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

Our secret driver – the read method

We will first show the relevant code for the read method – this is how a user space process (or thread) can read in the secret information housed within our driver (in its context structure):

static ssize_t
read_miscdrv_rdwr(struct file *filp, char __user *ubuf, size_t count, loff_t *off)
int ret = count, secret_len = strlen(ctx->oursecret);
struct device *dev = ctx->dev;
char tasknm[TASK_COMM_LEN];

dev_info(dev, "%s wants to read (upto) %zd bytes\n", get_task_comm(tasknm, current), count);

ret = -EINVAL;
if (count < MAXBYTES) {
[...] << we don't display some validity checks here >>

/* In a 'real' driver, we would now actually read the content of the
* [...]
* Returns 0 on success, i.e., non-zero return implies an I/O fault).
* Here, we simply copy the content of our context structure's
* 'secret' member to userspace. */
ret = -EFAULT;
if (copy_to_user(ubuf, ctx->oursecret, secret_len)) {
dev_warn(dev, "copy_to_user() failed\n");
goto out_notok;
ret = secret_len;

// Update stats
ctx->tx += secret_len; // our 'transmit' is wrt this driver
dev_info(dev, " %d bytes read, returning... (stats: tx=%d, rx=%d)\n",
secret_len, ctx->tx, ctx->rx);
return ret;

The copy_to_user() routine does its job – it copies the ctx->oursecret source buffer to the destination pointer, the ubuf user space buffer, for secret_len bytes, thus transferring the secret to the user space app. Now, let's check out the driver's write method.