasbperry Pi Imager

Insert the SD card into your computer.

Open Raspberry Pi Imager.

Click "Choose OS" button. Select "Use Custom" from the bottom of the menu. Select the '.img' file you just built in the `rpi-rt-kernel` directory.

Click "Choose Storage" button. Select your SD card.

Click the settings sprocket image button. Set your WLAN credentials, enable ssh, hostname, username/password, and timezone in the settings. I use

    hostname = linuxcnc
    enable ssh = checked
    use password authentication = selected
    set username and password = checked
        username = pi
        password = (password of your choice)
    configure wireless lan = checked
        SSID = (your wifi access point name)
        password = (your wifi password)
        wireless lan country = (your country code)
    set locale settings = checked
        timezone = (your timezone)

Click the "Write" button

1.4 Boot Raspberry Pi

Insert the micro SD card in your RPi 4 and power it up. If you correctly set your WLAN credentials, it should be available on your Wifi network in about a minute. You can watch for it to appear by running `ping`.

1.5 Connect to Raspberry Pi

Once the Rasberry Pi is online, `ssh` in to it

Enter your password when prompted.

2. On Rasbperrry Pi

The following instructions are based on gnipsel.com/linuxcnc/debian-11-emc.html

2.1 Fix locale setting

First check if you need to set the locale

If you don't get anything back, then run

2.2 Build LinuxCNC

2.3 Install LinuxCNC

2.4 Configure notification service

2.5 Install Qt Designer

2.6 Install Mesaflash

2.7 Install Mesa Configuration Tool

2.8 Improve latency

2.9 Fix virtual keyboard

2.10 (Optional) Fix USB ports on Raspberry Pi 4 Compute Module

2.11 (Optional) Improve latency by stopping unnecessary networking

This will disable Wifi, automatic IP address assignment, remote shell access, and local DNS naming. Make sure you have a monitor and keyboard connected before doing this.

And that's about it.

With 7 glxgears and a ping flood to my 7i96S running, base thread jitter is 25us. That's good enough for slow-ish software stepping according to the Wiki.

You should also mention the version of the RPI4, by now there are several versions with different hardware so all this might not work on all of them, as can be noted from complaints about the official image not working on some new RPI4.
Thank you.

tommylight wrote:

You should also mention the version of the RPI4, by now there are several versions with different hardware so all this might not work on all of them, as can be noted from complaints about the official image not working on some new RPI4.
Thank you.

I don't think the version matters as from the git files Pi 1 to Pi 4/4b are supported.

It does, new versions had issues with the official Raspbian, it did not work several months back.
I do not nor do i intend to use RPI4 in the foreseeable future, but i read everything on this forum and there are several topic with that subject.

Alangibson,
I have tested your recipe on raspberry Pi, compute module CM4108016, 8G RAM + 16G eMMc installed on mother board “Compute Module 4 IO Board” and mesa 7i96S
This is the cleanest OS 64 installation for linuxcnc 2.9 preRT with kernel

uname -a
Linux linuxcnc29 5.15.84-rt54-v8+ #1 SMP PREEMPT_RT Fri Dec 23 20:39:48 CET 2022 aarch64 GNU/Linux

Note: compute module carrier board IO must have
in the file /boot/config.txt

dtoverlay=dwc2,dr_mode=host

to enable usb PORTS.

The only hiccup I have experienced was with setting on the “locale”.

for users experiencing the issue
VERIFY IF YOU HAVE THOSE LINES
export LC_ALL="en_US.UTF-8"
export LC_CTYPE="en_US.UTF-8"

So far i have no issues with other programs residing in raspberrian.

My project is mobile robotic analytical platform with linuxcnc as the backbone for sample collection.
This is my third setup. The other systems were for big CNC 4x6 ft table with encoders in the loop using 7i96 + 7i85 raspberry pi 4M
and Grizzly CNC with Jetson TX1 and 7i95

Thank you and the other contributors.
RMS

Similar problem, but different enough to chime in. My tack was admittedly a bit hacky, but it worked.
My goal was to create a "fresh" rpi image (ie, modify the .img file, BEFORE flashing, rather than after, since after first boot, things change) with the rt preempt kernel, but based on the rpi images supplied by linuxcnc, rather than the vanilla raspbian. This was difficult, because the released images from linuxcnc don't seem to support the RPi 4*B*, which is what I'm running, and the raspbian image with a new kernel didn't seem to want to install linuxcnc. Both sets of software had something I wanted, but neither had everything, hence the deep-dive.

I found a way that worked for me, and have some (poorly documented) code here: github.com/InternetofAwesome/pm-940-cnc/...misc/distro-build.sh

Basically, the script is going to want a "fresh" image of the 2.8.1 build from linuxcnc for rpi, and the files from /boot from a more recent raspbian build.
Those are in the top of the script in a clearly marked config section, and named IMAGE_PATH and BOOTLOADER_PATH, respectively.

Once the script runs, you should have a ready-to-flash base OS image. From there, the apt repos are setup to grab from the mainline linuxcnc sources, and worked fine for me.

I didn't need/want to build LCNC from source, so the mainline releases were fine for me.