Character Drivers

In Linux

Live Coding

Bare bones module

Major and Minor

• Major: maps to driver associated with device
• Minor: which device within the driver
  • typedef dev_t
  • MAJOR(dev_t)
  • MINOR(dev_t)
  • MKDEV(major, minor)
• ls -l /dev
  • Look for c in first column

Allocating and Freeing Device numbers

• register_chrdev_region
  • N minors (static)
• alloc_chrdev_region
  • Allocates some minors and a major for a specified number of needed minors
• unregister_chrdev_region
  • Frees major and minor number
• /proc/devices

Dynamic allocation

• alloc_chrdev_region vs register_chrdev_region
  • Dynamic vs Static
  • Affects creation of device nodes
  • Static: easy node creation
  • Dynamic: bit more complex

Device registration

• Register char dev
  • cdev_alloc
  • my_cdev->ops = &my_fops;
  • cdev_init
  • cdev_add
• Remove char dev
  • cdev_del
• The (much) older way
  • register_chrdev
  • unregister_chrdev

Live Coding

Major + Minor, make it show up in /dev/

Important data structures

• struct file_operations
  • Holds pointer to the kernel methods (read, write, ioctl etc)
• struct file
  • Represents an open file (similar to FILE * or a file number in user space)
• struct inode
  • Represents a unique file within the directory structure (one or more paths may point to a single inode)
  • Many to one (file:inode)

file_operations struct

• owner = THIS_MODULE
  The Methods to the Madness
• llseek(2)
• read(2)
• write(2)
• ioctl(2)
• open(2)
• release(2)
• ...

file struct

• NOT the same as C library FILE pointer
  • It is its equivalent within the kernel
• Represents an open file
• Important fields
  • f_mode (opened for reading, writing etc (what you passed to open()))
  • f_pos (current location in file)
  • f_flags (flags you passed to open())
  • f_op (pointer to file operations the kernel will use with this file)
  • private_data (driver can use this to hold info it needs)

inode struct

• Used by kernel to represent files
• Two important fields
  • i_rdev: Contains device number
  • i_cdev: Contains pointer to char device

Live Coding

Stubs for driver functions

Memory usage

• kmalloc (often devm_kmalloc)
  • Typically used in probe/init
• kfree (only needed if you aren’t using devm)
  • Typically used in remove/exit
• Kinda like malloc/free with extra steps
  • No glibc!
  • Need to tell the kernel memory zone
  • How long can you afford to wait?

Open and Release

• Open
  • Initialize device if being used for the first time
  • Check for device specific errors
  • Update f_op
  • Set private data if needed
• Release
  • Deallocate anything allocated in open
• Kinda like kmalloc/kfree for the whole thing

Read and Write

• How do we get access to memory in user space?
• Read (write from kernel side)
  • copy_to_user
• Write (read from kernel side)
  • copy_from _user
• __user to mark a userspace ptr
  • Don’t just dereference!

Why not just dereference?

• In process context
  • Oops + death on a page fault
• Elsewhere, null ptr deref:
  = panic() (big death) (not from system call)
• Unchecked _* versions of copy*_user

Live Coding

Make it do something! Echo user input

Scull: Simple Character Utility for Loading Local Localities

• Example char driver used throughout LDD3
• RAM simulation of external storage
  • Platform/Hardware independent
• scull/main.c at master · starpos/scull · GitHub

Linux Driver Development with Raspberry Pi - ch4

• Chapter 4 has examples of complete char dev including miscellaneous character module
• miscdevice
  • Simpler way to write char device driver
  • Used for sense-hat display

Discussion