Linux Device Drivers

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Linux Device Drivers

• By Adam Ormond
• ormonda_at_msoe.edu

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Purpose

• Gain better understanding of Linux environment
• Become familiar with device drivers
• How they work
• How theyre written
• What they can do
• Learn more about Makefiles and gcc

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Resources

• Dr. Russell Meier Independent Study Advisor
• Rubini, Alessandro and Corbet, Jonathon. Linux
  Device Drivers. OReilly Sebastopol, CA. June
  2001.
• Victor Meyer Engineering Technician
• Welsh, Matt and Kaufman, Lar. Running Linux.
  OReilly Sebastopol, CA. August 1996.
• http//people.msoe.edu/ormonda/index.php?content
  projects/LDD/LDD

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Goals
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Development Machine

• Abit AB-ZM6 motherboard
• Pentium Celeron 333 MHz overclocked to 416 MHz
  (83Mhz FSB 5)
• 256 MB of SDRAM
• 9 GB Seagate Barracuda Model ST39175LU
• 4 GB Quantum Fireball Model ST4.3S
• Redhat Linux 7.2
• Kernel Version 2.4.7-10

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Simple Module

• Purpose
• Ensure test machine works properly
• Become familiar with insmod and rmmod
• Learn basic functions
• init_module
• cleanup_module
• printk

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Simple Module

• Issues/Difficulties
• Problem Messages using printk not displaying on
  console
• Solution set DEFAULT_CONSOLE_LOGLEVEL to 8 using
  echo 8 gt /proc/sys/kernel/print

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DEFAULT_CONSOLE_LOGLEVEL

• KERN_EMERG
• KERN_ALERT
• KERN_CRIT
• KERN_ERR
• KERN_WARNING
• KERN_NOTICE
• KERN_INFO
• KERN_DEBUG

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Simple Character Device

• Purpose
• Allocate memory
• Read/write memory
• Learn about device nodes
• mknod
• Device file system
• register_chrdev
• unregister_chrdev

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Simple Character Device

• Requirements
• Major number
• Minor number
• file_operations structure
• file structure
• open, release, read, and write methods

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file_operations

• Provides hooks for llseek, read, write, readdir,
  poll, ioctl, mmap, open, flush, release, fsync,
  fasync, lock, readv, and writev
• Uses a tagged structure struct file_operations
  Char_Dev read c_d_read, write
  c_d_write, open c_d_open, release
  c_d_release

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file

• Structure that represents an open file
• Struct members
• mode_t f_mode
• FMODE_READ
• FMODE_WRITE
• loff_t f_pos
• usnigned int f_flags
• O_RDONLY
• O_NONBLOCK
• O_SYNC
• struct file_operations f_op
• void private_data
• struct dentry f_dentry

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devfs

• devfs_handle_t devfs_mk_dir (devfs_handle_t dir,
  const char name, void info)
• devfs_handle_t devfs_register (devfs_handle_t
  dir, const char name, unsigned int flags,
  unsigned int major, unsigned int minor, umode_t
  mode, void ops, void info)
• devfs_unregister(devfs_handle_t de)

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Methods implemented

• open
• Initialization of device
• Increment usage count
• MOD_INC_USE_COUNT
• Check for problems with device
• Determine minor number and update file_operations
  struct
• Allocate memory for filp-gtprivate_data

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Methods implemented

• release
• Deallocate memory
• Shut down device if usage count 1
• Decrement usage count
• MOD_DEC_USE_COUNT

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Methods implemented

• read
• ssize_t read(struct file filp, char buff,
  size_t count, loff_t offp)
• down_interruptible
• unsigned long copy_to_user(void to, const void
  from, unsigned long count)

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Methods Implemented

• write
• ssize_t write(struct file filp, const char
  buff, size_t count, loff_t offp)
• down_interruptible
• unsigned long copy_from_user(voit to, const void
  from, unsigned long count)

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Issues/Difficulties

• gcc
• Problem Inline statements not being compiled
• Solution use O flag to expand inline statements
• ifdef
• Problem missing endif statements
• Solution add endif statements

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Advanced Character Device

• Purpose
• Utilize ioctl and llseek functionality
• Hardware device control is given through ioctl
• Ability to seek the device is given through llseek

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ioctl

• int ioctl(struct inode inode, struct file
  filp, unsigned int cmd, unsigned long arg)
• Defining commands
• Function usually uses a switch statement
• Defining commands to use 1-x not safe
• Unix method uses a magic number and then a plain
  number (example 0x6b01, 0x6b02, etc)
• New method uses a magic number, ordinal number,
  direction of transfer, and size of argument.
  These are set using a variety of macros (example
  _IO(k, 1)). Defined in ltasm/ioctl.hgt

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llseek

• loff_t llseek(struct file filp, loff_t off, int
  whence)
• Retrieve the file length of the device
• whence values
• SEEK_SET
• SEEK_CUR
• SEEK_END

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Parallel Port Device

• Purpose
• Transfer data across the parallel port
• Gain more experience with soldering
• Learn about memory barriers
• Requirements
• Working parallel port
• LED bank with interface to parallel port

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LED Bank
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Memory barriers

• Prevent compiler optimization and hardware
  reordering
• Keeps order of operations the same
• void barrier(void)
• void rmb(void)
• void wmb(void)
• void mb(void)

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Conclusion

• Accomplished most major goals
• Learned a lot about Linux device drivers
• More familiar with electrical components
• Enjoyed working independently, using instructors
  and books as resources

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Questions