cs6900 independent study bolin hsu. bolin hsu introduction relationship of linux kernel and linux...

CS6900 Independent study

Bolin Hsu<bolin_hsu@yahoo.com>

Bolin Hsu <bolin_hsu@yahoo.com>

Introduction Relationship of Linux kernel and

Linux device drivers Implementation of a Linux device

driver How to write your own device driver Bibliography

Concepts

Objective Let the users access the device by file

system calls:…fd=open(“/dev/myrd”,O_RDWR);…write(fd,buffer,sizeof(buffer));…read(fd,buffer,N);…close(fd);

Challenges How to connect the device file

“/dev/myrd” to the device How to make the system calls

access the device

Linking of device file and the device

Everything in UNIX is a file User interact with the device

through filesystem functions open, close, read, write, ioctl, etc A device file or a filesystem

Two interfaces to the device Interface to user: device file Interface to kernel: service routine

Device file concept Device file type:

Character (inode.type = 3) Block (inode.type = 4)

Device file number Major: device type

(2: floppy disk 3: hard disk, etc) Minor: device number

Examples:brw-rw---- 1 root disk 3, 0 May 19 2004 /dev/had

Creating the device file Create our ram disk device file

Type is block Major number is 100 Minor number is 0% mknod /dev/ b 100 0

Another option is to create device during device driver initialization.

Connecting system calls to the device driver

How Linux kernel uses device drivers

User mode

Kernel mode

Libraries

User applications

File subsystem

Buffer/page cache

Hardware control

Block device driverCharacter device driver

Block I/O and Page I/O Two fundamental I/O type:

Block I/O: bread() Page I/O: brw_page()

Block I/O: read/write a single block. Kernel uses it to handle file system superblock and inode.

Page I/O: read/write a page in a file. Used in read/write system calls.

Page/buffer cache A cache is a RAM area The cache holds data which usually

should be on a block device. I/O from the cache is faster than the

device. Kernel can merge and schedule

device I/O to improve system performance.

Sector, block, and buffer cache A sector is the basic data transfer unit of

a block device A block is a collection of adjacent sectors

transferred in one I/O request Every block device file has its own block

size Every block has a corresponding buffer

How device request uses buffer cache

request

Blocks on device

Buffer cache in RAM

Buffer head Buffer head Buffer head

request

Device driver API calling sequence

open close read write

buffer cache calls

block device switch table

open release

Interrupt vector

device interrupt

request

Interrupt service ruotine

check_media_change

revalidate

device driver API

How read() works The read() system call searches the page

in the page cache. If the page is in the cache and valid, it is copied to the user buffer. If no valid page is found in the cache, the process issues a request for the page and suspend itself.

When the page becomes available later, the kernel wakes up the suspended process. The process then copies the page to the user buffer.

Trace read() system call These functions are called in order:1. read()

2. sys_read()

3. def_blk_fops.read: generic_file_read()

4. do_generic_file_read()if the page is found in the page cache, it is copied to the user buffer. If it is not in the cache, call next function. This function could suspend the calling process.

5. def_blk_aops.readpage: blkdev_readpage()

6. block_read_full_page()

7. blkdev_get_block()

How write() works The write() system call copies the

user buffer to the page cache and returns.

Unlike read(), the write() system call doesn’t wait for the transfer between page cache and the device to complete. There is no data dependency in write operation.

Trace write() system call These functions are called in order:1. write()

2. sys_write()

3. def_blk_fops.write: generic_file_write()

4. def_blk_aops.prepare_write: blkdev_prepare_write()def_blk_aops.commit_write: blkdev_commit_write()

5. block_prepare_write()block_commit_write()

How open() works open() makes the file subsystem read the inode

of the file from the device. The kernel then checks the inode type and

knows it is a device file. The kernel then initializes the inode structure in RAM to contain file operations defined in the device driver. The kernel uses the major number of the device to find the file operations.

From this point on, file operation calls to this inode will cause the execution of the corresponding device driver functions.

Trace open() system call These functions are called in order:1. open()

2. sys_open()

3. file_open()

4. dengry_open()

5. def_blk_fops.open: blkdev_open()

6. do_open()

7. get_blkfops()retrieves blkdevs[MAJOR].bdops, stores in bdev.bd_op

The request queue of the device Requests to a block device is maintained

in a request queue of the device. BLK_DEFAULT_QUEUE(MAJOR) finds the

request queue by the major number. A request function from the device driver

handles the request. The request function is registered when

the queue is initialized:blk_init_queue(BLK_DEFAULT_QUEUE(MAJOR),request_function);

Implementation

What’s in a device driver A device driver is like a server. It

has: A set of functions to services Functions to publish the services to the

kernel Some internal data structures for

bookkeeping purposes

In-kernel or kernel module Two ways of adding device driver to

the kernel: Compile into the kernel Loadable kernel module

Benefit of kernel module No need to rebuild kernel No need to reboot system

Block device interface to Linux kernel/usr/src/linux/fs/block_dev.cstruct file_operations def_blk_fops = { open: blkdev_open, release: blkdev_close, llseek: block_llseek, read: generic_file_read, write: generic_file_write, mmap: generic_file_mmap, fsync: block_fsync, ioctl: blkdev_ioctl,};// Warning: nonstandard gcc structure initialization

File system interface of the block device/usr/src/linux/include/linux/fs.hstruct block_device_operations { int (*open) (struct inode *, struct file *); int (*release) (struct inode *, struct file

*); int (*ioctl) (struct inode *, struct file *,

unsigned, unsigned long); int (*check_media_change) (kdev_t); int (*revalidate) (kdev_t); struct module *owner;};

