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Machinekit-RT-Preempt-RPI.md

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Overview

The following steps are required to get a basic Machinekit / EtherCAT system on the Raspberry PI 3 Model B:

  • Download and boot into a Raspbian 8.0 system
  • Build and install a Raspberry Pi RT Preempt Linux Kernel
  • Build and install IgH EtherCAT master
  • Setup Machinekit

Download and boot into a Raspbian 8.0 system

Build and install a Raspberry Pi RT Preempt Linux Kernel

Download Raspberry Pi Kernel Sources

cd /opt
git ls-remote --heads https://github.com/raspberrypi/linux.git # show the branches
git clone -b 'rpi-4.4.y' --single-branch --depth 1 https://github.com/raspberrypi/linux.git

Patch Kernel with RT Preempt Patch

Next, patch the kernel with the RT Preempt patch. Choose the patch matching your kernel version. To this end, have a look at the Makefile. VERSION, PATCHLEVEL, and SUBLEVEL define the kernel version. At the time of writing this tutorial, the latest kernel was version 4.4.9. Patches for older kernels can be found in folder "older".

cd /opt/linux
wget https://www.kernel.org/pub/linux/kernel/projects/rt/4.4/patch-4.4.9-rt17.patch.gz
zcat patch-4.4.9-rt17.patch.gz | patch -p1
find -iname "*.rej" # check for rejected patches

Configure the Kernel

Next, configure the kernel for using RT Preempt.

For Raspberry Pi 2/3 Model B, execute these commands:

export KERNEL=kernel7
make bcm2709_defconfig

An alternative way is to export the configuration from a running Raspberry Pi:

sudo modprobe configs
zcat /proc/config.gz > .config

Next, prepare the system for kernel building:

sudo apt -y install bc libncurses-dev

Then, you can start to configure the kernel:

make -j$(nproc) menuconfig

In the kernel configuration, enable the following settings:

  • CONFIG_PREEMPT_RT_FULL: Kernel Features → Preemption Model (Fully Preemptible Kernel (RT)) → Fully Preemptible Kernel (RT)

kernel-config-rt-preempt-01.png

  • Enable HIGH_RES_TIMERS: General setup → Timers subsystem → High Resolution Timer Support (Actually, this should already be enabled in the standard configuration.)

kernel-config-rt-preempt-02.png

Build the Kernel

cd /opt/linux
make -j$(nproc) zImage
make -j$(nproc) modules
make -j$(nproc) dtbs
sudo make modules_install

Install Kernel Image, Modules, and Device Tree Overlay files

sudo su -
cp -rpv /boot /boot.bak
cd /opt/linux
export KERNEL=kernel7 # For Raspberry Pi 2/3 Model B
./scripts/mkknlimg ./arch/arm/boot/zImage /boot/$KERNEL.img
cp -pv  ./arch/arm/boot/dts/*.dtb /boot
cp -rpv ./arch/arm/boot/dts/overlays /boot
sudo nano /boot/cmdline.txt

Add the following option:

sdhci_bcm2708.enable_llm=0

Reboot:

sudo shutdown -r 1; exit

See also: http://www.frank-durr.de/?p=203

Latency Evaluation

For sure, you want to know the latency bounds achieved with the RT Preempt patch. To this end, you can use the tool cyclictest with the following test case:

  • 100 % load generated by running the following commands in parallel:
    • On the Pi:
    

    • 



pi@raspberrypi ~$ cat /dev/zero > /dev/null


* From another host:
```bash
user@host ~$ sudo ping -i 0.01 raspberrypi



cd /opt
git clone git://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git
cd rt-tests/
make all
sudo cp ./cyclictest /usr/bin/


On a Raspberry Pi 3 model B, I got the following results:


With Linux 4.4.11-rt17v7+ (my custom built kernel as described above):


$ uname -a
Linux raspberrypi 4.4.11-rt17-v7+ #1 SMP PREEMPT RT Sun May 29 12:59:15 UTC 2016 armv7l GNU/Linux
$ sudo cyclictest -t1 -p 80 -n -i 10000 -l 10000
# /dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 0.46 0.31 0.19 1/190 8964

T: 0 ( 8600) P:80 I:10000 C:  10000 Min:     15 Act:   17 Avg:   21 Max:      86


With Linux 4.4.4-rt9-v7+ from linux-image-rpi2-rt:


$ uname -a
Linux raspberrypi 4.4.4-rt9-v7+ #7 SMP PREEMPT RT Mon Mar 7 14:53:11 UTC 2016 armv7l GNU/Linux
$ sudo cyclictest -t1 -p 80 -n -i 10000 -l 10000
# /dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 1.50 1.01 0.71 3/197 5368           

T: 0 ( 3112) P:80 I:10000 C:  10000 Min:     13 Act:   17 Avg:   25 Max:      86


With some more tests, the worst case latency sometimes reached about 127 microseconds. Adding a safety margin, this should be safe for cycletimes of 1 ms.


For comparison see:



Build and install IgH EtherCAT master


git clone https://github.com/koppi/ec-debianize
cd ec-debianize
git checkout for-sittner
debian/configure `uname -r`
dpkg-checkbuilddeps
debuild -uc -us


In case you see an error during the package build process, do as a workaround:


touch master/soe_errors.c
debuild -uc -us -nc


Install the etherlabmaster Debian package:


sudo dpkg -i ../etherlabmaster*deb


Next, edit:


sudo vi /etc/default/sysconfig/ethercat


Add your ethX MAC address, given by ifconfig eth0:


MASTER0_DEVICE="b8:27:XX:XX:XX:XX" # your MAC address
DEVICE_MODULES="generic" #  the IgH EtherCAT master generic driver



Start the EtherCAT master:


sudo systemctl start ethercat.service
sudo systemctl status ethercat.service
● ethercat.service - EtherCAT Master Kernel Modules
   Loaded: loaded (/usr/lib/systemd/system/ethercat.service; enabled)
   Active: active (exited) since Mon 2016-05-30 18:53:20 UTC; 4s ago
  Process: 10020 ExecStop=/usr/sbin/ethercatctl stop (code=exited, status=0/SUCCESS)
  Process: 10030 ExecStart=/usr/sbin/ethercatctl start (code=exited, status=0/SUCCESS)
 Main PID: 10030 (code=exited, status=0/SUCCESS)


List the EtherCAT devices connected to the Raspberry PI 3:


ethercat slaves
0  0:0  PREOP  +  EK1100 EtherCAT-Koppler (2A E-Bus)
1  0:1  PREOP  +  EL2004 4K. Dig. Ausgang 24V, 0.5A
2  0:2  PREOP  +  EL2004 4K. Dig. Ausgang 24V, 0.5A


If the above worked, enable the service during system boot:


sudo systemctl enable ethercat.service


Setup Machinekit


The Machinekit setup is pretty straight forward – just compile Machinekit from source or install the armhf Debian packages provided by http://deb.machinekit.io/debian/dists/raspbian/ .


A quick Video shows Machinekit + EtherCAT on the Raspberry Pi 3 with some MQTT broker latency testing. Note that due to the way eth0 is connected internally on the Raspberry Pi 3, the EtherCAT frames sending and receiving may interfere with potential USB devices connected to the RPi (- for my setup this is not so relevant, because I'm not planning to connect any USB devices to the RPi3. – You have been warned).


ChangeLog


    

  • 2016-05-30 Initial version.
    • 

  • 2016-06-16 Add info about Machinekit setup.
    •