Linux Device Drivers & Project3 preview csc345 Project 3 Preview

Linux Device Drivers & Project3 preview

• CSC345

Project 3 Preview

• Write a device driver for a pseudo stack device

• Idea from http://www.cs.swarthmore.edu/~newhall/cs45/f01/proj5.html

• Linux character device type supports the following operations

• Open: only one is allowed.
• Write: writes an char string to top of the device stack. Error if stack is empty
• Read: reads an item from top of the device stack. Error if stack is empty
• Release: release the device

• Install with LKM.

• Test: It will be a dedicated standalone machine in the lab. Root password may be given out. If you mess up, you will re-install the

What is a device driver?

• A programming module with interfaces

• Communication Medium between application/user and hardware

• In Unix,

• Kernel module
• device driver interface = file interface
• What are normal operations?
• Block vs. character

User program & Kernel interface

Loadable Kernel Module (LKM)

• A new kernel module can be added on the fly (while OS is still running)

• LKMs are often called “kernel modules”

• They are not user program

Types of LKM

• Device drivers

• Filesystem driver (one for ext2, MSDOS FAT16, 32, NFS)

• System calls

• Network Drivers

• TTY line disciplines. special terminal devices.

• Executable interpreters.

Basic LKM (program)

• Every LKM consist of two basic functions (minimum) :

• int init_module(void) /*used for all initialition stuff*/
• {
• ...
• }
• void cleanup_module(void) /*used for a clean shutdown*/
• {
• ...
• }

• Loading a module - normally retricted to root - is managed by issuing the follwing command: # insmod module.o

LKM Utilities cmd

• insmod

• Insert an LKM into the kernel.

• rmmod

• Remove an LKM from the kernel.

• depmod

• Determine interdependencies between LKMs.

• kerneld

• Kerneld daemon program

• ksyms

• Display symbols that are exported by the kernel for use by new LKMs.

• lsmod

• List currently loaded LKMs.

• modinfo

• Display contents of .modinfo section in an LKM object file.

• modprobe

• Insert or remove an LKM or set of LKMs intelligently. For example, if you must load A before loading B, Modprobe will automatically load A when you tell it to load B.

Common LKM util cmd

• Create a special device file

• % mknode /dev/driver c 40 0

• Insert a new module

• % insmod modname

• Remove a module

• %rmmod modname

• List module

• % lsmod
• Or % more /proc/modules
• audio 37840 0
• cmpci 24544 0
• soundcore 4208 4 [audio cmpci]
• nfsd 70464 8 (autoclean)

Linux Device Drivers

• A set of API subroutines (typically system calls) interface to hardware

• Hide implementation and hardware-specific details from a user program

• Typically use a file interface metaphor

• Device is a special file

Linux Device Drivers (continued)

• Manage data flow between a user program and devices

• A self-contained component (add/remove from kernel)

• A user can access the device via file name in /dev , e.g. /dev/lp0

General implementation steps

• Understand the device characteristic and supported commands.

• Map device specific operations to unix file operation

• Select the device name (user interface)

• Namespace (2-3 characters, /dev/lp0)

• (optional) select a major number and minor (a device special file creation) for VFS interface

• Mapping the number to right device sub-routines

• Implement file interface subroutines

• Compile the device driver

• Install the device driver module with loadable kernel module (LKM)

• or Rebuild (compile) the kernel

Read/write (I/O)

• Pooling (or synchronous)

• Interrupt based

Device Driver interface

VSF & Major number

• principal interface between a device driver and Linux kernel

File operation structure

• struct file_operations Fops = {

• NULL, /* seek */
• xxx_read,
• xxx_write,
• NULL, /* readdir */
• NULL, /* select */
• NULL, /* ioctl */
• NULL, /* mmap */
• xxx_open,
• NULL, /* flush */
• xxx_release /* a.k.a. close */

• };