// file system hooks for the block devices

Important global array read_ahead // how many block

blk_size // device sizeblksize_size // block sizehardsect_size // sector size

These arrays are indexed by major number

Kernel fetches information about the device from these arrays

Initializing the device driver Done in init_module Allocate memory Initialize the device driver’s slots in

global arrays Initialize the request queue of the

device driver Register the device driver (API)

Initialization exampleInt Init_module(void) {

...myrd_storage = (char *)kmalloc(2*1024*1024*sizeof(char), GFP_KERNEL);...read_ahead[MAJOR_NR] = 2; // sectors

blk_size[MAJOR_NR] = kmalloc(sizeof(int), GFP_KERNEL);*blk_size[MAJOR_NR] = 2048; // device size (kilobytes);

blksize_size[MAJOR_NR] = kmalloc(sizeof(int), GFP_KERNEL);*blksize_size[MAJOR_NR] = 512; // block size (bytes)

hardsect_size[MAJOR_NR] = kmalloc(sizeof(int), GFP_KERNEL);*hardsect_size[MAJOR_NR] = 512; // sector size (bytes)

blk_init_queue(BLK_DEFAULT_QUEUE(MAJOR_NR), myrd_request);register_blkdev(MAJOR_NR,"myrd",&myrd_ops);register_disk(NULL, MKDEV(MAJOR_NR, 0), 1, &myrd_ops, 100);...

Cleanup examplevoid cleanup_module(void){

...fsync_dev(MKDEV(MAJOR_NR, 0));

blk_cleanup_queue(BLK_DEFAULT_QUEUE(MAJOR_NR));

kfree(hardsect_size[MAJOR_NR]);hardsect_size[MAJOR_NR] = NULL;

kfree(blk_size[MAJOR_NR]);blk_size[MAJOR_NR] = NULL;

kfree(blksize_size[MAJOR_NR]);blksize_size[MAJOR_NR] = NULL;

kfree(myrd_storage);

unregister_blkdev(MAJOR_NR,"myrd");}

Servicing a request The kernel passes to the request function

the beginning sector and the number of sectors to read.

The request function can calculate the starting location and size by:start_loc = sector * SECTOR_SIZE;size = nr_sectors * SECTOR_SIZE;

Assume the RAM disk stores the data in an byte array data[], the memory region to be copied is data[start_loc] to data[start_loc+size-1]

Request routine examplevoid myrd_request(request_queue_t *q){

...while(1){

INIT_REQUEST; // this macro does a return when queue is emptylocation = myrd_storage + CURRENT->sector * MYRD_HARDSECT_SIZE;size = CURRENT->current_nr_sectors * MYRD_HARDSECT_SIZE;

spin_lock(&myrd_lock); if (CURRENT->cmd == READ)

memcpy(CURRENT->buffer, location, size);else if (CURRENT->cmd == WRITE)

memcpy(location, CURRENT->buffer, size);spin_unlock(&myrd_lock);

end_request(1); // update the queue and requests}

Adding the device driver into the Linux source tree Modify the Config.in in the directory

Let user choose Y/N/M for this question:tristate “Configure My RAM disk’ CONFIG_MYRD

Add your code to the makefile obj_$(CONFIG_MYRD) += myrd.o

The above line adds the object to obj_m or obj_y, depending on the answer in “make config”

Roll your own device driver

Helpful courses If you don’t feel comfortable

working with the hardware, these two courses are very helpful

1. CS3432 Digital circuit design2. CS3434 Computer interfacing

A checklist of tasks Create the device file Define the default file operations:

def_blk_ops def_chr_fops

Initialize the global data arrays Register the device driver to the kernel Cleanup everything when you are done

How do I drive a real device The derive driver interface to the user and the

kernel remain the same. The request function will have to interact with

the device. This is usually done by reading and writing to device registers.

If the device is slow, you may want to use the interrupt driven approach. The process sets up an I/O operation and suspends. When the I/O completes, the device raise an interrupt request and the interrupt service routine in the device driver is called to handle the completed transfer.

How do I drive character device Needs to supply the file operations

defined by the file system. No cache is used. Thus no need of

the request function. Needs to initialize the global arrays

of the kernel and register the driver. Still needs to implement interrupt

service routine.

Looking ahead: the 2.6.x kernel The 2.6 kernel removed several

macro used in this presentation. The 2.6 kernel changed some global

bookkeeping arrays. The 2.6 kernel is preemptive. The nonstandard structure

initialization was removed in 2.6.x

Other types of drivers Sometimes character and block

driver models are not suitable Network interface card driver Bus driver

PCI, SCSI, etc. Need to deal with packets like in

networking. And more

Suggestions to future work Build a file system on the ram disk

Partition the data array into boot block, super block, inodes, data blocks.

Supply mount() and unmount() system calls, plus other system calls.

Port the RAM disk driver to kernel 2.6.x

Bibliography Linux device drivers, 2ed Alessandro Rubini &

Jonathan Corbet, O’Reilly, Sebastopol, CA Understanding the Linux kernel Daniel P.l

Bovet & Marco Cesati, O’Reilly, Sebastopol, CA <ftp://ftp.oreilly.com/pub/examples/linux/drivers2/book_pdf.rar>

The design of the UNIX operating system Maurice J. Bach, Prentice Hall, Englewood Cliffs, New Jersey

Writing a UNIX device driver, 2ed Janet I. Egan, Thomas J. Teixeira, John Wiley & Sons, Inc.

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