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#include #include #include #include #include #include #include #include #include #include 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#include 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#include #include #include #include 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。