Device Drivers in Linux T S PRADEEP KUMAR

Lecturer, SCS, VIT

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Key points

• History• Why Linux• Linux Kernel• Device Drivers

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History• Linux is a modern, free operating system

based on UNIX standards.

• First developed as a small but self-contained kernel in 1991 by Linus Torvalds, with the major design goal of UNIX compatibility.

• Collaboration by many users all around the world, corresponding almost exclusively over the Internet.

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What Linux in Embedded?

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Linux Kernel/System/Distribution

• Kernel– The OS Code that runs on privileged mode

• System– Essential system components, but runs in

user mode– Compilers, system libraries

• Linux Distribution– Precompiled and ready to install tools and

packages– Popular distributions: redHat, SuSE, Debain,

Mandrake,etc.

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Red Hat Directory Tree

/b oot

/b in

/in c lu de

/lib ex e c

/lib

/lo ca l

/sb in

/sh are

/X 1 1 R 6

/u sr

/u ser1

/u ser2....

/h o m e /d ev /b in

/in it.d

/rc .d

/h ttpd

/x in e t.d

/e tc /sb in

/lo ck

/log

/lib

/m a il

/w w w

/var /tm p /root /p roc

/

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User mode and kernel mode

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User Space

• At any given time, the CPU executes ineither User Mode or Kernel Mode• User Space

– Cannot execute privileged instructions– Cannot access kernel code and data– Cannot directly access hardware resources– Shell or other applications like Open

Office,etc.

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Kernel Space

• Full privileges, Full access• Any subroutines or functions

forming part of the kernel (modules and device drivers, etc) are considered to be part of kernel space.

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Kernel Modules or Device Drivers

• Multiple Processes trying to access a particular hardware

• Executed as part of the system kernel

• Drivers are either part of the kernel or loaded as a module

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Everything is File• You have probably heard the saying

that in UNIX, everything is a file.

• This is true, because ALL devices are provided as a file in the UNIX OS.

• The user can then perform basic file

operations on the device – read, write, open, close, ...

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Prerequisites

• Basic knowledge of kernel compilation

• A good deal of programmingexperience in C under Linux

• the right techniques of data structures, like linked list is essential along with their data types.

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Major & Minor Numbers

• Devices require unique identification in the system

• The major number identifies the type of device

• The minor number identifies any mode or subunit of the device.

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Blocks & Characters• Devices within the kernel can be separated

between character and block devices.

• A block device is something that can host a filesystem such as a disk. A block device can only be accessed as multiples of a block, where a block is usually 1kB of data .

• A character device is one that can be accessed like a file. – This driver implements the open, close, read and

write system calls.– Examples: Console, Parallel Ports, Serial Ports, etc.

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Device Drivers

Option 1• Build device driver into the kernel

– Advantage – Driver available at boottime– Disadvantage – My need to load drivers that

are rarely used

Option 2• Build device driver as a kernel module

– Advantage – Load When Needed– Advantage – Unload when not longer needed– Disadvantage – Potential attempts to load

“bad” modules into the kernel

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Device Drivers as a Module• If a device is provided as a module, then

it must be registered and unregistered with the kernel.

• To register a device driver– register_chrdev()– register_blkdev()

• To unregister a device driver – unregister_chrdev() or– unregister_blkdev()

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Accessing the Device

• Once created, the device is accessible through a standard interface.– Function pointers in a structure.

• Two types of structures typically exist,

one for character devices and one for block devices.

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Device Driver Events

Events User Functions Kernel Functions

Load Module insmod init_module()

Open Device fopen file_operations:open

Read Device fread file_operations:read

Write Device fwrite file_operations:write

Close Device fclose file_operations:release

Remove Device rmmod Cleanup_module()

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Demos• To list currently loaded kernel modules

– /sbin/lsmod• Example character device name

– ls l /dev/lp0• Example block device name

– ls l /dev/hda• Show list of registered character/block

devices– cat /proc/devices

• Show list of network interfaces– /sbin/ifconfig a

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Hello World Device Driver#include <linux/module.h>#include <linux/kernel.h>MODULE_LICENSE(“GPL”);static int major=0;static struct file_operations fops={};

static int init_module(void) { printk("hello, world\n"); major=register_chrdev(major,”mychr”,&fops);printk(“major=%d\n”,major);return 0; }

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static void cleanup_module(void){

unregister_chrdev(major,”mychr”);printk(“Bye”);

}

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Makefile (2.6 Kernel)

• objm := hellokm.o• KDIR := /lib/modules/$(shell uname r)• PWD := $(shell pwd)• default:

make C $(KDIR) SUBDIRS=$(PWD) modules

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Compilation (2.4 KERNEL)

root# gcc -c hello.croot# insmod ./hello.oHello, worldroot# rmmod helloGoodbye cruel worldroot#

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Thank you