UQC152H3 Advanced OS

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UQC152H3 Advanced OS

• Writing a Device Driver

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The SCULL Device Driver

• Simple Character Utility for Loading Localities
• 6 devices types
• Scull-03
• Scullpipe0-3
• Scullsingle
• Scullpriv
• Sculluid
• Scullwuid

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Types of SCULL

• Scull0-3
• Global persistent, can be opened many times
• Scullpipe0-3
• Works like a normal pipe example of blocking
• Scullsingle, scullpriv, sculluid, scullwuid
• Access versions of scull

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Major and Minor numbers

• Major number used for device identification
• Minor used by device
• Use ltlinux/fs.hgt
• Register_chdev(unsigned int major, const char
  name, struct file_ops fops)
• Registers major number in static array of devices
  128 entries!

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Mknod creating a file access point

• Once we have a device table entry we need some
  way for programs and utilities to access it.
• Use mknod to create file access point
• mknod /dev/scull c 127 0

Device name
Device type
Minor number
Major number
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Dynamic Allocation of Major Numbers

• It is better to use dynamic allocation of major
  numbers
• Use register_chrdev with intial arg0
• There is a limited number of driver numbers
• Many are already taken
• Requires script to run mknod

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Dynamically creating a file node
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Dynamically creating a file node
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Deregistering devices

• Unloaded drivers should be de-registered using
• Unregister_chrdev(unsigned int major, const char
  name)
• Can cause procs problems if not de-registered

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dev_t and k_dev

• System passes the major number to called code
  through the major number
• Passed as i_rdev in struct inode
• i_rdev is 16 bits
• Only allows 256 minor numbers
• Cant be changed!

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kdev_t

• Linux kernels greater then 1.2 has introduced
  kdev_t
• It allows larger minor numbers
• It has a number of macros
• MAJOR, get a major dev_t from kdev_t
• MINOR, get a minor dev_t from kdev_t
• MKDEV return a kdev_t from a dev_t

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File Operations

• Pointed to from struct file
• lseek
• read , write readdir
• Select
• mmap
• ioctl
• open release
• fsync fasync
• check_media_change revalidate

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Struct file

• Very important kernel data structure
• Not the same of FILE of library calls
• Has
• Pointer to file operations
• Flag data
• Read/write position
• Inode
• Mode

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Which minor number?
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Opening a device

• Open does the following
• Check for errors device not ready?
• Initialises the device if this is the first open
  call
• Identifies the minor numbers selects correct
  file op
• Allocates private data
• Increments the usage count

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Close or release method

• This should
• Decrement the usage count
• Deallocate anything allocated by open
• Last one out turn off the lights!

void scull_release (struct inode inode, struct
file filp) MOD_DEC_USE_COUNT