Book Image

Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization

By : Kaiwan N Billimoria
Book Image

Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization

By: Kaiwan N Billimoria

Overview of this book

Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization is an ideal companion guide to the Linux Kernel Programming book. This book provides a comprehensive introduction for those new to Linux device driver development and will have you up and running with writing misc class character device driver code (on the 5.4 LTS Linux kernel) in next to no time. You'll begin by learning how to write a simple and complete misc class character driver before interfacing your driver with user-mode processes via procfs, sysfs, debugfs, netlink sockets, and ioctl. You'll then find out how to work with hardware I/O memory. The book covers working with hardware interrupts in depth and helps you understand interrupt request (IRQ) allocation, threaded IRQ handlers, tasklets, and softirqs. You'll also explore the practical usage of useful kernel mechanisms, setting up delays, timers, kernel threads, and workqueues. Finally, you'll discover how to deal with the complexity of kernel synchronization with locking technologies (mutexes, spinlocks, and atomic/refcount operators), including more advanced topics such as cache effects, a primer on lock-free techniques, deadlock avoidance (with lockdep), and kernel lock debugging techniques. By the end of this Linux kernel book, you'll have learned the fundamentals of writing Linux character device driver code for real-world projects and products.
Table of Contents (11 chapters)
Section 1: Character Device Driver Basics
User-Kernel Communication Pathways
Handling Hardware Interrupts
Working with Kernel Timers, Threads, and Workqueues
Section 2: Delving Deeper

Checking for the presence of debugfs

First off, in order to make use of the powerful debugfs interface, it must be enabled within the kernel config. The relevant Kconfig macro is CONFIG_DEBUG_FS. Let's check whether it's enabled on our 5.4 custom kernel:

Here, we are assuming you have the CONFIG_IKCONFIG and CONFIG_IKCONFIG_PROC options set to y, thus allowing us to use the /proc/config.gz pseudo file to access the current kernel's configuration.
$ zcat /proc/config.gz | grep -w CONFIG_DEBUG_FS

Indeed it is; it's typically enabled by default in distributions.

Next, the default mount point of debugfs is /sys/kernel/debug. Thus, we can see that it is internally dependent on the sysfs kernel feature being present and mounted, which it is by default. Let's check where debugfs is mounted on our Ubuntu 18.04 x86_64 VM:

$ mount | grep -w debugfs
debugfs on /sys/kernel/debug type debugfs (rw,relatime)

It is available and mounted at the expected location...