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

Using the ioremap*() APIs

Now, as we saw in the Understanding the issue with direct access section, attempting to perform I/O directly on these physical or bus addresses simply won't work. The way we should do this is by telling Linux to map these bus addresses into the kernel's VAS so that we can access it through kernel virtual addresses (KVAs)! How do we do this? The kernel provides APIs for this express purpose; a very common one that driver authors use is the ioremap() API. Its signature is as follows:

#include <asm/io.h>
void __iomem *ioremap(phys_addr_t offset, size_t size)

The asm/io.h header becomes an arch-specific header file as required. Notice how the first parameter to ioremap() is a physical (or bus) address (it's data type is phys_addr_t). This is one of the rare cases in Linux where you, as a driver author, have to supply a physical – not a virtual  address (the other typical case...