There are cases where you may wish to build a new kernel, recompile modules or a device tree for the BeagleBone Black. For example, you may wish to patch the kernel with the RK extension for real-time support, or modify the in-kernel OMAP clock code. This tutorial covers obtaining, patching and building a kernel, module and device tree from scratch.
STEP 1 : Download Linaro toolchain (cross-compiler)
armhf cross-compiler in Linux has known bugs, which make it impossible to compile the Linux kernel. For this reason we will use a toolchain provided by the Linaro project. Start by downloading and unzipping this toolchain, and creating symbolic link to make things simpler:
cd <roseline>/kernel wget https://releases.linaro.org/14.11/components/toolchain/binaries/arm-linux-gnueabihf/gcc-linaro-4.9-2014.11-x86_64_arm-linux-gnueabihf.tar.xz tar -xJf gcc-linaro-4.9-2014.11-x86_64_arm-linux-gnueabihf.tar.xz ln -s gcc-linaro-4.9-2014.11-x86_64_arm-linux-gnueabihf linaro
STEP 2 : Download and patch the Linux Kernel
The first thing that is required is to download, decompress and create a convenient symbolic link for a stable version of Linux. We have chosen v3.18.9 because it is compatible with the RT patch set and includes drivers for our AT86RF233 radio.
cd <roseline>/kernel wget https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.18.9.tar.xz tar -xJf linux-3.18.9.tar.xz ln -s linux-3.18.9 linux
Then, download and apply the Real-time Linux (RT) patch. Please double-check that all patch hunks are successfully applied to the Kernel source tree.
cd <roseline>/kernel wget https://www.kernel.org/pub/linux/kernel/projects/rt/3.18/older/patch-3.18.9-rt4.patch.xz pushd linux xzcat ../patch-3.18.9-rt4.patch.xz | patch -p1 popd
Then, apply the ROSELINE patches to the kernel tree. These patches change the OMAP2 clock to enable TCLKIN, set up hardware interrupts for the radio, and modify the standard pin multiplexing for our board to enable SPI, i2c, TCLKIN and four timer input capture pins.
cd <roseline>/kernel pushd linux for i in ../patches/*.patch; do patch -p1 < $i; done popd
Before compiling it is necessary to download firmware for the board, which should be placed in the firmware directory. This is firmware released by TI that enables power management for the board.
cd <roseline>/kernel wget http://arago-project.org/git/projects/?p=am33x-cm3.git\;a=blob_plain\;f=bin/am335x-pm-firmware.bin\;hb=HEAD -O linux/firmware/am335x-pm-firmware.bin
We are now ready to copy over the kernel configuration and compile the kernel, modules and device tree. Note that the kernel configuration specified below assumes the toolchain is located at ../linaro. If you have a quad core processor and 8GB+ RAM you can compile with the -j8 in place of -j4.
cd <roseline>/kernel pushd linux cp ../kernel.config .config make ARCH=arm oldconfig make ARCH=arm zImage dtbs modules -j4 popd
If this finishes without error, then you have a new kernel, modules and device tree ready.
STEP 3 : Build the ROSELINE kernel module
At this stage it makes sense to build the out-of-tree kernel modules. The first of these is the ROSELINE module, which works along with the device tree to set up input capture on several timer pins, as well as act as an information proxy between applications and the synchronization algorithm running in user-space. The second kernel module, kmod-usrcc, allows the processor cycle counter to be read from user-space, which effectively provides a 1GHz timer clocked up from the 24MHz hardware timers.
To do this, just change to the modules directory and build. This should work automatically if you have the linux symbolic link pointing to the compiled Linux source tree in the kernel folder.
cd <roseline>/modules make
STEP 4 : Install kernel image, modules and device tree
Firstly, copy the kernel image and device tree to your boot location (/path/to/boot). If you are booting via memory card, then this will be on the first partition of the MicroSD card, replacing the precompiled files. If you are network-booting the platforms, then this will be to your TFTP root directory.
cd <roseline> sudo cp kernel/arch/arm/boot/zImage /path/to/boot sudo cp kernel/arch/arm/boot/dts/am335x-boneblack.dtb /path/to/boot
Secondly, copy the in and out of tree kernel modules to the root files ystem. Again, if you are booting off of the memory card, this will be the second partition of your card. Otherwise, for network-booting this will be the location of your NFS shared root file system.
cd <roseline> pushd kernel/linux sudo make ARCH=arm INSTALL_MOD_PATH=/path/to/rootfs modules_install sudo make ARCH=arm INSTALL_HDR_PATH=/path/to/rootfs/usr headers_install popd pushd modules sudo cp *.ko /path/to/rootfs/lib/modules/3.18.9-rt4/kernel/drivers popd