CM-T54: Linux: Kernel
Contents
Overview
Linux kernel for the CompuLab CM-T54 System-on-Module / Computer-on-Module provides support for on-board peripherals and abstracts the functionality provided by the hardware. Current support coverage is specified at O/S support coverage map page. CompuLab provides ready-to-run binary kernel images, and source code of the modifications and additions made to the Linux kernel to work properly with CM-T54 modules.
Device Tree
Device Tree (DT), is a data structure and language for describing hardware. More specifically, it is a description of hardware that is readable by an operating system so that the operating system doesn't need to hard code details of the machine.
A device tree file, named *.dts, is a text file that describes the hardware platform. It is compiled into a device tree blob, *.dtb, which is loaded into memory before the Linux kernel is started. The Linux kernel then uses that device tree blob to initialize itself at runtime.
CM-T54 DT related files are :
- arch/arm/boot/dts/omap5-cm-t54.dts - CM-T54 core CoM Device Tree
- arch/arm/boot/dts/omap5-sbc-t54.dts - SBC-T54 single board computer (eval board) Device Tree
For more details visit official Device Tree WiKi pages and make an overview of a Device Tree tutorial
Building kernel for CM-T54
Cross-Compiler
There are several options for cross-compilation toolchain setup. You can either compile your cross-compiler or use an already built cross-compiler. The cross-compiler should support the ARM embedded-application binary interface ("EABI")
- Pre-built toolchain (recommended):
- Tools for creating cross-compilers:
- Crosstool-ng: Builds a cross-compiler from source. Non-distribution specific.
- Crossdev: Gentoo's cross-compiler builder. Needs Gentoo.
Getting kernel sources
There are various ways to get Linux kernel sources that can be used as a baseline for CM-T54 kernel. For instance, a copy of linux-stable git tree can be created or a Linux kernel sources snapshot can be downloaded.
Assuming that the /home/development/cm-t54/kernel directory structure has been created for CM-T54 kernel development.
Snapshot download
- Download v4.0 snapshot with a web browser.
- Extract the downloaded archive linux-4.0.tar.gz
cd /home/development/cm-t54/kernel tar xvf /path/to/downloaded/linux-4.0.tar.gz mv linux-4.0 linux-cm-t54
- This will create /home/development/cm-t54/kernel/linux-cm-t54 directory containing linux kernel tree.
- Apply the CM-T54 patch
cd /home/development/cm-t54/kernel/linux-cm-t54 patch -p1 < /path/to/cm-t54-linux/kernel/linux-4.0-cm-t54-4.patch
Git clone
- Install git version control system.
- Create a clone of linux kernel tree
cd /home/development/cm-t54/kernel git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git linux-cm-t54
- Create a branch for CM-T54 development. The CM-T54 patches are generated vs. v4.0 tag (39a8804455fb23f09157341d3ba7db6d7ae6ee76 commit) in the Linux tree. It is recommended to use exactly the same baseline to avoid merge conflicts.
cd /home/development/cm-t54/kernel/linux-cm-t54 git checkout -b cm-t54-dev v4.0
- Apply the CM-T54 patch
git apply /path/to/cm-t54-linux/kernel/linux-4.0-cm-t54-4.patch
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Future stable kernel versions of 4.0 can also be used by adding the Linux kernel stable repository as a remote and checking out any later tag of the v4.0.y stable kernel branch. |
Building the kernel
export ARCH=arm
export CROSS_COMPILE=arm-none-linux-eabi-
make cm_t54_defconfig
make menuconfig
make && make zImage && make omap5-sbc-t54.dtb \
INSTALL_MOD_PATH=/home/development/cm-t54/rootfs make modules_install
In the example above:
- The Linux kernel image zImage will be created in /home/development/cm-t54/kernel/linux-cm-t54/arch/arm/boot
- The Device Tree binary omap5-sbc-t54.dtb will be created in /home/development/cm-t54/kernel/linux-cm-t54/arch/arm/boot/dts
- The loadable kernel modules will be installed into the /home/development/cm-t54/rootfs/lib/modules directory.
When the CM-T54 is booted with the kernel image and Device Tree binary created as described above and with networked root filesystem at /home/development/cm-t54/rootfs/, the system will be able to load the kernel modules properly, thus avoiding module version problems.
