An introduction to the linux kernel and device drivers (NTU CSIE 2016.03)

Note: The Copyrights of the referenced materials and photos go to their original authors. As a result, this slide is for personal reference only.
For the contents created in this document, the Copyright belongs to William W.-Y. Liang. © 2005-2016 All Rights Reserved.
NTU CSIE Open Source System Software
2016.03.22
An Introduction to the Linux Kernel
with Device Drivers
William W.-Y. Liang (梁文耀), Ph. D.
http://www.ntut.edu.tw/~wyliang
for 台大資工系開源系統軟體課程
hosted by Prof. Shih-Hao Hung

© 2016 William W.-Y. Liang, All Rights Reserved.
General-purpose Operating Systems
Characteristics
Development of applications can be logically separated from hardware
Complete software stacks and middleware can be created to support easy
development of complex and versatile applications.
Example:
Linux (with many Distributions), Android, Chrome OS, Firefox OS,
Brillo, uClinux, MacOS, iOS, Windows, etc.
Application development
System independent application development: General applications,
GUI/Window programming, Web programming
System dependent software development: System programming, Device
drivers, Protocol software
2 NTU OSSSP 2016: Linux Kernel

The Linux Operating System
Open source and free
Maintained by numerous volunteer experts and hackers
Robust, efficient, and reliable
Modular, configurable, and scalable
Tons of applications available
Suitable for application types from low-end embedded systems, mid-range consumer
products, personal computer, and up to the high performance supercomputers
https://en.wikipedia.org/wiki/Linux_kernel

Some References
Linux Kernel Wiki
https://en.wikipedia.org/wiki/Linux_kernel
Linux Kernel Source
git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
Reference books for Beginners
Linux Device Driver, 3rd Edition, by Jonathan Corbet, Alessandro
Rubini and Greg Kroah-Hartman, O'Reilly (Also available online
https://lwn.net/Kernel/LDD3/)
Linux Kernel in a NutShell, by Greg Kroah-Hartman, O'Reilly (Also
available online http://www.kroah.com/lkn/)
Understanding the Linux Kernel, 3rd Edition, by Daniel P. Bovet, Marco
Cesati, O'Reilly

Chitchat about Linux and the Kernel
* Taiwanese slang :^)

A Systems View -- from Hardware to Software
Hardware
Software development environment
Tool chain and Library
Boot loader
OS Kernel / RTOS
Middleware*
Applications
* Example: In Android, the
middleware is called the
Application Framework.
Source: Qing Li and Caroline Yao, “real-time concepts for embedded systems”
Embedded System Development Environment Example

The Episode: System Booting
Source: Qing Li and Caroline Yao, “real-time concepts for embedded systems”

Boot Sequence Example
http://www.at91.com/linux4sam/bin/view/Linux4SAM/GettingStarted

Booting the Kernel – Before the C code

Booting the Kernel – Entering the C World

Prepare for Tracing the Kernel
Tools, always the most important thing
git, repo, vim, ctags, grep, find, etc.
Basics for understanding the Kernel
Data structures

Introduction to the Linux Kernel
Let’s try to understand the Linux
kernel by introducing the basics
for the device drivers and the
kernel features, and discussing
on some related issues.

Software View from User Program to Hardware
For a general purpose OS
such as Linux
User level (user space)
Coding
Compilation
Linking
Execution
Kernel (kernel space)
Program loading
System calls (API)
Device drivers
Hardware manipulation
Bare metal hardware

Device Drivers in Linux
Device drivers are usually treated as black boxes for
the application developers.
They resemble as a software layer
lying between the applications and
the actual devices.
http: //www.ni.com/tutorial/3789/en/

Device Drivers in Linux Kernel
ALESSANDRO RUBINI and JONATHAN CORBET, Linux Device Drivers, Second Edition

Major Classes of Devices for Linux
Character devices
A character (char) device is one that can be accessed as a stream
of bytes (just like a normal file).
Block devices
A block device is accessed in a unit of fixed-size block.
The device can be accessed randomly.
Network interfaces
Others: File Systems, Protocol Stack, USB, PCI, etc.
Refers to <kernel-source>/Documentation/devices.txt”

Device Files
File Operations vs. Device Accesses
Device Initialization vs. File Open
Device Accesses vs. File Read/Write
Device Control vs. File Control
Device Termination vs. File Close
UNIX/Linux systems implement device files to enable
users to employ familiar commands and functions such
as open, read, write, and close when working with
some peripherals.
int open( char *pathname, int flags, … );
int read( int fd, void *buf, size_t count );
int write( int fd, void *buf, size_t count );
int ioctl(struct inode *, struct file *, unsigned int, unsigned long );
int close( int fd );

How Device File and Driver Modules Work?
☼ Discussion: taking “Hello, World” as an example

