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

Constraints when using a threaded handler

One last thing regarding threaded handlers: the kernel won't blindly allow you to use a threaded handler for any IRQ; it honors some constraints. At the time of registering your thread handler (via the [devm_]request_threaded_irq() APIs), it performs several validity checks, one of which we've mentioned already: IRQF_ONESHOT must be present for a threaded handler.

It also depends on the actual IRQ line; for example, I once tried using a threaded handler for IRQ 1 on x86 (it's typically the i8042 keyboard/mouse controller chip's interrupt line). It failed, with the kernel showing the following:

genirq: Flags mismatch irq 1. 00002080 (driver-name) vs. 00000080 (i8042)

So, from the preceding output, we can see that the i8042 will only accept the 0x80 bitmask for the IRQ flags, whereas I passed a value of 0x2080; a little checking will show that the 0x2000 flag is indeed the IRQF_ONESHOT...