1. Linux Device Drivers Inroduction

2. What’s a ‘device-driver’? A special kind of computer program Intended to control a peripheral device Needs to execute ‘privileged’ instructions Must be integrated into the OS kernel Interfaces both to kernel and to hardware Program-format specific to a particular OS

3. Linux device-drivers A package mainly of ‘service functions’ The package is conceptually an ‘object’ But in C this means it’s a ‘struct’ Specifically:struct file_operations { …; }; Definition is found in a kernel-header: ‘/usr/src/linux/include/linux/fs.h’

4. Types of Device-Drivers Character drivers: - the device processes individual bytes (e.g., keyboard, printer, modem) Block drivers: - the device processes groups of bytes (e.g., hard disks, CD-ROM drives)

5. Linux has other driver-types Network drivers Mouse drivers SCSI drivers USB drivers Video drivers ‘Hot-swap’ drivers … and others

6. Linux treats devices as files Programmers accustomed to the file API open(), seek(), read(), write(), close(),... Requires creating a filename in a directory (special ‘/dev’ directory is for devices)

7. Driver Identification Character/Block drivers: Use ‘major-number’ to identify the driver Use ‘minor-numbers’ to distinguish among several devices the same driver controls Kernel also needs a driver-name Users need a device-node as ‘interface’

8. Developing a device-driver Clarify your requirements Devise a design to achieve them Test your design-concept (‘prototype’) ‘Debug’ your prototype (as needed) Build your final driver in iteratively Document your work for future use

9. ‘Open Source’ Hardware Some equipment manufactures regard their designs as ‘intellectual property’ They don’t want to ‘give away’ their info They believe ‘secrecy’ is an advantage They fear others might copy their designs BUT: This hinders systems programmers!

10. Non-Disclosure Agreements Sometimes manufacturers will let ‘trusted’ individuals, or commercial ‘partners’, look at their design-specs and manuals. College professors usually are ‘trusted’ BUT: Just to be sure, an NDA is required -- which prevents professors from teaching students the design-details that they learn.

11. Advantage of ‘open’ designs Microsoft and Apple used to provide lots of technical information to programmers. They wanted to encourage innovations that made their products more valuable. Imagine hundreds of unpaid ‘volunteers’ creating applications for your system! BUT: Were they ‘giving away the store’?

12. Some designs are ‘open’ The IBM-PC designs were published Other companies copied them And those companies prospered! While IBM lost market-share! An unfortunate ‘lesson’ was learned

13. Standard PC/AT Keyboard Controlling the three LEDs

14. References on PC keyboard Hans-Peter Messmer, “The Indispensable PC Hardware Book (Third Edition)”, Addison-Wesley (1997), pp. 1007-1030. Frank van Gilluwe, “The Undocumented PC (Second Edition)”, Addison-Wesley (1997), pp. 309-390.

15. 8042 Keyboard Controller Programming Interface (4 registers): control register (0x64, write-only) status register (0x64, read-only) input buffer register (0x60, write-only) output buffer register (0x60, read-only) Device Interface (2 registers + 2 signals): input port registertiming signals output port registerinterrupt signals

16. Keyboard Controller’s Status Eight individual status-bits (read-only) Visible to cpu by using: ‘inb( 0x64 ); bit #0: output-buffer full bit #1: input-buffer full Other bits are unimportant for LEDs

17. Two-Step Sequence Step 1: Send command-byte Step 2: Send parameter-byte If this sequence were to interrupted, then another process might try to send a byte, causing hardware to perform wrong action!

18. “Critical Section” Code-fragment whose correct functioning depends on it not being interrupted Subject to “race conditions” if exclusive access to resourses isn’t assured

19. On Uniprocessor Systems Disable interrupts during “critical section”: cli();// do not recognize interrupts … /* critical code-fragment goes here */ … sti();// ok to recognize interrupts

20. Algorithm for setting LEDs Block interrupts (Enter “Critical Section”) Wait till controller’s input-buffer empties Output the ‘write_LED’ command-byte Wait till controller’s input-buffer empties Output the LED indicator-byte Wait till controller’s input-buffer empties Allow interrupts (Leave “Critical Section”)

21. On Multiprocessor Systems Not enough to block interrupts on one cpu Must also insure other cpus can’t interfere

22. Module ‘Boilerplate’ Must have ‘init_module()’ function (to ‘register’ service-functions with kernel) Must have ‘cleanup_module()’ function (to ‘unregister’ your service-functions)

23. More ‘boilerplate’ Must include certain kernel header-files (e.g., #include ) Must define certain constants (e.g., #define __KERNEL__, MODULE)

24. Rapid Prototyping We will be writing lots of modules We can ‘automate’ the boilerplate We should write a ‘wizard’ program It will save us LOTS of time!