Writing Linux Kernel Modules from Scratch

🚧 Work in Progress — This post is a placeholder. Full content coming soon.

Linux kernel module development sits at the intersection of systems programming and hardware control. For embedded systems work — especially bringing up custom FPGA IP cores under Linux on platforms like the Zynq — understanding how to write a clean kernel module is an essential skill. This post documents the fundamentals without assuming prior kernel development experience.

Planned Content

1. Kernel Space vs. User Space

• Virtual memory separation and why it matters
• System call interface: the boundary crossing
• Why drivers live in kernel space (direct hardware access, interrupt context)
• When not to write a kernel module: UIO, VFIO, sysfs alternatives

2. The Linux Kernel Build System (Kbuild)

• Out-of-tree module Makefile structure
• KERNELDIR, PWD, and cross-compilation for ARM
• MODULE_LICENSE, MODULE_AUTHOR, MODULE_DESCRIPTION

3. Module Lifecycle

module_init(my_driver_init);
module_exit(my_driver_exit);

• insmod / rmmod / modprobe
• printk and log levels: KERN_INFO, KERN_ERR, etc.
• Kernel symbol table: EXPORT_SYMBOL

4. Character Device Driver

• struct file_operations: open, read, write, release
• Allocating a major number dynamically with alloc_chrdev_region
• cdev_init and cdev_add
• Creating a device node automatically with class_create / device_create
• copy_to_user / copy_from_user: the safe crossing

5. Interrupt Handling

• Requesting an IRQ: request_irq
• Top half vs. bottom half: hardirq context limitations
• Tasklets and work queues for deferred processing
• Shared interrupts

6. Worked Example: GPIO Character Driver

• Reading a push-button state via interrupt
• Exposing events to userspace via read() blocking with wait_event_interruptible
• Testing with a simple Python userspace client

7. Debugging Kernel Code

• dmesg and journalctl -k
• ftrace for function tracing
• KGDB for source-level debugging (brief)
• Common pitfalls: sleeping in interrupt context, reference counting errors

Platform

• Raspberry Pi 4 with Raspberry Pi OS (64-bit)
• Cross-compilation from x86_64 Linux host

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See also: FPGA AXI DMA on Zynq (coming soon) for a production kernel driver example.