Device special file

• Device number

• Major (used to VFS mapping to right functions)
• Minor (sub-devices)

• mknod /dev/stk c 38 0

• ls –l /dev/tty

• crw-rw-rw- 1 root root 5, 0 Apr 21 18:33 /dev/tty

Register and unregister device

• int init_module(void) /*used for all initialition stuff*/
• {
• /* Register the character device (atleast try) */
• Major = register_chrdev(0,
• DEVICE_NAME,
• &Fops);
• :
• }
• void cleanup_module(void) /*used for a clean shutdown*/
• {ret = unregister_chrdev(Major, DEVICE_NAME);
• ...
• }

Register and unregister device

• compile

• -Wall -DMODULE -D__KERNEL__ -DLINUX –DDEBUG -I /usr/include/linux/version.h -I/lib/modules/`uname -r`/build/include

• Install the module

• %insmod module.o

• List the module

• %lsmod

• If you let the system pick Major number, you can find the major number (for special creation) by

• % more /proc/devices

• Make a special file

• % mknod /dev/device_name c major minor

Device Driver Types

• A character device driver ( c )

• Most devices are this type (e.g.Modem, lp, USB
• No buffer.

• A block device driver (b)

• through a system buffer that acts as a data cache.
• Hard drive controller and HDs

Implementation

• Assuming that your device name is Xxx

• Xxx_init() initialize the device when OS is booted

• Xxx_open() open a device

• Xxx_read() read from kernel memory

• Xxx_write() write

• Xxx_release() clean-up (close)

• init_module()

• cleanup_module()

kernel functions

• add_timer()

• Causes a function to be executed when a given amount of time has passed

• cli()

• Prevents interrupts from being acknowledged

• end_request()

• Called when a request has been satisfied or aborted

• free_irq()

• Frees an IRQ previously acquired with request_irq() or irqaction()

• get_user*()

• Allows a driver to access data in user space, a memory area distinct from the kernel

• inb(), inb_p()

• Reads a byte from a port. Here, inb() goes as fast as it can, while inb_p() pauses before returning.

• irqaction()

• Registers an interrupt like a signal.

• IS_*(inode)

• Tests if inode is on a file system mounted with the corresponding flag.

• kfree*()

• Frees memory previously allocated with kmalloc()

• kmalloc()

• Allocates a chu nk of memory no larger than 4096 bytes.

• MAJOR()

• Reports the major device number for a device.

• MINOR()

• Reports the minor device number for a device.

kernel functions

• memcpy_*fs()

• Copies chunks of memory between user space and kernel space

• outb(), outb_p()

• Writes a byte to a port. Here, outb() goes as fast as it can, while outb_p() pauses before returning.

• printk()

• A version of printf() for the kernel.

• put_user*()

• Allows a driver to write data in user space.

• register_*dev()

• Registers a device with the kernel.

• request_irq()

• Requests an IRQ from the kernel, and, if successful, installs an IRQ interrupt handler.

• select_wait()

• Adds a process to the proper select_wait queue.

• *sleep_on()

• Sleeps on an event, puts a wait_queue entry in the list so that the process can be awakened on that event.

• sti()

• Allows interrupts to be acknowledged.

• sys_get*()

• System calls used to get information regarding the process, user, or group.

• wake_up*()

• Wakes up a process that has been put to sleep by the matching *sleep_on() function.

Stop here 5/1/2003

Pitfalls

• Using standard libraries: can only use kernel functions, which are the functions you can see in /proc/ksyms.      

• Disabling interrupts You might need to do this for a short time and that is OK, but if you don't enable them afterwards, your system will be stuck

• Changes from version to version

Resources

• Linux Kernel API: http://kernelnewbies.org/documents/kdoc/kernel-api/linuxkernelapi.html

• Kernel development tool http://www.jungo.com/products.html

• Linux Device Drivers 2nd Edition by Rubini & Corbet, O'Reilly  Pub, ISBN 0-596-00008-1