Part III – Linux kernel and Xenomai patch

In order to cross compile the kernel we can use the one provided by Buildroot or download a new one with the command:

userk@dopamine:~/RaspberryPi/Xenomai-RPI$ wget https://github.com/raspberrypi/tools/archive/master.tar.gz
userk@dopamine:~/RaspberryPi/Xenomai-RPI$ tar xzf master.tar.gz

Xenomai provides a real-time sub-system seamlessly integrated to Linux, therefore the first step is to download it and then to build it as part of the target kernel.

userk@dopamine:~/RaspberryPi/Xenomai-RPI$ wget -q -O - http://download.gna.org/xenomai/stable/xenomai-2.6.3.tar.bz2 | tar -xjf -

Let’s create a new directory and download the linux kernel version 3.8.13 from the Linux Kernel Archives.

userk@dopamine:~/RaspberryPi/Xenomai-RPI$ git clone -b rpi-3.8.y git://github.com/raspberrypi/linux.git linux-3.8.13
userk@dopamine:~/RaspberryPi/Xenomai-RPI$ cd linux-3.8.13
userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$

Xenomai needs special kernel support to deliver fast and deterministic response time to external interrupts, and also to provide real-time services highly integrated with the standard Linux kernel.

This support is provided by the interrupt pipeline (aka I-pipe) in the form of a kernel patch.
You can find the latest patches in the xenomai-2.6.3/ksrc/arch/arm/patches/raspberry folder.
Let’s apply the first one.

userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ patch -Np1 < ../xenomai-2.6.3/ksrc/arch/arm/patches/raspberry/ipipe-core-3.8.13-raspberry-pre-2.patch

Now, let’s apply the ipipe patch. Hopefully the prepare-kernel.sh script located in the scripts folder will do the job. We just need to specify the path of the target kernel source tree, the Adeos patch to apply against the tree and the target architecture.

userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ ../xenomai-2.6.3/scripts/./prepare-kernel.sh --arch=arm --linux=./ --adeos=../xenomai-2.6.3/ksrc/arch/arm/patches/ipipe-core-3.8.13-arm-3.patch

The above command prepares the Linux tree located at ./linux-3.8.13 in order to include the Xenomai support. From Xenomai installation guide:

Once the target kernel has been prepared, the kernel should be configured following its usual configuration procedure. All Xenomai configuration options are available from the “Real-time subsystem” toplevel menu.

Let’s apply the last patch:

userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ patch -Np1 < ../xenomai-2.6.3/ksrc/arch/arm/patches/raspberry/ipipe-core-3.8.13-raspberry-post-2.patch

We can finally compile the kernel. Please note that this version, 3.8.13, is the same one we have used during the Part II of this tutorial.

Clean the configuration file and check the one for the Raspberry pi.

userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ make ARCH=arm mrproper
userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ make ARCH=arm help | grep rpi
userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ make ARCH=arm bcmrpi_quick_defconfig

Since most of the sensors provide SPI and I2C transmission protocol, we need to enable the kernel support for these communication systems.

userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ make menuconfig

• Go to Device Drivers menu and check the I2C support pressing ‘Y’.
• Enter the I2C menu, go to I2C device interface and Press ‘M’ for module support.
• Enter the I2C Hardware Bus support menu and press ‘M’ BCM2708 BSC
• Check the SPI support pressing ‘Y’ and enter the sub menu
• Press ‘M’ near the BCM2708 SPI controller driver

Here are a few guidelines, from the Learning system, in order to set up a kernel for Xenomai in a dual kernel configuration.

Frequency scaling creates issues with Xenomai timing code, as well as unpredictable run-time for your real-time threads, and possibly high latencies when CPU frequency is changed dynamically

Disable this option in CPU Power Management -> CPU Frequency scaling. Press ‘n’ to disable.
Next, disable CPU idle PM support.

CONFIG_CPU_IDLE allows the CPU to enter deep sleep states, increasing the time it takes to get out of these sleep states, hence the latency of an idle system. Also, on some CPU, entering these deep sleep states causes the timers used by Xenomai to stop functioning.

Save the configuration and cross-compile.

userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ make ARCH=arm CROSS_COMPILE=../tools-master/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin/arm-linux-gnueabihf-

The kernel image is now available to be transferred in the Raspberry Pi Boot partition. The image called ‘zImage’ is located in the arch/arm/boot/ folder. Copy it in the Boot partition.

userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ ls arch/arm/boot/ | grep zImage
zImage
userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ sudo cp arch/arm/boot/zImage /media/$USER/Boot/zImage

Kernel modules are installed into the /lib/modules/x.y.z directory on the target system. Copy them into the SD card in /media/$USER/Root.

userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$ sudo make ARCH=arm INSTALL_MOD_PATH=/media/$USER/Root modules_install

Next, let’s compile Xenomai user space. First define the absolute path of the cross-compiler.

userk@dopamine:~/RaspberryPi/Xenomai-RPI/linux-3.8.13$$ cd ../xenomai-2.6.3
userk@dopamine:~/RaspberryPi/Xenomai-RPI/xenomai-2.6.3$ PATH=$PATH:/home/$USER/RaspberryPi/Xenomai-RPI/tools-master/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin/
userk@dopamine:~/RaspberryPi/Xenomai-RPI/xenomai-2.6.3$ ./configure --host=arm-linux-gnueabihf CFLAGS='-march=armv6' LDFLAGS='-march=armv6'
userk@dopamine:~/RaspberryPi/Xenomai-RPI/xenomai-2.6.3$ make
userk@dopamine:~/RaspberryPi/Xenomai-RPI/xenomai-2.6.3$ make DESTDIR=$(pwd)/RPI install

Compress the sbin, bin and lib folder and copy them to the Root partition

userk@dopamine:~/RaspberryPi/Xenomai-RPI/xenomai-2.6.3$ tar cjf xenomai-rpi.tar.bz2 usr/xenomai/bin/ usr/xenomai/sbin/ usr/xenomai/lib/ usr/xenomai/include/
userk@dopamine:~/RaspberryPi/Xenomai-RPI/xenomai-2.6.3$ sudo cp xenomai-rpi.tar.bz2 /media/$USER/Root/
userk@dopamine:~/RaspberryPi/Xenomai-RPI/xenomai-2.6.3$ cd /media/%USER/Root/
userk@dopamine:~/media/%USER/Root/$ sudo tar xjf xenomai-rpi.tar.bz2 && sudo rm xenomai-rpi.tar.bz2 && cd ..
userk@dopamine:~/media/%USER/$ sudo umount /dev/sdb1 /dev/sdb2

Finally, we have our dual kernel embedded system with Xenomai!