Linux应用层想要操作kernel层的API，比方想操作相关GPIO或寄存器，能够通过写一个字符设备驱动来实现。

1、先在rootfs中的 /dev/ 下生成一个字符设备。注意主设备号 和 从设备号。可用例如以下shell脚本生成：

if [ ! -e audioIN ];then
     sudo mknod audioIN c 240 0     
fi

生成的设备为 /dev/audioIN ，主设备号240，从设备号0。

2、写audioINdriver.ko ，audioINdriver.c 基本代码框架例如以下：代码中定义了设备名audioIN，设备号240, 0 ，与之前创建的设备一致。

/************************************************************************** * audioINdriver.c
 *
 * kang_liu <liukang325@qq.com>
 * 2014-07-15
\**************************************************************************/

#include <asm/uaccess.h>
#include <asm/errno.h>
#include <linux/types.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/cdev.h>
#include <mach/gpio.h>
//#include <mach/at91_rstc.h> /* debug */
//#include <mach/at91_pmc.h>
//#include <mach/at91_rstc.h>
//#include <mach/at91_shdwc.h>
#include <mach/irqs.h>
//#include "generic.h"
//#include "clock.h"
#include <mach/w55fa92_reg.h>
#include <asm/io.h>

#define DEV_MAJOR 240
#define DEV_MINOR 0
#define NUM_MINORS 1
#define DEVICE_NAME "audioIN"

#define ERR(fmt, args...) printk(KERN_ALERT __FILE__ ": " fmt, ##args)
#define MSG(fmt, args...) printk(KERN_INFO __FILE__ ": " fmt, ##args)
#define DBG(fmt, args...) printk(KERN_DEBUG __FILE__ ": " fmt, ##args)

static ssize_t user_gpio_read(struct file *fp, char __user *buff,
                       size_t count, loff_t *offp)
{
    char str[32] = {0};
    char out[32] = {0};
    int n, err;
//    printk("lk~~~~~~~read buff = %s\n",buff);
    err = copy_from_user(str, buff, count);
//    printk("lk~~~~~~~read str = %s\n",str);
    if (err)
        return -EFAULT;

	sprintf(out,"return values");
	memset(buff, 0, count);
	err = copy_to_user(buff, out, sizeof(out));
	if (err)
	    return -EFAULT;	

    return n;
}

static ssize_t user_gpio_write(struct file *fp, const char __user *buff,
                        size_t count, loff_t *offp)
{
    int err;
    char tmp[32];
   
//    printk("lk~~~~~~~write buff = %s\n",buff);
    err = copy_from_user(tmp, buff, count);
//    printk("lk~~~~~~~write tmp = %s\n",tmp);

    if (err)
        return -EFAULT;
	if('1' == tmp[0])
	{
		//LINE IN
		printk("line in\n");
	}
	else if('0' == tmp[0])
	{
		//MIC IN
		printk("mic in\n");
	}

    return count;
}

static ssize_t user_gpio_open(struct inode *inode,struct file *fp) 
{
//  printk("open gpio devices\n"); 
  
  return 0; 
} 

static struct file_operations user_gpio_file_ops = 
{
  .owner = THIS_MODULE,
  .write = user_gpio_write,
  .read = user_gpio_read,
  .open = user_gpio_open, 
};

static struct cdev *dev;

static void __exit user_audioIN_exit(void)
{
	printk("exit audioIN\n");
    dev_t devno = MKDEV(DEV_MAJOR, DEV_MINOR);

    unregister_chrdev_region(devno, NUM_MINORS);

    cdev_del(dev);

    return;
}

static int __init user_audioIN_init(void)
{
	printk("init audioIN\n");
    int err = 0;
    int i;
    dev_t devno = MKDEV(DEV_MAJOR, DEV_MINOR);

    err = register_chrdev_region(devno, NUM_MINORS, DEVICE_NAME);

    if (err)
        goto fail_devno;

    dev = cdev_alloc();
    dev->ops = &user_gpio_file_ops;
    dev->owner = THIS_MODULE;
   
    err = cdev_add(dev, devno, NUM_MINORS);

    if (err)
        goto fail_cdev;
    
    return err;
fail_cdev:
fail_devno:
    unregister_chrdev_region(devno, NUM_MINORS);
fail_gpio:
    return err;
}

module_init(user_audioIN_init);
module_exit(user_audioIN_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("kang_liu <liukang325@qq.com>");
MODULE_DESCRIPTION("Access GSEIO from userspace."); 

