Linux Device Drivers

1
Linux Device Drivers

• CSlab _at_ NTUA
• ???ast???? ?p?????st???? S?st?µ?t??
• ?????? ?ets?ß?? ????te??e??

2
Device Drivers

• Black boxes
• Hide details
• Standardized calls
• Map standard calls to specific hardware
• Build separately from kernel plugged in at
  runtime

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Why Do We Need Drivers?

• New devices constantly available
• Hardware vendors
• Learn things most people dont know ?
• Learn how machine works
• Learn about OS kernel

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Mechanism vs. Policy

• device driver provides mechanism, not policy
• X server vs. window manager
• TCP/IP (sockets interface is mechanism, servers
  provide policy)
• Floppy driver (mechanism to see data blocks,
  filesystem provides policy)

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User Programs Needed

• Kernel programming not enough
• User apps needed to communicate with kernel
• e.g. sound volume control (sndctl)

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Kernel

• Big chunk of executable code
• Process management
• Memory management
• Filesystems
• Device control
• Networking

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Modules

• Ability to extend features offered by kernel at
  runtime
• Module is piece of code
• insmod/rmmod
• Device driver can be a module

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Classes of Devices

• Char devices
• Block devices
• Network devices

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Classes of Modules

• Device drivers
• filesystems

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Building and Running Modules
include ltlinux/module.hgt int init_module()
printk("Hello, world\n")   return 0 void
cleanup_module() printk(Bye Bye\n")
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Building and Running Modules (2)

• Kernel functions only, no C library
• Dont use printf, only printk
• insmod ./hello.o
• dmesg to see kernel messages
• rmmod hello.o
• Dmesg to see kernel messages
• Insmod/rmmod called by super-user (root)

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User Space and Kernel Space

• Modules run in kernel space
• Applications in user space
• Different memory mapping-address space
• User?kernel space by system call or hardware
  interrupt

Kernel code is executed in process context,
acceses data in process address space
Runs in no process context
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Character Devices
mknod /dev/lunix c 69 0
Example of ls l /dev on my system

• crw------- 1 gtsouk users 14, 3 Apr 17
  1999 /dev/dsp
• crw-rw-rw- 1 root root 69, 0 May 5
  1998 /dev/lunix
• crw-rw-rw- 1 root root 1, 3 May 5
  1998 /dev/null
• crw-r--r-- 1 root root 1, 8 May 5
  1998 /dev/random
• crw------- 1 sotirop users 4, 0 May 5
  1998 /dev/tty0
• crw-r----- 1 root uucp 4, 64 Mar 19
  1456 /dev/ttyS0
• crw-rw-rw- 1 root root 1, 5 May 5
  1998 /dev/zero

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Character Devices

• int (open) (struct inode , struct file )
• void (release) (struct inode , struct file )
• int (read) (struct inode , struct file , char
  , unsigned long)
• int (write) (struct inode , struct file ,
  const char , unsigned long)
• int (lseek) (struct inode , struct file , long
  long, int)
• int (ioctl) (struct inode , struct file ,
  unsigned int, unseigned long)

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Character Devices

• static struct file_operations lunix_fops
• open (void )lunix_open,
• release (void )lunix_release,
• read (void )lunix_read,
• write (void )lunix_write,
• llseek (void )lunix_llseek,
• ioctl (void )lunix_ioctl,

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Character Devices

• register_chrdev()
• unregister_chrdev()

• Character devices
• 1 mem
• 2 pty
• 69 lunix_driver
• 180 usb
• Block devices
• 2 fd
• 33 ide2

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Exercise and Notes

• http//www.cslab.ntua.gr/courses
• http//ray.cslab.ntua.gr/dd
• Labs exercise and notes in .pdf.
• Give it a try!

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Linux Device Drivers, 2nd Edition
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Bibliography

• Writing Character Device Driver for Linux
• ftp//ftp.llp.fu-berlin.de/pub/linux/LINUX-LAB/whi
  tepapers/drivers.ps.gz
•  
• The Linux Kernel Module Programming Guide
  http//www.ntua.gr/LDP/guides.html
•  
• Linux Device Drivers, 2nd Edition, Alessandro
  Rubini and Jonathan Corbet. http//www.xml.com/ldd
  /chapter/book/