Example: Applications using a Device

Device Number
Example
crw-rw-rw- 1 root root 1, 3 Mar 2 2015 null
crw-rw-rw- 1 root root 1, 5 Mar 2 2015 zero
crw------- 1 root root 4, 1 Mar 2 2015 tty1
crw-rw-rw- 1 root tty 4, 64 Mar 2 2015 ttys0
crw-rw---- 1 root uucp 4, 65 Mar 2 2015 ttyS1
crw------- 1 root root 10, 1 Mar 2 2015 psaux
crw-rw-rw- 1 root root 123, 0 Mar 2 2015 androint
A System Administrator may create device files with
the “mknod” command. For example,
mknod /dev/androint c 123 0
‘/dev/androint’ is the file’s pathname, ‘c’ indicates that it’s a
character-mode device, 123 is its (unique) ‘major number’,
and 0 is its ‘minor number’.
☼ Discussion: relationship between device files and drivers

Loadable Kernel Modules
A great mechanism for OS ‘extensibility’
No need to recompile and then reboot
A kernel module differs from a normal C program
A kernel module cannot call any of the familiar
functions from the standard C runtime libraries
A module will be run as part of the kernel
Any ‘bug’ can cause a system malfunction or a
complete crash!
☼ Discussion: GPL issues with the Linux kernel modules

Example
☼ Discussion: how multiple program entries work?

How to get a module to work in Kernel
Driver Functions

Example: Device Registration

Driver Methods -- File Operations
The kernel uses the file_operations structure to access the
driver’s methods.
int (*open) (struct inode *, struct file *);
ssize_t (*read) (struct file *, char *, size_t, loff_t *);
ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned
long);
int (*release) (struct inode *, struct file *);
…
☼ Discussion: drivers vs. the object-orient concept

Kernel and User Space
☼ Discussion: address space for kernel and processes

Virtual to Physical Space
https://en.wikipedia.org/wiki/Operating_system
☼ Discussion: how the memory space is utilized?

Data Transfer between User and Kernel Space
Read: kernel->user
Write: user->kernel
☼ Discussion: issues for cross space data accesses
ALESSANDRO RUBINI and JONATHAN CORBET, Linux Device Drivers, Second Edition ,

Example: User & Kernel Space Data Transfer

Kernel Memory Allocation
Small size allocation
kmalloc and kfree
dptr->data = kmalloc(qset * sizeof(char *), GFP_KERNEL);
memset(dptr->data, 0, qset * sizeof(char *));
Limitations
Large size allocation
vmalloc and vfree
Restrictions
Page allocation
Slab and memory pool
☼ Discussion: performance issues in memory management

Multi-tasking: the Central Part of the OS
Process states in OSes
Running: own CPU
Ready: To be scheduled,
in ready queue (run-queue)
Waiting:
Pending on I/O or Event
Delay itself for some duration
Request a resource but is not yet available
Linux Task States
Tasks: user-process, user-thread, kernel thread
States: TASK_RUNNING, TASK_INTERRUPTIBLE, etc.
Scheduling Policy
SCHED_OTHER, SCHED_FIFO, SCHED_RR, etc
☼ Discussion: scheduling and preemption issues

Concurrency Problems
Problems: Data inconsistency, Deadlock
Race Condition: Occurs when multiple tasks want to
access the shared resources, such as shared data or
hardware devices.
☼ Discussion: what’s the exact cause?

Thread Safe
Potential Causes of Thread-Unsafe Problems
Global variables
Static variables
Indirect accesses
Solutions
Reentrancy
Atomic operations
Thread-local storage
Mutual exclusion
Concurrency in Kernel and Device Drivers
 Driver code vs. Tasks
 Single Core vs. Multicore
 IRQ impact
 Soft-IRQ
☼ Discussion: performance issues for synchronizations

Synchronization Operations
Semaphore
down(), up(), etc.
Mutex
mutex_lock(), mutex_unlock(), etc.
Spin locks
spin_lock(), spin_unlock(), etc.
Atomic operations
atomic_set(), atomic_add(), etc
Bit operations
set_bit(), clear_bit(), etc.
☼ Discussion: semaphore vs. spinlock, usage & performance

Interrupt Handling
☼ Discussion: cost and performance issues
of IRQ handling and solutions

Task Blocking and Waiting Queue
Processes are usually blocked due to I/O waiting.
No data available for read, or no space
available for write or device busy
Waiting queues
Blocked task is put into a waiting queue
before it is woke up.
36 NTU OSSSP 2016: Linux Kernel Linux Device Drivers, Second Edition
☼ Discussion: how it works?
Linux Device Drivers, Second Edition

The Endless Journey on Discovering Linux
To be Continued…
This brief introductory lecture is just a beginning.
Still many interesting things out there
Take it as interesting adventure, by learning from the
wonderful open source world!
https://en.wikipedia.org/wiki/Linux_kernel#/media/File:Linux_kernel_map.png
Let’s keep the discussion next week
on the next topic – Android Framework.

Note: The Copyrights of the referenced materials and photos go to their original authors. As a result, this slide is for personal reference only.
For the contents created in this document, the Copyright belongs to William W.-Y. Liang. © 2005-2016 All Rights Reserved.
Q & A
william.wyliang@gmail.com
http://www.ntut.edu.tw/~wyliang
http://www.facebook.com/william.wyliang

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