这里就能够调用kernel层的一些API进行底层的操作。

Makefile：生成audioINdriver.ko

# Comment/uncomment the following line to disable/enable debugging
#DEBUG = y
BUILD_TOOLS_PRE = arm-linux-

CC=$(BUILD_TOOLS_PRE)gcc
LD=$(BUILD_TOOLS_PRE)ld
# Add your debugging flag (or not) to CFLAGS
ifeq ($(DEBUG),y)
  DEBFLAGS = -O -g -DSCULL_DEBUG # "-O" is needed to expand inlines
else
  DEBFLAGS = -O2
endif

KERNEL_DIR = ../../../linux-2.6.35.4

EXTRA_CFLAGS += $(DEBFLAGS)
EXTRA_CFLAGS += -I$(LDDINC)
EXTRA_CFLAGS +=-I$(KERNEL_DIR)/arch/arm/mach-w55fa92/include
EXTRA_CFLAGS +=-I$(KERNEL_DIR)/arch/arm
EXTRA_CFLAGS +=-I$(KERNEL_DIR)/arch/arm/include
EXTRA_CFLAGS +=-I$(KERNEL_DIR)/arch/arm/include/linux

ifneq ($(KERNELRELEASE),)
# call from kernel build system

audioIN-objs := audioINdriver.o

obj-m	:= audioINdriver.o

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

modules:
	$(MAKE) ARCH=arm CROSS_COMPILE=$(BUILD_TOOLS_PRE) -C $(KERNELDIR) M=$(PWD) LDDINC=$(PWD)/../include modules

endif

clean:
	rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versions modules.order Module.symvers

depend .depend dep:
	$(CC) $(CFLAGS) -M *.c > .depend


ifeq (.depend,$(wildcard .depend))
include .depend
endif

3. 生成好 .ko 以后，就能够在ARM板上，载入驱动。

insmod audioINdriver.ko

4、载入驱动成功后，就能够在应用层直接操作设备 /dev/audioIN，来实现相关功能，将一些參数传到驱动层，运行相关kernel层的代码。

应用层測试程序例如以下：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#define BUF_LEN 32
int s_audioInFd = 0;

int InitAudioInDevice()
{
	s_audioInFd = open("/dev/audioIN",O_RDWR);

	if (s_audioInFd > 0)
	{
		return 1;
	}
	else
	{
		printf("Can't open the GSE IO device\n");
		return 0;
	}
}

void UninitAudioInDevice()
{
	if (s_audioInFd > 0)
		close(s_audioInFd);
}

int getAudioIn()
{
	char buffer[BUF_LEN] = {0};
	if (s_audioInFd > 0)
	{
		memcpy(&buffer[0], "lk_test", 7);
		//		printf("get buffer = %s\n", buffer);
		int len = read(s_audioInFd, buffer, 7);
		//		printf("get buffer = %s, len = %d\n", buffer, len);
		return len;
	}

	return -1;
}

int setAudioIn(int micLine)
{
	char buffer[BUF_LEN] = {0};
	if (s_audioInFd > 0)
	{
		sprintf(buffer, "%d", micLine);
		int len = write(s_audioInFd, buffer, sizeof(buffer));
		if (len > 0)
			return 1;
	}

	return 0;
}

当中的read 和 write函数，可从驱动中获取一些返回值，也可将字符串传到驱动中。

驱动的入口为：

module_init(user_audioIN_init);
module_exit(user_audioIN_exit);