How to Add Device a Custom Device

Revision History

Introduction

The Prerequisite for the below steps is that the new device already is supported by the Linux kernel version we are about to modify. To confirm driver support for a specific kernel version, search the kernelconfig website.

To add a custom device to the MistySOM distribution, multiple steps are required. They can be separated as follows:

1. Update Linux kernel configuration to compile the driver for the new device
2. Patch the device tree to configure the driver for the new device.
3. Rebuild the kernel & device tree
4. Load & boot the rebuilt files for testing

1. Update Linux kernel configuration to compile the driver for the new device

The prerequisite for this step is, that the build container from the repository has been cloned and started with the dev shell argument. To find out more about it, please see Description of advanced container options in the Getting Started Guide and start the container with the -n argument. When the container is started, the directory is changed and the build-environment is sourced, the present directory should be build/. From here,

bitbake linux-renesas -c menuconfig

can be invoked to launch menuconfig(a ncruses based configuration utility) to customize the Linux kernel options. Configuration options can be difficult to find and you can use kernelconfig.io to find if support for your device is part of the Linux kernel at a selected version. It will also give directions, about where the configuration option can be found and enabled.
After the option has been enabled, save the configuration (default filename .config is fine)and exit the menuconfig utility.
Back on the shell,

bitbake linux-renesas -c diffconfig

can be invoked to create a fragment of the configuration differences. Copy the fragment from $WORK/build/tmp/work/smarc-rz_rzv2l-poky-linux/linux-renesas/[kernelversion...]/ to $WORK/meta-renesas/recipes-common/recipes-kernel/linux/linux-renesas/ and rename it for ease of management (e.g. MY_DEVICE.cfg). if you look at the contents of your MY_DEVICE.cfg it should look similar to:

cat $WORK/meta-renesas/recipes-common/recipes-kernel/linux/linux-renesas/MY_DEVICE.cfg 
CONFIG_TYP_MY_DEVICE=y

After the fragment has been added to, edit the kernel bitbake recipe located at $WORK/meta-renesas/recipes-common/recipes-kernel/linux/linux-renesas_#.##.bb and add MY_DEVICE.cfg to the configuration fragments listed under SRC_URI_append = " \ like so:

SRC_URI_append = " \
file://MY_DEVICE.cfg \ (new)
" 

invocation of

bitbake mistysom-image

will now build the kernel with the newly added device driver included.

2. Patch the device tree to configure the driver for the new device.

An excellent tutorial that relates to editing the device tree, can be found here: Intro to Embedded Linux Part 5 - How to Enable I2C in the Yocto Project.
Also see this article: Customize Device tree

3. Rebuild the kernel & device tree

Invoke

bitbake virtual/kernel -c compile -f && bitbake virtual/kernel

to re-compile the kernel and the device tree

4. Load & boot the rebuilt files for testing

Prepare a uSD card according to instructions here, copy the Linux kernel and the device tree blob (.dtb) files to the first partition (see here for details about the output files) and extract the root filesystem to the second partition.