Embedded System Programming

1
Embedded System Programming

• Introduction to Device Drivers

2
What are device drivers?

• Software interfaces to hardware resources in the
  kernel
• Mirrors variety of hardware resources
• Can change constantly
• Will require reconfiguration
• Different interfaces into the same devices
• Require kernel resident programs
• Uses module interface

3
Split level kernel
User programs and applications
Application level
Kernel level
Process mgment
Memory mgment
File systems
Device Control
Networking
Kernel parts
Multi-tasking
Files Dirs
Connectivity
features
V Memory
TTYS devs
software
Fs types
Networks
Memory Mang.
Achture code
Character Devices
hardware
Block Dvc
If drivers
Hardware level
Disks CDs
Console S Ports
Network Interfaces
CPU
RAM
4
Types of device driver

• There are some general types or characteristics
  of device drivers
• Character devices
• Serial data stream
• Block devices
• Accessed via blocked data stream
• Network interfaces
• Although serial, has no mapping into file system

5
Recognising device drivers

• Apart from network devices most drivers appear
  like file in the Unix system
• ls l /dev
• crw-r--r-- 1 root root 10, 134 Jun7 1996 apm_bios
• brw-rw---- 1 root floppy 2, 0 May 14 1996 fd0

File Name
Device type
Minor number
Major number
6
Character Device Drivers

• Data is a serial stream
• No random access
• Even a block devices can be a character one!
• Printers

7
Block Devices

• Data is read in blocks
• Block size depends upon the device
• Random access is supported
• Files systems can only be mounted block devices
• Block devices can have character interfaces
• fsck works on character raw interface to file
  system

8
Network Interfaces

• Network interfaces act in some ways like a serial
  character orientated device, however they dont
  exist in the file system
• For example eth0 is queried through the ifconfig
  command
• Weirdly, network devices can support block
  features for example network mounted file systems

9
Character Block Device number assignment

• Character devices
• 1 Memory
• 4 Terminal
• 6 Parallel interfaces
• 7 Vitual consoles
• 9 SCSI tapes
• 10 Bus mice
• 12 QIC02 Tape
• 13 PC speaker driver

• Block devices
• 1 RAM disk
• 2 Floppy disk
• 3 IDE disk
• 8 SCSI disk
• 11 SCSI CD-ROM
• 13XT 8-bit hard disk

10
Loadable modules

• Historically there has been a split between
  monolithic kernels
• All code is contained in the kernel, fixed at
  build or boot time
• This means that kernel can be fast and efficient
• And Micro kernels
• Small lightweight kernel that load features in at
  run time, when required
• The kernel is smaller, easier to port, the feaure
  list of more flexible
• Linux loadable modules gives a compromise
  position
• Mainly monolithic kernel but can load modules in
  when needed
• The use of modules for devices is clearly
  important

11
Kernel Modules

• The provision of kernel modules allows code to be
  introduced into a running kernel.
• This requires the kernel to be built with this
  capability, it also requires the commands
• Insmod and rmmod (plus lsmod, depmod and
  modprobe)
• Modules can be loaded on demand automatically.

12
Module programming

• The 2.6 kernel has changed the way module
  programming is handled.
• We will look at this later on for the moment we
  will deal with 2.4
• Modules under 2.4 are just ordinary unlinked
  object files (cc o)
• Although they must link with the kernel and can
  bring it down, so they are rather special.

13
Module programs

• Requires header files
• These will include others
• Needs an init_module and cleanup_module function
• The return value is important
• Only return 0.
• Use of printk

14
Using macros

• Use of init exit macros
• Use of __init and __initdata

15
Features of kernel programming

• Dont use libraries only kernel code
• Printk not printf
• Set use of headers
• /usr/include/asm /usr/include/linux
• Beware of namespace pollution
• Code shares names with the kernel
• Use static