S5PV210（TQ210）学习笔记——按键驱动程序

经过前面的配置，S5PV210开发已经可以成功进入Linux控制台了，那么，有了这个环境就可以开始学习Linux驱动的编写和测试了。学习Linux设备驱动，通常是从字符设备驱动开始。由于linux驱动开发具有比较系统的体系结构，我很难在一篇文章中阐述其开发思路，为了简单起见，从本文开始，自行编写的驱动将直接附上代码，对开发过程中感触比较深的地方稍作陈述。

我写的第一个驱动程序是Led的，但是感觉没有必要发出来了，S5PV210（TQ210）的按键驱动程序源码，仅供参考：

#include <linux/types.h>
#include <linux/module.h>
#include <linux/cdev.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <linux/gpio.h>
#include <linux/irq.h>
#include <linux/interrupt.h>
#include <linux/sched.h>
#include <linux/wait.h>
#include <linux/uaccess.h>

static dev_t devno;
static struct cdev cdev;
static struct class* buttons_class;
static struct device* buttons_device;

static wait_queue_head_t button_waitq;

static volatile int pressed = 0;
static unsigned char key_val;

struct key_desc{
unsigned int  pin;
unsigned char value;
};

static struct key_desc key_descs[8] = {
[0] = {
.pin = S5PV210_GPH0(0),
.value = 0x00,
},

[1] = {
.pin = S5PV210_GPH0(1),
.value = 0x01,
},

[2] = {
.pin = S5PV210_GPH0(2),
.value = 0x02,
},

[3] = {
.pin = S5PV210_GPH0(3),
.value = 0x03,
},

[4] = {
.pin = S5PV210_GPH0(4),
.value = 0x04,
},

[5] = {
.pin = S5PV210_GPH0(5),
.value = 0x05,
},

[6] = {
.pin = S5PV210_GPH2(6),
.value = 0x06,
},

[7] = {
.pin = S5PV210_GPH2(7),
.value = 0x07,
},
};

static irqreturn_t buttons_irq(int irq, void *dev_id){
volatile struct key_desc *key = (volatile struct key_desc *)dev_id;

if(gpio_get_value(key->pin)){
key_val = key->value|0x80;
}
else{
key_val = key->value;
}

pressed = 1;
wake_up_interruptible(&button_waitq);

return IRQ_RETVAL(IRQ_HANDLED);
}

static int buttons_open(struct inode *inode, struct file *file){
int ret;

ret = request_irq(IRQ_EINT(0),   buttons_irq, IRQ_TYPE_EDGE_BOTH, "key1", &key_descs[0]);
if(ret)
return ret;
ret = request_irq(IRQ_EINT(1),   buttons_irq, IRQ_TYPE_EDGE_BOTH, "key2", &key_descs[1]);
if(ret)
return ret;
ret = request_irq(IRQ_EINT(2),   buttons_irq, IRQ_TYPE_EDGE_BOTH, "key3", &key_descs[2]);
if(ret)
return ret;
ret = request_irq(IRQ_EINT(3),   buttons_irq, IRQ_TYPE_EDGE_BOTH, "key4", &key_descs[3]);
if(ret)
return ret;
ret = request_irq(IRQ_EINT(4),   buttons_irq, IRQ_TYPE_EDGE_BOTH, "key5", &key_descs[4]);
if(ret)
return ret;
ret = request_irq(IRQ_EINT(5),   buttons_irq, IRQ_TYPE_EDGE_BOTH, "key6", &key_descs[5]);
if(ret)
return ret;
ret = request_irq(IRQ_EINT(22),  buttons_irq, IRQ_TYPE_EDGE_BOTH, "key7", &key_descs[6]);
if(ret)
return ret;
ret = request_irq(IRQ_EINT(23),  buttons_irq, IRQ_TYPE_EDGE_BOTH, "key8", &key_descs[7]);
if(ret)
return ret;
return 0;
}

static ssize_t buttons_read(struct file * file, char __user *data, size_t count, loff_t *loff){
if(count != 1){
printk(KERN_ERR "The driver can only give one key value once!\n");
return -ENOMEM;
}

wait_event_interruptible(button_waitq, pressed);
pressed = 0;

