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

Available softirqs and what they are for

The work that's carried out by a given softirq is statically compiled into the kernel image (it's fixed). This coupling of the softirq and the action it takes (in effect, the code it runs, via the action function pointer) is done via the following code:

// kernel/softirq.c
void open_softirq(int nr, void (*action)(struct softirq_action *))
softirq_vec[nr].action = action;

The following diagram is a conceptual representation of the available softirqs and their priority level on Linux (as of kernel version 5.4), with 0 being the highest and 9 the lowest softirq priority level:

Figure 4.11 – The 10 softirqs on Linux in order of priority (0:highest, 9:lowest)

The following table sums up the individual kernel's softirqs in order of their priority (0: HI_SOFTIRQ being the highest priority one), along with the action or vector, its functionality, and a comment mentioning what its use case is:

Softirq# Softirq Comment...