CM-T54: Linux: Kernel

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):
  • Sourcery CodeBench Lite
• 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 v3.16.2 snapshot with a web browser.
• Extract the downloaded archive linux-3.16.2.tar.gz

cd /home/development/cm-t54/kernel
tar xvf /path/to/downloaded/linux-3.16.2.tar.gz
mv linux-3.16.2 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-3.16.2-cm-t54-3.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/stable/linux-stable.git linux-cm-t54

• Create a branch for CM-T54 development. The CM-T54 patches are generated vs. v3.16.2 tag (62de88e8e65811010deac5375f8f0d8b14dc4d94 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 v3.16.2

• Apply the CM-T54 patch

git apply /path/to/cm-t54-linux/kernel/linux-3.16.2-cm-t54-3.patch

Future stable kernel versions of 3.16 can also be used by checking out any later tag of the v3.16.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.

See also

• CM-T54: Linux: Getting started
• CM-T54: Linux: Debian