1. Introduction to Linux Device Drivers
Class 2: Linux Character Device Drivers
August 5, 2014 Khem Raj

2. Introduction What is a Device Driver
Operates and controls a given device type
Part of kernel runs with same privileges
Abstracts out interfaces to be common read/write like files

3. Introduction Device Driver Types
Character Device drivers e.g. tty drivers
Block Device drivers
Network Device drivers
User-space device drivers

4. Character Device Drivers
Drivers for byte-oriented operations
Serial driver
Audio/video drivers
Camera drivers

5. Character Device Files
Used to communicate with device Drivers
Uses Virtual File system
Applications write/read usual way on device files
File operations translated by VFS to device operations
Even though application operations are normal output may not be same
E.g. an immediate read after write may not be same

6. Character Device Drivers
User Space
kernel space
device space
App1
Character Device Files
Virtual File System
Char Device Driver
Char Device Driver
Char device A
Char device B

7. Character Device Operations
Application e.g. minicom
Device File e.g. /dev/ttyUSB0
Device Driver
Device

8. Major and Minor Numbers
Application talks to device file via file name
/dev/console
Connection between Device File and Device Driver
Device file number <major, minor>
Defined in linux/types.h
MAJOR(dev_t dev), MINOR(dev_t dev)
MKDEV(int major, int minor) – Creates device

9. Major and Minor Numbers
Connecting device file to device driver
Register for given major,minor device files
register_chrdev_region ()
Static allocation
alloc_chrdev_region()
Assign the device number dynamically
Link device file ops to device driver functions

10. Simple serial driver module

11. Compiling module Needs prepared kernel source tree Now Build module
zcat /proc/config.gz > .config
make prepare
make scripts
Make
Now Build module
make –C <path-to-kernel-srcs> M=$PWD modules

12. Installing and running
sudo insmod –f fakemod.ko
Lsmod | grep fakemod
rmmod fakemod

13. Running the driver

14. File Creation and operations
Automatic creation of devices
Using devfs APIs
struct class *class = class_create(THIS_MODULE, "<device class name>");
Populate device major/minor
Device_create(class, NULL, mydevice, NULL, "<device name format>", ...)

15. File Creation and operations
Destroy device and class
Calls should be made in reverse order
device_destroy(class, mydevice);
class_destroy(class);

16. File Operations File operations use same APIs from userspace
VFS decodes file types and transfers file operations to right bus/channel
Driver should register file operations with VFS
Fill fileops struct with desired operations (Struct file_operations )
dev_open, dev_close, dev_read, dev_write
Initialize character device struct cdev_init()
Hand over to VFS
cdev_add()

17. File Operations

18. Thanks
Questions?