I/O Devices and Device Drivers I/O Devices and Device Drivers
(Topic 11) (Topic 11)
홍 성 수
서울대학교 공과대학 전기 공학부
Real-Time Operating Systems Laboratory
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Inside Your PC Inside Your PC
• Your PC is equipped with
– CPU and memory – many I/O devices
• VGA card, network card, disk controller, …
Main board
CPU
Memory Network card
Hard disk
Device Connection Device Connection
• Devices are attached to I/O bus
– ISA, PCI, EISA, SCSI, …
I/O BUS
CPU VGA Ethernet Sound
Inside I/O Device Inside I/O Device
• I/O device consists of
– Controller, Logic – Register set – Bus interface
Register Register
BUS interface
I/O BUS Address
Controller External interface
Logic
Data Control
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Register Register
BUS interface Controller External interface
Logic
Device Control Device Control
• How CPU control device
– CPU write register address on the bus
• Address spaces are assigned to each device – Device write / read data on the bus
I/O BUS
Address Data Control Address Data Control
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Device Driver Device Driver
• Software layer between application and device
• Implements operations exposed to users
– (Ex) open, close, read, write, ioctl, …
Register Register
BUS interface Controller External interface
Logic
I/O BUS
Address Data Control Address Data Control
Application
CPU Device driver
OS
What is Device Driver?
What is Device Driver?
• Software layer between application and device
• Implements operations exposed to users
– (Ex) open, close, read, write, ioctl, …
Types of Linux Devices Types of Linux Devices
• Character Devices
– Transfer unit: character (byte)
– Can be accessed like a file (open, close, read, write, …) – (Ex) console, keyboard, mouse, … (/dev/tty1, /dev/lp1, …)
• Block Devices
– Transfer unit: block (usually some kilobytes)
– Can be accessed like a file (open, close, read, write, …) – External view to users is same as character devices
– Internally, block buffer is used for efficiency (contrast to character devices)
– (Ex) hard disk drive, CD-ROM drive, … (/dev/hda1, …)
• Network Interfaces
– Can not be easily mapped to neither character or block devices
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Device Files Device Files
• Represent an I/O device – /dev/tty0 = first serial port
• Two device files can represent same I/O devices (but may be implemented in a different way)
– /dev/psaux, /dev/psmouse = serial mouse
• Attributes
– Type: block or character
– Major number: specifies device driver
– Minor number: argument to device driver, kernel don’t care
Name Type Major Minor Desc
/dev/fd0 Block 2 0 Floppy Disk
/dev/had Block 3 0 First IDE disk
/dev/hda2 Block 3 2 Second partition of first IDE disk
/dev/hdb Block 3 64 Second IDE disk
/dev/console Char 5 1 Console
/dev/null char 1 3 Null device
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Structure of Structure of
Character Device Drivers Character Device Drivers
Name File operations
… Character Device Table
0 1 2 Index = major number
Character Device Driver
Open Read Write Close Ioctl
…
Read code write code Open code
… Device Name
Registered when device being
installed
Character Device
Character Device Drivers Character Device Drivers
• To applications:
– Device file = Regular file
• To file system:
– Regular file: read from or write to disk drive – Device file: invoke device driver operations
• Open: initialize device
• Read: device data Æuser buffer
• Write: user buffer Ædevice
• Close: called when removed from device table
• Ioctl: device specific control
– (Ex) baud rate change, access permission set,
Block Device Drivers Block Device Drivers
• Difference with Character Device Drivers
– Actual transfer unit: block(some kilobytes)
– <Device Driver : Device> transfer unit cannot be character.
• How to implement character oriented read/write?
– Kernel provides general block consciousread/write functions
• translate character I/O to block I/O
– Device driver only need to implement handle block request
• sometimes called as ‘strategy routine’
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Structure of Block Device Drivers Structure of Block Device Drivers
Block Device Table Block Device Driver Open
Read Write Close Ioctl
…
Open code
…
Block Device
Handling request code (strategy routine)
Buffer Cache Manager Block Read Block Write
Buffer Buffer
Buffer Block Buffer (Buffer Cache) Request queue
char
block
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Interrupt Handling in Device Interrupt Handling in Device
Drivers Drivers
• Register ‘interrupt handling routine’ when opening
• Kernel invokes registered handler when interrupt occurs
• Linux interrupt handler
– Interrupt Handler
• High priority function (such as acknowledging to PIC)
• Run with interrupt disabled
• Invoked every time interrupt occurred
• Marks a bottom half as active – Bottom Half
• Low priority function (such as transferring data from device)
• Run with interrupt enabled
• Execution may be deferred
Bottom Half Bottom Half
Proc
Int handler 1
Ack INTR 1, activate bottom half 1
Bottom half 1 INTR 2
Int handler 2
Ack INTR 2, activate bottom half 1
Proc
Intr disabled
Example of bottom half activated twice, but executed once
INTR 1
General Device Driver Routines (1) General Device Driver Routines (1)
• Open
– Reads minor number – Initialize appropriate device
– Initialize device driver internal data structures
– If needed, change the file operation table (according to minor) – Increase usage counter
– If needed, register interrupt handler, enable irq
• Close
– Free dynamically allocated data structures – Decrease usage counter
– Un-register the interrupt handler
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General Device Driver Routines (2) General Device Driver Routines (2)
• Read / Write
– Polling vs Interrupt Driven
• Polling
– checks device if data can be read or written – Usually for character device
• Interrupt Driven
– sleep process until data can be read or written – Usually for character and block device – Blocking vs Non-Blocking
• Blocking
– If data cannot be read or written: wait until ready
• Non-Blocking:
– If data cannot be read or written: immediately returns
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General Device Driver Routines (3) General Device Driver Routines (3)
Character Device, Polling and Blocking I/O
Check Device
Device ready?
No
Read Data (device Æuser)
Return to User Yes Read
Character Device, Polling and Non-Blocking I/O
Check Device
Device ready? No
Read Data (device Æuser)
Return to User Yes Read
General Device Driver Routines (4) General Device Driver Routines (4)
Character Device, Interrupt Drivenand Blocking I/O
Check Device
Device Ready?
Sleep process No
Read
Yes
Read ready interrupt
Wakeup process
Read Data (device Æuser)
Return to User
General Device Driver Routines (5) General Device Driver Routines (5)
Block Device, Interrupt Drivenand Blocking I/O
Check read buffer
Buffer full ?
Issue Read Command
Sleep process No
Read
Yes Sleep process
Wakeup by another process
Read complete interrupt
Wakeup process
Return to User
