How to customize the Linux kernel

1 Purpose of article[edit]

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

2 Pre-requesites[edit]

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:

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

3 Adding kernel customization (including Linux kernel device tree, configuration, driver modification)[edit]

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

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

3.1 Adding kernel configuration modifications[edit]

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

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

Update accordingly <name of kernel recipe>.bbappend:

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 How to cross-compile with the Distribution Package#modifying_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/4.14/
Update ../meta-my-custo-layer/recipes-kernel/linux/linux-stm32mp.bbappend accordingly by adding these lines:

KERNEL_CONFIG_FRAGMENTS_append += "${WORKDIR}/fragments/4.14/fragment-cma-size.config"
SRC_URI_append = " file://4.14/fragment-cma-size.config;subdir=fragments "

3.2 Adding kernel driver or device tree modifications[edit]

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/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:

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

Copy these patch files into the custom layer

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

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://<kernel version>/<kernel version>.<revision>/0001-DT-leds-change.patch \
   file://<kernel version>/<kernel version>.<revision>/0002-Driver-change.patch \
   "