How to customize the Linux kernel

Applicable for

STM32MP13x lines, STM32MP15x lines, Unknown MPU, Unknown MPU, Unknown MPU

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1. Purpose of article

This article gives the main steps needed to add kernel customization within the Yocto build process (with a Distribution Package).

2. Pre-requisites

You are already familiar with the Yocto build process and OpenSTLinux distribution.

You have already created a customer layer (How to create a new open embedded layer) to update, for your own needs, the OpenSTLinux distribution.

We describe here what you must do once you have:

• modified the kernel configuration
• modified the Linux kernel device tree
• modified a built-in device driver

so that these modifications are taken into account in your build process.

3. Adding kernel customization (including Linux kernel device tree, configuration, driver modification)

• First, create (in your custom layer) a <name of kernel recipe>.bbappend file

touch ../meta-my-custo-layer/recipes-kernel/linux/<name of kernel recipe>.bbappend

3.1. Adding kernel configuration modifications

• Identify all new configs you set or unset with: bitbake <name of kernel recipe> -c menuconfig
• Put them inside a new fragment file and copy the fragment here:

cp <custom-fragment>.config  ../meta-my-custo-layer/recipes-kernel/linux/<name of kernel recipe>/<kernel version>/

• Update accordingly <name of kernel recipe>.bbappend:

# Needed to tell Yocto to search inside this path
FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
KERNEL_CONFIG_FRAGMENTS:append = " ${WORKDIR}/fragments/<kernel version>/<custom-fragment>.config"
SRC_URI:append = " file://<kernel version>/<custom-fragment>.config;subdir=fragments "

For the use case described in the modified the kernel configuration example, you should:

• Create fragment-cma-size.config with the following line:

CONFIG_CMA_SIZE_MBYTES=256

• Copy fragment-cma-size.config to ../meta-my-custo-layer/recipes-kernel/linux/linux-stm32mp/<kernel version>
• Update ../meta-my-custo-layer/recipes-kernel/linux/linux-stm32mp.bbappend accordingly by adding these lines:

KERNEL_CONFIG_FRAGMENTS:append = " ${WORKDIR}/fragments/${LINUX_VERSION}/fragment-cma-size.config"

SRC_URI:append = " file://${LINUX_VERSION}/fragment-cma-size.config;subdir=fragments"

3.2. Adding kernel driver or device tree modifications

Information

The example given below is associated with the STM32MP15 Evaluation board, but the method is independent of the board.

Once you have made the changes for the device tree in <build dir>/workspace/sources/<name of kernel recipe>/arch/arm/boot/dts/st/stm32mp157c-ed1.dts AND, for built-in device driver in <build dir>/workspace/sources/<name of kernel recipe>/drivers/gpu/drm/stm/drv.c, you must:

• Create the corresponding patch files:

cd <build dir>/workspace/sources/<name of kernel recipe>/
git format-patch -2

• Copy these patch files into the custom layer

cp *.patch ../meta-my-custo-layer/recipes-kernel/linux/<name of kernel recipe>/<kernel version>/<kernel version>.<revision>/

Warning

Patches are linked to a kernel version, which means that these patches are rebuilt if the kernel version changes, and copied to the according kernel version sub-folder

• Update <name of kernel recipe>.bbappend accordingly:

SRC_URI:append = " \
   file://${LINUX_VERSION}/${LINUX_VERSION}.${LINUX_SUBVERSION}/0001-DT-leds-change.patch \
   file://${LINUX_VERSION}/${LINUX_VERSION}.${LINUX_SUBVERSION}/0002-Driver-change.patch \
   "

Microprocessor unit

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Device tree