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How do you use Linux device drivers to interface with hardware peripherals?

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Linux is a popular operating system for embedded software development, as it offers a rich set of features, tools, and libraries. However, to fully leverage the capabilities of Linux, you need to understand how to use Linux device drivers to interface with hardware peripherals. In this article, you will learn the basics of Linux device drivers, how to write and compile a simple driver module, how to load and unload it from the kernel, and how to communicate with it from user space.

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Thusinthaka Thinesh Hewa Pathiranna

Embedded Engineer @ Zebra Technologies | Embedded Memory Optimization | AOSP

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1 What are Linux device drivers?

A device driver is a piece of software that controls a specific hardware device, such as a sensor, a motor, or a display. It provides a standard interface for the kernel and user space applications to access the device, hiding the details of its implementation and operation. Linux device drivers are typically written in C or C++, and compiled as loadable kernel modules (LKMs). LKMs are dynamic objects that can be inserted into or removed from the kernel at runtime, without rebooting the system.

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2 How to write a Linux device driver?

To write a Linux device driver, you need to follow some basic steps. First, you need to include the appropriate header files, such as <linux/module.h>, <linux/kernel.h>, and <linux/init.h>, which define the macros and functions for module initialization and cleanup. Second, you need to define the module information, such as its name, author, license, and description, using the MODULE_* macros. Third, you need to implement the module_init() and module_exit() functions, which are called when the module is loaded or unloaded. These functions typically register and unregister the device driver with the kernel, using the functions register_chrdev() and unregister_chrdev() for character devices, or register_blkdev() and unregister_blkdev() for block devices. Fourth, you need to implement the file operations for the device driver, such as open, read, write, close, and ioctl, and assign them to a file_operations structure. These operations define how the device driver responds to the system calls from user space applications.

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Thusinthaka Thinesh Hewa Pathiranna

Embedded Engineer @ Zebra Technologies | Embedded Memory Optimization | AOSP
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$make modules_install command is used to install compiled kernel modules into the appropriate directory within the module tree. This command : 1- initiates the installation process. 2- Targets the "modules_install rule": This specific rule within the kernel build system's Makefile is designed to handle module installation. 3- Installs modules: The rule copies the compiled module files (usually with .ko extensions) into the designated directory: /lib/modules/$(uname -r)/kernel/drivers: This is the standard location for kernel modules, where: /lib/modules/ is the base directory for kernel modules.

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3 How to compile a Linux device driver?

To compile a Linux device driver, you need to create a Makefile that specifies the source files, the target module name, and the kernel build directory. The Makefile should use the kernel build system, which is invoked by the command make -C /lib/modules/$(uname -r)/build M=$(PWD) modules. This command will compile the source files and generate the module file, which has the extension .ko. You can also use the command make modules_install to install the module in the /lib/modules/$(uname -r)/kernel/drivers directory.

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Thusinthaka Thinesh Hewa Pathiranna

Embedded Engineer @ Zebra Technologies | Embedded Memory Optimization | AOSP
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In Brief: -Enter the kernel build system directory. -Instruct the kernel build system to build modules from the source code located in the current working directory. -Use the correct kernel version and build configuration for the currently running kernel. Must have the kernel headers installed for the specific kernel version you're targeting. Source code should be organized according to kernel module conventions (Makefile and a .c file).

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4 How to load and unload a Linux device driver?

To load and unload a Linux device driver, you need to use the commands insmod and rmmod, respectively. The insmod command takes the module file as an argument, and inserts it into the kernel. It also assigns a major number to the device driver, which is used to identify it in the /dev directory. The rmmod command takes the module name as an argument, and removes it from the kernel. It also frees the major number and any resources allocated by the device driver. You can use the command lsmod to list the loaded modules, and the command modinfo to display the module information.

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5 How to communicate with a Linux device driver?

To communicate with a Linux device driver, you need to use the standard file operations, such as open, read, write, close, and ioctl, from user space applications. You can either use the system calls directly, or use the library functions provided by the C standard library, such as fopen, fread, fwrite, fclose, and fcntl. You can also use the command cat to read from or write to the device driver. To access the device driver, you need to use its device file, which is usually located in the /dev directory. The device file has the format /dev/<device_name>, where <device_name> is the name of the device driver. You can use the command mknod to create the device file, or use the udev system to automatically create it when the module is loaded.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Peter Lyons Kehl

Rust Software Engineer; Investor; Speaker at Rustlang Conf42; Technical Reviewer; Loving Rust
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Suggest Rust: Helpful for drivers (but much less so for the kernel itself). Rust enforces memory and thread safety (though it doesn't prevent thread deadlocks). Yes, Rust (still) requires LLVM, so we can't use it in device-independent parts of the kernel (yet). Also, we won't be able to use Rust for many parts of the kernel anyway. Why? Because many kernel data types have multiple ownership (bidirectional lists...), and Rust enforcessingle ownership/RAII (Resource Acquisition Is Initialization). However, Rust is suitable for drivers (which are hardware-dependent, hence OK with the current LLVM requirement). It helps you write correct code, and it has good FFI (Foreign Function Interface).

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How do you use Linux device drivers to interface with hardware peripherals?

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1 What are Linux device drivers?

2 How to write a Linux device driver?

3 How to compile a Linux device driver?

4 How to load and unload a Linux device driver?

5 How to communicate with a Linux device driver?

6 Here’s what else to consider

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