Agenda
What are Device Drivers???
Scope of this Module
The Role of a Device Driver
Splitting the Kernel
Classes of Devices and Modules
Version Numbering
References
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Introduction to Device Drivers
User Space Mouse
Application 1 Driver
Operating Keyboard
System Driver
User Space
Application 2 Monitor
Driver
A simple black box that allows a user space application
interaction with a hardware, without need for the knowledge of
the hardware functioning
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Why the craze????
The rate at which new hardware becomes available (and
obsolete!) alone guarantees that driver writers will be busy
for the foreseeable future
To gain access to a particular device of interest
Hardware vendors, by making a linux driver available for
their products, can add the large and growing Linux user
base to their potential market
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Kernel Classifications and Linux
Kernels
Monolithic Kernel Micro Kernel
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Monolithic Kernels
All the parts of a kernel like the device drivers, scheduler, file
system, memory handling, networking stacks, etc., are maintained
in one unit within the kernel
Advantages
Faster processing as message passing is not required
Disadvantages
If any one module crashes, the entire system may go down
Examples
Windows, Linux, etc...
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Micro Kernels
Only the very important parts like IPC, basic scheduler, basic
memory handling, basic I/O primitives etc., are put into the kernel.
The other parts like the complete scheduler, memory handling,
filesystems and networking stacks are maintained as user space
applications.
Advantages
The parts running in the user space may crash without causing
major damage to the functioning of the entire system
Disadvantages
Message passing mechanisms between the kernel and user
space render slower functioning of the system
Examples: QNX, Mach (CMU), Minix 3, EROS, KeyKOS
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The Linux Kernel
Kernel Module
Linux Kernel
Linux is a monolithic kernel but allows a functionality of
modules.
Kernel modules may be added dynamically to the kernel
whenever required and unloaded whenever it is not in use.
This may be done either at kernel configuration time or later,
once the system has booted
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The Role of a Device Driver
The role of a driver is to provide mechanisms and not
policies.
Mechanisms????
“what capabilities are to be provided”
Policies?????
“how these capabilities can be used”
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The Role of a Device Driver
Most software problems are split into the above discussed
ideologies. If the two issues are addressed by by separate
parts of a program or by two separate programs altogether,
the software package is easier to develop
Examples of this modularization:
Unix management of the graphic display being split
into the X-server and the windows & sessions
manager
Advantages:
Users can use the same window manager on different hardware
Multiple window configurations can run on the same system
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The Role of a Device Driver
Another example is the the layered structure of TCP/IP
networking: The OS offers a socket abstraction, which
implements no policy regarding the data being transferred,
while servers (like the ftp server) are in charge of their services
(their associated policies)
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Characteristics of Policy free drivers
Include support for synchronous and asynchronous
operation
The ability to be opened multiple times
The ability to exploit full capabilities of the hardware
Lack of software layers to “simplify things” or provide
policy-related operations
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User Space Applications and
Kernel Space Modules
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Comparison of User and Kernel
Spaces
Event Driven Execution: User space applications run to
completion and once executed, they die out. Kernel modules
on the other hand just register themselves with the kernel
and wait until they are called to be executed. In short, kernel
modules may also be called event driven modules
Libraries: There are no libraries present in the kernel, unlike
user space applications
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Comparison of User and Kernel
Spaces
Floating Point: There is no floating point support in the
kernel.
Segmentation Faults: Segmentation faults in the kernel may
in the least kill the current process running or may even kill
the whole system
Kernel Debugging: No specialized mechanisms of
debugging in the kernel
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Comparison of User and Kernel Spaces
Kernel Stack: The stack in the kernel is limited to about 4k
bytes. Therefore the stack is to be used very sparingly
within the kernel
Infinite Looping: Infinite looping should not be done in the
kernel
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Splitting the Kernel
Process Management
creating and destroying processes, handling connection
to the outside world, communication among different
processes.... scheduler controls how processes share
the CPU
Memory Management
memory usage and virtual address space
Filesystems
managing files, supporting multiple file systems
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Splitting the Kernel
Device Control
device drivers
Networking
packet reception, handling, transmission, routing, address
resolution
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Classes of Devices and Modules
The linux way of looking at devices distinguishes between
three fundamental device types
All modules usually implement one of these types and thus
are classifiable as
character module
block module
network module
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Character Devices
Device that can be accessed as a stream of bytes. A char driver
is in charge of implementing this behaviour
Implements the basic functionalities such as open, read, write,
close system calls
Examples: text console, serial ports..etc..,
Char devices are accessed by user space applications through
device nodes implemented by the filesystem such as /dev/ttyS0,
/dev/tty1
The relevant difference between a char device and a usual file of
the system is the backward seeking not being possible in a char
device.
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Block Devices
A block device is one that can host a filesystem. A block
device can handle I/O operations that transfer one or more
whole blocks
Like char devices, block devices are accessed through
filesystem nodes in the /dev directory
Linux allows the application to read and write a block device
like a char device -it permits the transfer of any number of
bytes at a time
To a user, both the char device and block device look similar
but differ in interface to the kernel
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Network Interfaces
Any network interactions are made through an interface, that
is, a device that can exchange data with other hosts.
A network interface is in charge of sending and receiving
data packets, driven by the network subsystem of the kernel,
without knowing how individual transactions map to the
actual packets being transmitted.
A network driver does not know anything about
connections. It only handles packets
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Network Interfaces
A network interface isn’t easily mapped to a node in the
filesystem. A unique name is assigned to them (such as
eth0) that does not have an entry in the filesystem
Communications between the kernel and a network device is
completely different as compared to a char or block device.
Functions related to packet transmission are issued in place
of read and write in network drivers
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Other Driver Classifications
There are other classifications to driver modules that are
orthogonal to the previously discussed. In general, some
types of drivers work with additional layers of kernel support
functions for a given type of device
Example:
Every USB device is driven by a USB module that
works with the USB subsystem, but the device itself
shows up in the system as a char device (USB serial
port), a block device (a USB memory card reader) or,
a network device (a USB Ethernet Interface)
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Software Drivers
In addition to device drivers, there are other modules in the
kernel that do not relate to any specific hardware. They
simply map the organization of a device to higher level
structures.
A common example is a filesystem implementation which
maps lower level structures to higher level structures of the
filesystem. Such a module is called a Software Driver.
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Version Numbering
Every software package used in a Linux system has its own
release number, and there are often interdependencies
across them
Even numbered kernel numbers are stable versions and are
intended for general distributions. eg., linux 2.6, 2.4
Odd numbered kernel versions are development snapshots
and represent the current status of development
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References
Jonathan Corbet, Alessandro Rubini & Greg Kroah-Hartman,
“Linux Device Drivers”, 3rd Edition, O’reilly Publications,
November 2005
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