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Kernel Development

This guide covers Linux kernel development for embedded systems, focusing on the Rock 5B+ platform.

Understanding the Linux Kernel​

The Linux kernel is the core of any Linux system, responsible for:

  • Process Management: Scheduling, memory management, inter-process communication
  • Device Management: Hardware abstraction, device drivers
  • Memory Management: Virtual memory, physical memory allocation
  • File Systems: VFS, block devices, storage management
  • Network Stack: TCP/IP, network protocols, network interfaces

Kernel Architecture​

Monolithic vs Microkernel​

Linux uses a monolithic kernel architecture:

┌──────────────────────────────────────┐
│           User Space                 │
├──────────────────────────────────────┤
│           System Calls               │
├──────────────────────────────────────┤
│           Kernel Space               │
│  ┌─────────────────────────────────┐ │
│  │        Process Management       │ │
│  ├─────────────────────────────────┤ │
│  │        Memory Management        │ │
│  ├─────────────────────────────────┤ │
│  │        Device Drivers           │ │
│  ├─────────────────────────────────┤ │
│  │        File Systems             │ │
│  └─────────────────────────────────┘ │
└──────────────────────────────────────┘

Kernel Development Setup​

1. Prepare Development Environment​

# Install kernel development tools
sudo apt install -y build-essential libncurses-dev libssl-dev
sudo apt install -y flex bison libelf-dev bc rsync
sudo apt install -y device-tree-compiler

# Install cross-compilation tools
sudo apt install -y gcc-aarch64-linux-gnu g++-aarch64-linux-gnu

2. Clone Kernel Source​

# Clone Rock 5B+ kernel
git clone https://github.com/radxa/kernel.git -b stable-5.10-rock5
cd kernel

# Configure for Rock 5B+
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- rockchip_linux_defconfig

3. Build the Kernel​

# Build kernel image
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -j$(nproc)

# Build device tree
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- dtbs

Writing Your First Kernel Module​

1. Create a Simple Module​

// hello_kernel.c
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A simple kernel module");
MODULE_VERSION("0.1");

static int __init hello_init(void)
{
    printk(KERN_INFO "Hello, Kernel World!\n");
    return 0;
}

static void __exit hello_exit(void)
{
    printk(KERN_INFO "Goodbye, Kernel World!\n");
}

module_init(hello_init);
module_exit(hello_exit);

2. Create Makefile​

# Makefile
obj-m += hello_kernel.o

all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

3. Build and Test​

# Build the module
make

# Load the module
sudo insmod hello_kernel.ko

# Check kernel messages
dmesg | tail

# Unload the module
sudo rmmod hello_kernel

Device Tree Development​

1. Understanding Device Tree​

Device Tree is a data structure describing hardware:

// rock5b.dts
/dts-v1/;

#include "rk3588.dtsi"

/ {
    model = "Radxa ROCK 5B";
    compatible = "radxa,rock-5b", "rockchip,rk3588";

    chosen {
        stdout-path = "serial2:1500000n8";
    };

    memory@0 {
        device_type = "memory";
        reg = <0x0 0x0 0x0 0x80000000>;
    };
};

2. Adding Custom Hardware​

// Add custom GPIO device
&gpio0 {
    status = "okay";
    
    custom_device {
        compatible = "custom,device";
        gpios = <&gpio0 18 GPIO_ACTIVE_HIGH>;
        status = "okay";
    };
};

Kernel Debugging​

1. Using printk​

// Different log levels
printk(KERN_EMERG "Emergency message\n");
printk(KERN_ALERT "Alert message\n");
printk(KERN_CRIT "Critical message\n");
printk(KERN_ERR "Error message\n");
printk(KERN_WARNING "Warning message\n");
printk(KERN_NOTICE "Notice message\n");
printk(KERN_INFO "Info message\n");
printk(KERN_DEBUG "Debug message\n");

2. Using GDB for Kernel Debugging​

# Install kernel debugging tools
sudo apt install -y gdb-multiarch

# Start kernel with debugging
qemu-system-aarch64 -M virt -cpu cortex-a57 \
    -kernel arch/arm64/boot/Image \
    -append "console=ttyAMA0" \
    -nographic -s -S

3. Using ftrace​

# Enable ftrace
echo 1 > /sys/kernel/debug/tracing/tracing_on

# Set trace function
echo function > /sys/kernel/debug/tracing/current_tracer
echo schedule > /sys/kernel/debug/tracing/set_ftrace_filter

# View trace
cat /sys/kernel/debug/tracing/trace

Performance Optimization​

1. Kernel Configuration​

# Optimize kernel for embedded systems
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- menuconfig

# Key optimizations:
# - Disable unnecessary drivers
# - Enable CONFIG_PREEMPT_RT for real-time
# - Optimize memory management
# - Enable power management features

2. Real-Time Kernel​

# Apply RT patch
wget https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.10/patches-5.10.rt1.tar.xz
tar -xf patches-5.10.rt1.tar.xz
cd patches-5.10.rt1
./apply-patches.sh ../kernel

Common Kernel Development Tasks​

1. Adding System Calls​

// Add custom system call
asmlinkage long sys_my_syscall(unsigned long arg)
{
    // Your system call implementation
    return 0;
}

2. Interrupt Handling​

// Interrupt handler
static irqreturn_t my_interrupt_handler(int irq, void *dev_id)
{
    // Handle interrupt
    return IRQ_HANDLED;
}

// Register interrupt
request_irq(irq, my_interrupt_handler, IRQF_SHARED, "my_device", dev);

3. Memory Management​

// Allocate kernel memory
void *ptr = kmalloc(size, GFP_KERNEL);
if (!ptr) {
    // Handle allocation failure
}

// Free memory
kfree(ptr);

Best Practices​

1. Code Style​

  • Follow Linux kernel coding style
  • Use proper error handling
  • Document your code
  • Test thoroughly

2. Security Considerations​

  • Validate all inputs
  • Use proper locking mechanisms
  • Avoid buffer overflows
  • Follow security guidelines

3. Performance Tips​

  • Minimize context switches
  • Use efficient data structures
  • Optimize critical paths
  • Profile your code

Next Steps​

Resources​