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 write method

The end user can change the secret by writing a new secret into the driver, via a write(2) system call to the driver's device node. The kernel redirects the write (via the VFS layer) to our driver's write method (as you learned in the Understanding the connection between the process, the driver, and the kernel section):

static ssize_t
write_miscdrv_rdwr(struct file *filp, const char __user *ubuf, size_t count, loff_t *off)
int ret = count;
void *kbuf = NULL;
struct device *dev = ctx->dev;
char tasknm[TASK_COMM_LEN];

if (unlikely(count > MAXBYTES)) { /* paranoia */
dev_warn(dev, "count %zu exceeds max # of bytes allowed, "
"aborting write\n", count);
goto out_nomem;
dev_info(dev, "%s wants to write %zd bytes\n", get_task_comm(tasknm, current), count);

ret = -ENOMEM;
kbuf = kvmalloc(count, GFP_KERNEL);
if (unlikely(!kbuf))
goto out_nomem;
memset(kbuf, 0, count);

/* Copy in the user supplied buffer 'ubuf' - the data content
* to write ... */
ret = -EFAULT;
if (copy_from_user(kbuf, ubuf, count)) {
dev_warn(dev, "copy_from_user() failed\n");
goto out_cfu;

/* In a 'real' driver, we would now actually write (for 'count' bytes)
* the content of the 'ubuf' buffer to the device hardware (or
* whatever), and then return.
* Here, we do nothing, we just pretend we've done everything :-)
strscpy(ctx->oursecret, kbuf, (count > MAXBYTES ? MAXBYTES : count));
// Update stats
ctx->rx += count; // our 'receive' is wrt this driver

ret = count;
dev_info(dev, " %zd bytes written, returning... (stats: tx=%d, rx=%d)\n",
count, ctx->tx, ctx->rx);
return ret;

We employ the kvmalloc() API to allocate memory for a buffer to hold the user data that we will copy in. The actual copying is done via the copy_from_user() routine, of course. Here, we use it to copy the data passed by the user space app to our kernel buffer, kbuf. We then (via the strscpy() routine) update our driver's context structure's oursecret member to this value, thus updating the secret! (A subsequent read on the driver will now reveal the new secret.) Also, do notice the following:

  • How we now consistently use the dev_xxx() helpers in place of the usual printk routines. This is recommended for device drivers.
  • The (now typical) usage of goto to perform optimal error handling.

This covers the meat of the driver.