Yocto Kernel Configuration - Part 1: Getting Started with Devshell

This is the first part of a comprehensive series on managing kernel configuration in Yocto. In this article, we’ll cover the basics of using bitbake -c devshell virtual/kernel for kernel configuration work with linux-variscite.

Overview

When working with Yocto and linux-variscite kernel configuration, the devshell is your primary tool for:

• ✅ Interactive kernel configuration with proper environment
• ✅ Testing configuration changes before committing
• ✅ Understanding the build environment and directory structure
• ✅ Generating defconfig files for your Yocto layer

Prerequisites

Required Tools

# Ensure you have git configured
git config --global user.name "Your Name"
git config --global user.email "your.email@example.com"
 
# Yocto build environment
source oe-init-build-env
 
# Required packages for kernel configuration
sudo apt-get install libncurses5-dev libssl-dev bc flex bison

Understanding Your linux-variscite Setup

First, identify your kernel source and version:

# In your Yocto build directory
bitbake -e linux-variscite | grep "^S="
# Output: S="/path/to/tmp/work/imx8mp_var_som-poky-linux/linux-variscite/5.15.71+gitAUTOINC+hash/git"
 
# Check kernel version and branch
bitbake -e linux-variscite | grep "^PV="
# Output: PV="5.15.71+gitAUTOINC+hash"
 
bitbake -e linux-variscite | grep "SRCBRANCH"
# Output: SRCBRANCH="lf-5.15.y_var01"

Working with Yocto Devshell

Starting the Devshell

# Start kernel devshell (this is your starting point)
bitbake -c devshell virtual/kernel
 
# You're now in the kernel source directory with all environment variables set

Understanding the Devshell Environment

# In devshell, check important variables:
echo "Current directory: $PWD"
echo "Architecture: $ARCH"
echo "Cross compiler: $CROSS_COMPILE"
 
# Typical output:
# Current directory: /workdir/tmp/work-shared/imx8mp-var-som-mlx75027/kernel-source  (SOURCE directory)
# Architecture: arm64
# Cross compiler: aarch64-poky-linux-
 
# IMPORTANT: You're in the SOURCE directory, but make commands use BUILD directory
# Build directory: /workdir/tmp/work/imx8mp_var_som_mlx75027-poky-linux/linux-variscite/5.15.60+gitAUTOINC+740e6c7a7b-r2/build

Key Directory Structure

# Source directory (where you are in devshell)
/workdir/tmp/work-shared/imx8mp-var-som-mlx75027/kernel-source/
 
# Build directory (where .config and defconfig are created)
/workdir/tmp/work/imx8mp_var_som_mlx75027-poky-linux/linux-variscite/5.15.60+gitAUTOINC+740e6c7a7b-r2/build/

Basic Kernel Configuration Workflow

Step 1: Prepare Configuration

# In devshell, the .config file is already present
# IMPORTANT: Always run olddefconfig first!
make olddefconfig
 
# This step is crucial because:
# 1. It adds any new CONFIG options that weren't in your defconfig
# 2. It resolves dependencies automatically
# 3. It ensures your .config is complete and valid
# 4. It prevents menuconfig from showing unnecessary prompts

Step 2: Interactive Configuration

# Now run interactive configuration
make menuconfig
 
# Navigate through the menu system:
# - Use arrow keys to navigate
# - Use space to toggle options
# - Use '/' to search for specific options
# - Use '?' for help on any option
# - Press 'S' to save, then 'Q' to quit

Step 3: Example - Enabling ToF Camera Support

# In menuconfig, navigate to:
# Device Drivers --->
#   Multimedia support --->
#     Media drivers --->
#       [*] Media Controller API
#       [*] V4L2 sub-device userspace API
#       Camera sensor devices --->
#         <M> MLX75027 ToF sensor support
 
# For MIPI CSI support:
# Device Drivers --->
#   Multimedia support --->
#     Media drivers --->
#       Platform devices --->
#         <M> i.MX8 MIPI CSI-2 receiver
#         <M> i.MX8 Image Sensing Interface (ISI)
 
# Save and exit (S, then Q)

Step 4: Generate defconfig

# Generate the minimal defconfig
make savedefconfig
 
# IMPORTANT: defconfig is created in BUILD directory, not source directory
# Find the build directory from the make output:
# make[1]: Entering directory '/workdir/tmp/work/imx8mp_var_som_mlx75027-poky-linux/linux-variscite/5.15.60+gitAUTOINC+740e6c7a7b-r2/build'
 
BUILD_DIR="/workdir/tmp/work/imx8mp_var_som_mlx75027-poky-linux/linux-variscite/5.15.60+gitAUTOINC+740e6c7a7b-r2/build"
 
# Check defconfig in build directory
ls -la "$BUILD_DIR/defconfig"
# Output: -rw-r--r-- 1 1000 1000 32091 Jul 10 14:04 defconfig

Step 5: Verify Your Changes

# Check if your options are enabled
grep -E "CONFIG_VIDEO_MLX7502X|CONFIG_MXC_MIPI_CSI" .config
 
# Expected output:
# CONFIG_VIDEO_MLX7502X=m
# CONFIG_MXC_MIPI_CSI=m
# CONFIG_IMX8_MIPI_CSI2=m
 
# Exit devshell when done
exit

Common Issues and Solutions

Issue 1: Environment Variables Not Set

# If ARCH or CROSS_COMPILE are not set in devshell:
export ARCH=arm64
export CROSS_COMPILE=aarch64-poky-linux-

Issue 2: defconfig Not Found

# Remember: defconfig is created in BUILD directory, not source directory
# Look for the build directory path in make output
# Example: make[1]: Entering directory '/path/to/build'

Issue 3: Configuration Changes Not Saved

# Always run olddefconfig before menuconfig
make olddefconfig
make menuconfig
 
# Always save in menuconfig (press 'S' before 'Q')

Next Steps

In the next part of this series, we’ll cover:

• Part 2: Creating machine-specific defconfig files for your Yocto layer
• Part 3: Managing bbappend files for different machines
• Part 4: Git-based version control for configuration changes

Quick Reference

# Essential devshell workflow:
bitbake -c devshell virtual/kernel
make olddefconfig
make menuconfig
make savedefconfig
# Copy defconfig from build directory to your Yocto layer
exit

This workflow forms the foundation for all kernel configuration management in Yocto. Master these basics before moving on to more advanced topics in the next parts of this series.