自己动手写驱动程序

来源：互联网发布：linux https 编辑：程序博客网时间：2024/05/28 15:36

一、编码思维导图

二、编写代码
memdev.c

#include <linux/module.h>
#include <linux/init.h>
#include <linux/cdev.h>
#include <linux/fs.h>
#include <asm/uaccess.h>
struct cdev mdev; //1.静态分配cdev
dev_t devno;
int dev1_regs[5];
int dev2_regs[5];
loff_t mem_lseek(struct file *filep, loff_t offset, int whence)
{
loff_t new_pos = 0;
switch(whence) {
case SEEK_SET:
new_pos = 0 + offset;
break;
case SEEK_CUR:
new_pos = filep->f_pos + offset;
break;
case SEEK_END:
new_pos = 5*sizeof(int) + offset;
break;
default:
break;
}
filep->f_pos = new_pos;
return new_pos;
}
size_t mem_read(struct file *filep, char __user *buf, size_t size, loff_t *ppos)
{
int *reg_base = filep->private_data;
copy_to_user(buf, reg_base+(*ppos), size);
filep->f_pos += size;
return size;
}
size_t mem_write(struct file *filep, const char __user *buf, size_t size, loff_t *ppos)
{
int *reg_base = filep->private_data;
copy_from_user(reg_base+(*ppos), buf, size);
filep->f_pos += size;
return size;
}
size_t mem_open(struct inode *node, struct file *filep)
{
int num = MINOR(node->i_rdev);
if(num == 0)
filep->private_data = dev1_regs;
else if(num == 1)
filep->private_data = dev2_regs;
return 0;
}
int mem_close (struct inode *node, struct file *filep)
{
return 0;
}
struct file_operations memfops =
{
.llseek = mem_lseek,
.open = mem_open,
.write = mem_write,
.read = mem_read,
.release = mem_close,
};
int memdev_init()
{
cdev_init(&mdev, &memfops); //2.初始化cdev
alloc_chrdev_region(&devno, 0, 2, "memdev"); //3.动态分配设备号到devno
cdev_add(&mdev, devno, 2); //4.注册cdev
return 0;
}
void memdev_exit()
{
cdev_del(&mdev); //5.设备注销
unregister_chrdev_region(devno, 2); //6.设备号注销
}
module_init(memdev_init); //初始化入口
module_exit(memdev_exit); //模块退出

三、函数学习
设备初始化：cdev_init
原型：void cdev_init(struct cdev *cdev, const struct file_operations *fops);
cdev:待初始化的cdev结构
fops:设备对应的操作函数集

动态分配设备号：alloc_chrdev_region
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,const char *name);
dev:分配后设备号的地址
baseminor:起始地设备序号
count:分配的个数
name:分配后的设备别名

设备注册：cdev_add
int cdev_add(struct cdev *p, dev_t dev, unsigned count);
p:待添加的字符设备结构
dev:设备号
count：该类设备的设备个数

注销设备：cdev_del
void cdev_del(struct cdev *p)；
p:待注销的设备结构

释放设备号：unregister_chrdev_region
void unregister_chrdev_region(dev_t from, unsigned count)；
from:待释放设备号
count:释放的设备个数

四、驱动访问模型
1.应用程序访问read
arm-linux-objdump -d read_mem

00008228 <main>:
8228: e92d4800 push {fp, lr}
822c: e28db004 add fp, sp, #4 ; 0x4
8230: e24dd008 sub sp, sp, #8 ; 0x8
8234: e3a03000 mov r3, #0 ; 0x0
8238: e50b3008 str r3, [fp, #-8]
823c: e3a03000 mov r3, #0 ; 0x0
8240: e50b300c str r3, [fp, #-12]
8244: e59f0044 ldr r0, [pc, #68] ; 8290 <main+0x68>
8248: e3a01002 mov r1, #2 ; 0x2
824c: eb0028a3 bl 124e0 <__libc_open>
8250: e1a03000 mov r3, r0
8254: e50b3008 str r3, [fp, #-8]
8258: e24b300c sub r3, fp, #12 ; 0xc
825c: e51b0008 ldr r0, [fp, #-8]
8260: e1a01003 mov r1, r3
8264: e3a02004 mov r2, #4 ; 0x4
8268: eb0028c0 bl 12570 <__libc_read>

>>libc_read:

00012570 <__libc_read>:
12570: e51fc028 ldr ip, [pc, #-40] ; 12550 <__libc_open+0x70>
12574: e79fc00c ldr ip, [pc, ip]
12578: e33c0000 teq ip, #0 ; 0x0
1257c: 1a000006 bne 1259c <__libc_read+0x2c>
12580: e1a0c007 mov ip, r7
12584: e3a07003 mov r7, #3 ; 0x3
12588: ef000000 svc 0x00000000

将3放入r7寄存器中
调用svc，进入内核空间
通过查表找到read并使用

在entry-common.S中

ENTRY(sys_call_table)
#include "calls.S"
#undef ABI
#undef OBSOLETE
/*============================================================================
* Special system call wrappers
*/
@ r0 = syscall number
@ r8 = syscall table
sys_syscall:
bic scno, r0, #__NR_OABI_SYSCALL_BASE
cmp scno, #__NR_syscall - __NR_SYSCALL_BASE
cmpne scno, #NR_syscalls @ check range
stmloia sp, {r5, r6} @ shuffle args
movlo r0, r1
movlo r1, r2
movlo r2, r3
movlo r3, r4
ldrlo pc, [tbl, scno, lsl #2]
b sys_ni_syscall
ENDPROC(sys_syscall)

利用3这个编号找到sys_read
>>>read_write.c

ssize_t vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
{
ssize_t ret;
if (!(file->f_mode & FMODE_READ))
return -EBADF;
if (!file->f_op || (!file->f_op->read && !file->f_op->aio_read))
return -EINVAL;
if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))
return -EFAULT;
ret = rw_verify_area(READ, file, pos, count);
if (ret >= 0) {
count = ret;
if (file->f_op->read)
ret = file->f_op->read(file, buf, count, pos); //这里调用了file_operations的read
else
ret = do_sync_read(file, buf, count, pos);
if (ret > 0) {
fsnotify_access(file->f_path.dentry);
add_rchar(current, ret);
}
inc_syscr(current);
}
return ret;
}

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