Transcript Device Drivers

Device Drivers
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B. RAMAMURTHY
4/6/2016
Introduction
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 A device driver is computer program that allows a system
to interface with hardware devices.
 Example driver: printer driver, bluetooth driver, pipe
driver
 Example devices: your USB stick, sensors: accelerometer
 It is a translator between the operating system and
applications the use the devices and the devices.
 A typical operating system has many device drivers built
into it.
 A device driver converts general IO instructions into
device specific operations.
 Device drivers operate in a privileged mode requires
careful design
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Why Device Driver?
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 A typical computing system (lap top, computer, cell
phone, PDA, Point of sale system) deals with a variety of
devices.
 Making a hardware device work as expected is a
cumbersome task.
 Instead adding this code every application, operating
system provides a single point interface for all devices by
hosting the device drivers.
 Adding it under the operating systems provides the
protection and security needed for the device drivers
from malicious use.
 The device drivers are essentially shared dynamically
linked libraries.
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File abstraCtion
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 What do you with a device? {read, write}, {read
only}, {write only}
 Lets look at some examples: USB device, CD-ROM,
LED Display,
 What do you do with a file? open, close, read, write, ..
 File is an excellent abstraction for devices.
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/dev partial listing
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total 380
lrwxrwxrwx 1 root
30 Mar 7 2004 allkmem -> ../devices/pseudo/mm@0:
allkmem
lrwxrwxrwx 1 root
27 Aug 15 2001 arp -> ../devices/pseudo/arp@0:arp
lrwxrwxrwx 1 root
7 Aug 15 2001 audio -> sound/0
lrwxrwxrwx 1 root
10 Aug 15 2001 audioctl -> sound/0ctl
lrwxrwxrwx 1 root
11 Oct 4 03:06 bd.off -> /dev/term/b
drwxr-xr-x 2 root
512 Aug 17 2001 cfg
lrwxrwxrwx 1 root
31 Aug 15 2001 conslog -> ../devices/pseudo/log@0
:conslog
lrwxrwxrwx 1 root
30 Aug 15 2001 console -> ../devices/pseudo/cn@0:
console
drwxr-xr-x 2 root
512 Aug 15 2001 cua
drwxr-xr-x 2 root
2048 Aug 31 2002 dsk
lrwxrwxrwx 1 root
29 Aug 15 2001 dump -> ../devices/pseudo/dump@0:d
ump
lrwxrwxrwx 1 root
50 Aug 15 2001 ecpp0 -> ../devices/pci@1f,4000/eb
us@1/ecpp@14,3043bc:ecpp0
lrwxrwxrwx 1 root
8 Aug 15 2001 fb0 -> fbs/ffb0
drwxr-xr-x 2 root
512 Aug 15 2001 fbs
dr-xr-xr-x 2 root
528 Nov 9 11:51 fd
lrwxrwxrwx 1 root
30 Apr 7 2002 fssnapctl -> ../devices/pseudo/
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Device SPACE
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 Typically there are multiple devices of the same type.
 All the devices controlled by the same device driver is
given the same “major number”
 A “minor number” distinguishes among the devices
of the same type.
 Example: printers have a major number since
purpose is same, minor# is denote a specific printer
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Examples from XINU
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 Take a look at files in the include directory:
 device.h
 tty.h
 uart.h
 Also in the system directory devtable.c, initialize.c
 Bottom line is this, for a device xyz:
1. Include a file in include directory: xyz.h
-- define the operations/functions for the device
2. Add a directory xyz
-- implement all functions each in its own file
3. Add an entry in the devtable.c for the device (note that this
has the “minor” device number along with other things)
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Lets Analyze the XINU UART Driver
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 Starting point: uart.h in include directory
 uart directory functions
 system directory devtable.c, initialize.c
 Usage of the devices is through device table:
 Ex:
pdev = &devtab[i];
(pdevinit)(pdev);
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UART Driver in EXINU
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General device driver related files: device.h,
devtable.c
2. Uart files: uart.h defining the physical features of
the uart
3. All the files in the uart directory that implement
the operations related to the uart.
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uartControl.c uartInit.c uartIntr.c
uartPutChar.c uartWrite.c uartGetChar.c uartRead.c
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Device Drivers
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 On board devices are called internal peripherals
and one outside are called external peripherals
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UART Chip (internal)
TTY (external)
 UART transceiverRS232D-9 connector 
laptop serial socket

WRT54GL board and modifications
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Device drivers (contd.)
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 Embedded processor interacts with a peripheral
device through a set of control and status registers.
 Registers are part of the peripheral device.
 Registers within a serial controller are different
from those in a timer.
 These devices are located in the memory space of
the processor or I/O space of the processor-- two
types: memory-mapped or I/O mapped
respectively.
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Device driver (contd.)
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 The keyword volatile should be used when declaring
pointers to device drivers.
 Bit patterns for testing, setting, clearing, toggling,
shifting bits, bitmasks, and bitfields.
 Struct overlays:
 In embedded systems featuring memory mapped IO
devices, it is common to overlay a C struct on to each
peripheral’s control and status registers.
 This will provide the offsets for the various registers from
the base address of the device.
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Device Driver Philosophy
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 Hide the hardware completely: hardware abstraction
 If the device generates any interrupts include interrupt
controllers.
 Device driver presents a generic interface for applications
at higher level to access the devices: device.h
 Device drivers in embedded systems are different from
general purpose operating systems: See diagram in slide
#14
 Applications in general purpose systems accesses OS
(Operating Systems) which in turn accesses device
drivers.
 Applications in embedded systems can directly access
device drivers.
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General Purpose OS vs. Embedded
System
Application
process
Application
process
Operating
System:
dev/xyz
Device
driver
Device
driver
Physical
Device
hardware
Physical
Device
hardware
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Device Driver development steps
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1.
2.
3.
4.
5.
An interface to the control and status registers.
Variables to track the current state of the physical and
logical devices
-- Major and minor device number, device name
A routine to initialize the hardware to known state
An API for users of the device driver
-- Read, write, seek
Interrupts service routines
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Example: A serial device driver
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 Read the text for explanation and general example of
a timer
 Now lets look at the UARTdriver of the embedded
xinu and WRT54GL.
 Study the tty driver that is a logical device that is
layered on top of the UART driver.
 In our Lab 2 you will write a device driver for a
framebuffer.
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Shift Operators
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 << left shift
 >> right shift
Usage:
unsigned int x = 70707;
//x = 00000000 00000001 00010100 00110011
unsigned int y, z;
y = x << 2;
// y = 00000000 00000100 01010000 11001100
z = x >> 2;
//z = 00000000 00000000 01000101 00001100
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Logic Operators
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 Bitwise & (AND)
 Bitwise inclusive | (OR)
 Bitwise exclusive ^ (XOR)
 Bitwise negation ~
Usage:
unsigned exp1 = 1;
unsigned exp2 = 4;
printf (“ %d\n”, exp1 | exp2);
printf (“ %d\n”, exp1 & exp2);
printf (“ %d\n”, exp1 ^ exp2);
printf (“ %d\n”, ~exp1);
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Relevance of shift and logic operators
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 Bitwise operations are necessary for much low-level
programming, such as writing to device drivers, lowlevel graphics, communications protocol packet
assembly and decoding.
 Device drivers use these operators to test the
presence or absence of a bit in a serial port or a
device input, for example. (checking for on or off)
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Summary
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 We studied the design and development of device
drivers.
 We analyzed the code for a sample UART driver.
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