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

A misc driver with a secret

Now that you understand how to copy data between user and kernel space (and the reverse), let's write another device driver (ch1/miscdrv_rdwr) based on our previous skeleton (ch1/miscdrv/) miscellaneous driver. The key difference is that we use a few global data items (within a structure) throughout, and actually perform some I/O in the form of reads and writes. Here, let's introduce the notion of a driver context or private driver data structure; the idea is to have a conveniently accessible data structure that contains all relevant information in one place. Here, we name this structure struct drv_ctx (see it in the code listing that follows). On driver initialization, we allocate memory to and initialize it.

Okay, there's no real secret here, it just makes it sound interesting. One of the members within this driver context data structure of ours is a so-called secret message (it's the drv_ctx.oursecret member, along with some (fake) statistics and config words). This is the simple "driver context" or private data structure we propose using:

// ch1/miscdrv_rdwr/miscdrv_rdwr.c
[ ... ]
/* The driver 'context' (or private) data structure;
* all relevant 'state info' reg the driver is here. */
struct drv_ctx {
struct device *dev;
int tx, rx, err, myword;
u32 config1, config2;
u64 config3;
#define MAXBYTES 128 /* Must match the userspace app; we should actually
* use a common header file for things like this */
char oursecret[MAXBYTES];
static struct drv_ctx *ctx;

Great; now let's move on to seeing and understanding the code.