if(copy_to_user(data, &key_val, 1)){
printk(KERN_ERR "The driver can not copy the data to user area!\n");
return -ENOMEM;
}

return 0;
}

static int buttons_close(struct inode *inode, struct file *file){
free_irq(IRQ_EINT(0),  &key_descs[0]);
free_irq(IRQ_EINT(1),  &key_descs[1]);
free_irq(IRQ_EINT(2),  &key_descs[2]);
free_irq(IRQ_EINT(3),  &key_descs[3]);
free_irq(IRQ_EINT(4),  &key_descs[4]);
free_irq(IRQ_EINT(5),  &key_descs[5]);
free_irq(IRQ_EINT(22), &key_descs[6]);
free_irq(IRQ_EINT(23), &key_descs[7]);
return 0;
}

struct file_operations buttons_ops = {
.open    = buttons_open,
.read    = buttons_read,
.release = buttons_close,
};

int buttons_init(void){
int ret;

cdev_init(&cdev, &buttons_ops);
cdev.owner = THIS_MODULE;

ret = alloc_chrdev_region(&devno, 0, 1, "buttons");
if(ret){
printk(KERN_ERR "alloc char device region faild!\n");
return ret;
}

ret = cdev_add(&cdev, devno, 1);
if(ret){
printk(KERN_ERR "add char device faild!\n");
goto add_error;
}

buttons_class = class_create(THIS_MODULE, "buttonsdrv");
if(IS_ERR(buttons_class)){
printk(KERN_ERR "create class error!\n");
goto class_error;
}

buttons_device = device_create(buttons_class, NULL, devno, NULL, "buttons");
if(IS_ERR(buttons_device)){
printk(KERN_ERR "create buttons device error!\n");
goto device_error;
}

init_waitqueue_head(&button_waitq);

return 0;

device_error:
class_destroy(buttons_class);
class_error:
cdev_del(&cdev);
add_error:
unregister_chrdev_region(devno,1);

return -ENODEV;
}

void buttons_exit(void){
device_destroy(buttons_class, devno);
class_destroy(buttons_class);
cdev_del(&cdev);
unregister_chrdev_region(devno, 1);
}

module_init(buttons_init);
module_exit(buttons_exit);
MODULE_LICENSE("GPL");

测试程序代码：

#include <stdio.h>
#include <fcntl.h>

int main(){
int fd = open("/dev/buttons", O_RDWR);
if(fd < 0){
printf("open error");;
return 0;
}

unsigned char key;
while(1){
read(fd, &key, 1);
printf("The key = %x\n", key);
}

close(fd);
}

相比轮询方式的按键驱动程序，中断方式编写的按键驱动程序可以很大程度上节省CPU资源，因此，推荐使用中断方式。

但是，这种方式有个弊端，如果一直接收不到按键，程序就会永远阻塞在这里，幸运的是，linux内核提供了poll机制，可以设置延迟时间，如果在这个时间内受到按键消息则取得键值，反之则超时退出。使内核支持poll非常简单，为file_operations的poll成员提供poll处理函数即可。

使内核支持poll还需要以下几步：

添加poll头文件

编写poll处理函数：

static unsigned buttons_poll(struct file *file, poll_table *wait){
unsigned int mask = 0;
poll_wait(file, &button_waitq, wait);

if (pressed)
mask |= POLLIN | POLLRDNORM;

return mask;
}

将poll处理函数添加给file_operations：

.poll    = buttons_poll,

这样，驱动程序就支持poll机制了。下面是poll方式的测试程序：

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <poll.h>

int main(int argc, char **argv){
int fd;
unsigned char key_val;
int ret;

struct pollfd fds[1];

fd = open("/dev/buttons", O_RDWR);
if (fd < 0){
printf("can't open!\n");
}

fds[0].fd     = fd;
fds[0].events = POLLIN;
while (1){
ret = poll(fds, 1, 5000);
if (ret == 0){
printf("time out\n");
}
else{
read(fd, &key_val, 1);
printf("key_val = 0x%x\n", key_val);
}
}

return 0;
}

这样按键驱动程序就完成了。如果您在编写测试阶段发现了其他问题，欢迎留言讨论。