Linux UK Communication: Character Device

這是同事 Taco 撰寫的範例，在經過作者同意後厚顏無恥的列在 Blog 上。畢竟人家都寫好了啊，要我自己在寫一份有點懶。為了紀念原作者（明明就還活著，說啥紀念），所以沒有任何地方更動（應該是懶惰吧）

這個範例和我平常的習慣不同，一般來說我只會寫一個範例檔，不喜歡拆成兩個，而且愈簡單愈好。 但這同事和我的習慣不同，也很好，因為可以很簡單的看出兩個檔案的分工，特別是 taco_1.c，可以直接拿來建立各樣的 kernel module 範例，這個習慣值得我學習。

taco_1.c

#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>

MODULE_AUTHOR("tacolin");
MODULE_DESCRIPTION("CHARACTOR DEVICEL TEST MODULE 2");
MODULE_LICENSE("GPL");

extern int register_device(int major, int minor);
extern int unregister_device(int major, int minor);

static int __init kernel_module_init(void)
{
 printk("[KERNEL-PART] %s\n", __func__);
 if ( 0 > register_device(125, 0) )
 {
  printk("[KERNEL-PART-ERROR] register device failed\n");
  return -1;
 }
    return 0;
}

static void __exit kernel_module_exit(void)
{
 printk("[KERNEL-PART] %s\n", __func__);
 if ( 0 > unregister_device(125, 0) )
 {
  printk("[KERNEL-PART-ERROR] unregister device failed\n");
  return;
 }
 return;
}

module_init( kernel_module_init );
module_exit( kernel_module_exit );

taco_2.c

#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>

#include <linux/cdev.h>
#include <asm/uaccess.h>

#include <linux/device.h>

static int _device_open(struct inode*, struct file*);
static int _device_release(struct inode*, struct file*);
static ssize_t _device_write(struct file*, const char*, size_t, loff_t*);
static ssize_t _device_read(struct file*, char*, size_t, loff_t*);

static struct file_operations _ops = {
 open:  _device_open,
 release: _device_release,
 write:  _device_write,
 read:  _device_read,
};

static struct class*  _p_class = NULL;
static struct cdev   _cdevice;

static unsigned char  _kbuf[128];

static int _device_open(struct inode* p_node, struct file* p_file)
{
 printk("[KERNEL-PART] %s\n", __func__);
 return 0;
}

static int _device_release(struct inode* p_node, struct file* p_file)
{
 printk("[KERNEL-PART] %s\n", __func__);
 return 0;
}

static ssize_t _device_write(struct file* p_file, const char* p_data, size_t count, loff_t* p_position)
{
 printk("[KERNEL-PART] %s\n", __func__);

 count = min( (unsigned long)128, (unsigned long)count );

 if ( 0 > copy_from_user( _kbuf, p_data, count) )
 {
  printk("[KERNEL-PART-ERROR] copy from user failed\n");
  return -EFAULT;
 }

 printk("[KERNEL-PART] %s\n", _kbuf); 

 return count;
}

static ssize_t _device_read(struct file* p_file, char *p_buf, size_t len, loff_t* p_position)
{
 size_t read_len = min( (size_t)128, len );

 printk("[KERNEL-PART] %s\n", __func__);

 if ( 0 > copy_to_user( p_buf, _kbuf, read_len ) )
 {
  printk("[KERNEL-PART-ERROR] copy to user failed\n");
  return -EFAULT;
 }
 
 return read_len;
}


int register_device(int major, int minor)
{
 dev_t device_number;
 int   ret;

 device_number = MKDEV(major, minor);
 if ( !MAJOR(device_number) )
 {
  ret = alloc_chrdev_region( &device_number, MINOR(device_number), 1, "tacobuf");
  printk("[KERNEL-PART] alloc_chrdev_region\n");
 }
 else
 {
  ret = register_chrdev_region(device_number, 1, "tacobuf");
  printk("[KERNEL-PART] register_chrdev_region\n");
 }

 if ( ret < 0 )
 {
  printk("[KERNEL-PART-ERROR] register device failed\n");
  return ret;
 }

 memset(&_kbuf, 0, 128);
 sprintf(_kbuf, "nothing");
 memset(&_cdevice, 0, sizeof(struct cdev));

 cdev_init( &_cdevice, &_ops );

 _cdevice.owner = THIS_MODULE;
 _cdevice.ops   = &_ops;

 ret = cdev_add( &_cdevice, device_number, 1 );
 if ( ret != 0 )
 {
  unregister_chrdev_region( device_number, 1 );
  printk("[KERNEL-PART-ERROR] add device failed\n");
  return ret;
 }
 
 _p_class = class_create(THIS_MODULE, "taco_class");
 if ( _p_class == NULL )
 {
  printk("[KERNEL-PART-ERROR] class create failed\n");
  unregister_chrdev_region( device_number, 1 );
  return -1;
 }

 if ( NULL == device_create(_p_class, NULL, device_number, NULL, "tacobuf") )
 {
  printk("[KERNEL-PART-ERROR] device create failed\n");
  unregister_chrdev_region( device_number, 1 );
  return -1;
 }

 return 0;
}

int unregister_device(int major, int minor)
{
 dev_t device_number;

 device_number = MKDEV(major, minor);

 cdev_del( &_cdevice );

 unregister_chrdev_region(device_number, 1);

 device_destroy(_p_class, device_number);
 
 class_destroy(_p_class);

 return 0;
}

Makefile

ifneq ($(KERNELRELEASE),)

obj-m := taco.o
taco-objs := taco_1.o taco_2.o

else

KERNELDIR ?= /lib/modules/$(shell uname -r)/build
PWD       := $(shell pwd)

default:
 @$(MAKE) -C $(KERNELDIR) M=$(PWD) modules


endif

clean:
 @$(RM) -rf *.o *.ko *~ core .depend *.mod.c .*.cmd .tmp_versions .*.o.d *.symvers *.markers *.order