Modify, rebuild and reload the Linux® kernel

1. Overview

This stage will explain you how modify, rebuild and reload the Linux® kernel.
You will first be guide to install the Linux® kernel source code in the Developer Package directory. Then step by step you will execute procedures to modify, rebuild and reload the Linux® kernel.

2. Download the the Linux® kernel source code

• Download the STM32MP15-Ecosystem-v1.0.0 Developer Package Sources in your $HOME/STM32MPU_workspace/STM32MP15-Ecosystem-v1.0.0/Developer-Package directory
• Uncompress the tarball file to get the Linux® kernel tarball, the ST patches and the ST configuration fragments

 cd $HOME/STM32MPU_workspace/STM32MP15-Ecosystem-v1.0.0/Developer-Package
 tar xvf SOURCES-stm32mp1-openstlinux-4.19-thud-mp1-19-02-20.tar.xz

3. Prepare the Linux® kernel source code

• Extract the Linux® kernel source

 cd stm32mp1-openstlinux-4.19-thud-mp1-19-02-20/sources/arm-openstlinux_weston-linux-gnueabi/linux-stm32mp-4.19-r0
 tar xvf linux-4.19.9.tar.xz

• Apply the ST patches

 cd linux-4.19.9/
 for p in `ls -1 ../*.patch`; do patch -p1 < $p; done

• Apply fragments

 make multi_v7_defconfig fragment*.config
 for f in `ls -1 ../fragment*.config`; do scripts/kconfig/merge_config.sh -m -r .config $f; done
 yes '' | make oldconfig

4. Build the Linux® kernel source code for the first time

Information

The first time the kernel is build it could take several minutes.

• Build kernel images (uImage and vmlinux) and device tree (dtbs)

 make uImage vmlinux dtbs LOADADDR=0xC2000040

• Build kernel module

 make modules

• Generate output build artifacts

 mkdir -p $PWD/install_artifact/
 make INSTALL_MOD_PATH="$PWD/install_artifact" modules_install

8. Modifying a built-in Linux kernel device driver

This simple example adds unconditional log information when the display driver is probed.

• Check that there's no log information when the display driver is probed

 dmesg | grep -i stm_drm_platform_probe

• Go to the <Linux kernel source directory>

 cd <Linux kernel source directory>

• Edit the ./drivers/gpu/drm/stm/drv.c source file
• Add a log information in the stm_drm_platform_probe function

static int stm_drm_platform_probe(struct platform_device *pdev)
{
	struct device *dev = &pdev->dev;
	struct drm_device *ddev;
	int ret;
	[...]

	DRM_INFO("Simple example - %s\n", __func__);

	return 0;
	[...]
}

• Go to the <Linux kernel build directory>

 cd <Linux kernel build directory>

• Cross-compile the Linux kernel (please check the load address in the README.HOW_TO.txt helper file)

 make uImage LOADADDR=0xC2000040
 cp arch/arm/boot/uImage install_artifact/boot/

• Update the Linux kernel image on board

 scp install_artifact/boot/uImage root@<board ip address>:/boot/

Information

If the /boot mounting point doesn't exist yet, please see how to create a mounting point

• Reboot the board

 reboot

• Check that there is now log information when the display driver is probed

 dmesg | grep -i stm_drm_platform_probe
[    5.005833] [drm] Simple example - stm_drm_platform_probe

Direct Rendering Manager (kernel module that gives direct hardware access to DRI clients, find more information on official DRI web site http://dri.freedesktop.org/wiki/DRM)