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 V Memory TTYS & devs features Fs types Networks software Memory Mang. Achture code Character Devices Block Dvc If drivers hardware 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 rootfloppy 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 don’t 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 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 Character & Block Device number assignment

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. 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 Module programs

14. Use of init & exit macros Use of __init and __initdata Using macros

15. Features of kernel programming • Don’t 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