Understanding Device Drivers: A Comprehensive Guide for University Students, Study notes of Design

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Topic 10: Device Driver

Tongping Liu

Administration

• Today it is the deadline of Project
• Homework5 is posted, due on 05/
• Bonus points: ECE570 – 3 points, ECE670- 5 points - Design exam questions - Process&threads, scheduling, synchronization, IPC, memory management, device driver/virtualization - Due: 05/
• Survey project (ECE570/ECE670): 05/

Outline

• Basic concepts
• Kernel module
• Writing device driver

Device Driver

• A special kind of computer program that operates or controls a particular type of device that is attached to a computer

Whole System Stack

Note: This picture is excerpted from Write a Linux Hardware Device Driver, Andrew O’Shauqhnessy, Unix world

Another View from OS

Linux Device Driver

• Manage data flow between user programs and device
• Typically a self-contained kernel module
  • Add and remove dynamically
• Device is a special file in /dev that user can access, e.g. /dev/lp
• Can be only programed with C/assembly language

Device Files

• Unix-like systems handle device files with separate subdirectory of /dev for each type of device: disk, block etc.
• Device files are created with mknod : mknod filename type major minor
  • filename is the device file to be created.
• type is c for a character device or b for a block device.
• major and minor are the major and minor device numbers.

Device Jump Table

• Each driver supports some or all of the following functions: probe attach open close read reset stop select strategy dump psize write
• Addresses of these functions are stored in a structure called a jump table, which are indexed by major number

• Two tables exists
  • One for character device
  • One for block devices.
• Upon operations on a device file
  • Kernel catches the reference, looks up the appropriate function name in the jump table, and transfers control to it.
• For unusual operation (ejecting floppy disk)
  • ioctl() to pass a message directly from user space into the driver

Device Jump Table

Block Device Driver

• Accessed through a cache so buffering is required.
• Seeking is possible, which can move back and forth between any location
• Kernel provides an entire subsystem, since it is too complex
• Reads and write done by buffer cache mechanism by bread(), bwrite(). These request may be asynchronous.

Outline

• Basic concepts
• Kernel module
• Writing device driver

Kernel Modules

• Definition: modules are pieces of code that can be loaded and unloaded into the kernel upon demand. - They extend the functionality of the kernel without the need to rebuild/reboot the system. - Without modules, we would have to build monolithic kernels and add new functionality directly into the kernel image.
• Advantages:
  • We do not need to build the entire kernel, saving time
  • Help us to save memory, due to load on demand

Kernel Modules vs. Device Drivers

• Most modules are device drivers
  • But some are not, such as IPv6 support or some file systems
• Device drivers: drive the hardware
  • They can be built as kernel modules so that it can be dynamically loaded later.
  • They can also be built statically into the kernel file on disk.