1. Linux Details: Device Drivers
David Ferry, Chris Gill
CSE 522S - Advanced Operating Systems
Washington University in St. Louis
St. Louis, MO 63130

2. Goal of a Device Driver
Hide details of how a device works behind a standard interface
User Code
User Space
Standard Interface
Black Box Device Driver
Kernel Space
Device
Hardware
CSE 522S – Advanced Operating Systems

3. Goal of a Device Driver
Hide details of how a device works behind a standard interface
Be modular and flexible
User Code
User Code
Standard Interface
Standard Interface
Black Box Device Driver
Modified Driver
Device
Device (version 2)
CSE 522S – Advanced Operating Systems

4. Driver Design Philosophy
Support mechanism, not policy
Mechanism: technical capabilities
Policy: how those capabilities are used
Example – sockets:
Sockets abstract many types of connections
Synchronous or asynchronous access
FTP or UDP
Variety of protocols
By being flexible, sockets place little constraint on how they are used.
Expect hardware devices to be used in many contexts!
CSE 522S – Advanced Operating Systems

5. Traditional Driver Types
Character Devices
Reads and writes a stream of characters
May only implement read, write, open, close
Console drivers, keyboard, etc.
Block Devices
Reads and writes whole blocks of bytes
E.g. a hard drive only reads and writes 4K bytes at a time
Hard drives, CD-ROM drives, floppy drives
Network Devices
Provides network interfaces (like eth0 or loopback)
Packet based read & write
CSE 522S – Advanced Operating Systems

6. CSE 522S – Advanced Operating Systems
Driver Interface
A driver typically communicates through (how else?) a special file.
struct file – kernel representation of an open file
struct file_operations – list of function pointers to be called when file is accessed via file system calls (i.e. read, write, flush, etc.), used in file
CSE 522S – Advanced Operating Systems

7. Accessing Hardware Devices
Control over peripheral devices is done by writing and the device’s registers
How do we read and write those?
I/O Ports – R/W special processor instructions
I/O Memory – R/W special memory addresses
Memory mapping is preferred (and faster)!
Maps large regions (e.g. video frame buffer)
Some architectures don’t support I/O Ports
CSE 522S – Advanced Operating Systems

8. Memory Mapped I/O Functions
#include <linux/ioport.h> request_mem_region() – allocates a memory region suitable for I/O memory map ioremap() – maps peripheral physical I/O space into kernel memory space ioread32(), iowrite32(), io_memcpy() – family of functions for reading and writing memory mapped I/O
CSE 522S – Advanced Operating Systems

9. Devices and Interrupts
Syscalls provide a synchronous interface to hardware devices
Interrupts allow asynchronous operations
Many devices are much slower than processor
Processor
3) Interrupt notifies CPU that work is done
4) Deferred processing
1) Writes requested block to device
2) Seeks and writes block into I/O mapped memory
Hard-drive
CSE 522S – Advanced Operating Systems

10. CSE 522S – Advanced Operating Systems
More Details
There are whole books about how to write good device drivers, plus lots of example code: Linux Device Drivers, 3rd ed. by Corbet, Rubini, and Kroah-Hartmann Available online (free) under creative commons license
CSE 522S – Advanced Operating Systems