poll机制的引入
使用休眠-唤醒的方式等待某个事件发生时，有一个缺点：等待的时间可能很久。我们可以加上一个超时时间，这时就可以使用
poll机制
APP不知道驱动程序中是否有数据，可以先调用poll函数查询一下，poll函数可以传入超时时间；
APP进入内核态，调用到驱动程序的poll函数，如果有数据的话立刻返回；
如果发现没有数据时就休眠一段时间；
当有数据时，比如当按下按键时，驱动程序的中断服务程序被调用，它会记录数据、唤醒APP；
当超时时间到了之后，内核也会唤醒APP；
APP根据poll函数的返回值就可以知道是否有数据，如果有数据就调用read得到数据
poll机制内核实现流程
drv_poll要把自己这个线程挂入等待队列wq中；假设不放入队列里，那以后发生中断时，中断服务程序去哪里找到你嘛？
总结：
内核提供poll_wait()函数实现了sys_poll的过程
drv_poll要把线程挂入队列wq，但是并不是在drv_poll中进入休眠，而是在调用drv_poll之后休眠
drv_poll要返回数据状态, 返回状态给应用程序，应用程序根据这个状态判断是超时还是有按键到来
APP调用一次poll，有可能会导致drv_poll被调用2次
线程被唤醒的原因有2：中断发生了去队列wq中把它唤醒，超时时间到了内核把它唤醒
APP要判断poll返回的原因：有数据，还是超时。有数据时再去调用read函数。
相关函数
poll_wait：把当前进程添加到wait参数指定的等待列表(poll_table)中
void poll_wait(struct file *filp, wait_queue_head_t *queue, poll_table *wait);
参数：
queue：等待队列，要把自己这个线程挂入等待队列wq中；假设不放入队列里，那以后发生中断时，中断服务程序去哪
里找到你嘛？
wait：返回设备资源的可获取状态 POLLIN POLLOUT
poll实现按键中断实现
#include <linux/module.h>
#include <linux/poll.h>
#include <linux/fs.h>
#include <linux/errno.h>
#include <linux/miscdevice.h>
#include <linux/kernel.h>
#include <linux/major.h>
#include <linux/mutex.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/stat.h>
#include <linux/init.h>
#include <linux/device.h>
#include <linux/tty.h>
#include <linux/kmod.h>
#include <linux/gfp.h>
#include <linux/gpio/consumer.h>
#include <linux/platform_device.h>
#include <linux/of_gpio.h>
#include <linux/of_irq.h>
#include <linux/interrupt.h>
#include <linux/irq.h>
#include <linux/slab.h>
struct gpio_key{
int gpio;
struct gpio_desc *gpiod;
int flag;
int irq;
} ;
static struct gpio_key *gpio_keys_100ask;
static int major = 0;
static struct class *gpio_key_class;
#define BUF_LEN 128
static int g_keys[BUF_LEN];
static int r, w;
#define NEXT_POS(x) ((x+1) % BUF_LEN)
static int is_key_buf_empty(void)
{
return (r == w);
}
static int is_key_buf_full(void)
{
return (r == NEXT_POS(w));
}
static void put_key(int key)
{
if (!is_key_buf_full())
{
g_keys[w] = key;
w = NEXT_POS(w);
}
}
static int get_key(void)
{
int key = 0;
if (!is_key_buf_empty())
{
key = g_keys[r];
r = NEXT_POS(r);
}
return key;
}
static DECLARE_WAIT_QUEUE_HEAD(gpio_key_wait);
static ssize_t gpio_key_drv_read (struct file *file, char __user *buf, size_t size, loff_t *offset)
{
int err;
int key;
wait_event_interruptible(gpio_key_wait, !is_key_buf_empty());
key = get_key();
err = copy_to_user(buf, &key, 4);
return 4;
}
static unsigned int gpio_key_drv_poll(struct file *fp, poll_table * wait)
{
printk("%s %s line %d\n", __FILE__, __FUNCTION__, __LINE__);
poll_wait(fp, &gpio_key_wait, wait);
return is_key_buf_empty() ? 0 : POLLIN | POLLRDNORM;
}
static struct file_operations gpio_key_drv = {
.owner = THIS_MODULE,
.read = gpio_key_drv_read,
.poll = gpio_key_drv_poll,
};
static irqreturn_t gpio_key_isr(int irq, void *dev_id)
{
struct gpio_key *gpio_key = dev_id;
int val;
int key;
val = gpiod_get_value(gpio_key->gpiod);
printk("key %d %d\n", gpio_key->gpio, val);
key = (gpio_key->gpio << 8) | val;
put_key(key);
wake_up_interruptible(&gpio_key_wait);
return IRQ_HANDLED;
}
static int gpio_key_probe(struct platform_device *pdev)
{
int err;
struct device_node *node = pdev->dev.of_node;
int count;
int i;
enum of_gpio_flags flag;
printk("%s %s line %d\n", __FILE__, __FUNCTION__, __LINE__);
count = of_gpio_count(node);
if (!count)
{
printk("%s %s line %d, there isn't any gpio available\n", __FILE__, __FUNCTION__, __LINE__);
return -1;
}
gpio_keys_100ask = kzalloc(sizeof(struct gpio_key) * count, GFP_KERNEL);
for (i = 0; i < count; i++)
{
gpio_keys_100ask[i].gpio = of_get_gpio_flags(node, i, &flag);
if (gpio_keys_100ask[i].gpio < 0)
{
printk("%s %s line %d, of_get_gpio_flags fail\n", __FILE__, __FUNCTION__, __LINE__);
return -1;
}
gpio_keys_100ask[i].gpiod = gpio_to_desc(gpio_keys_100ask[i].gpio);
gpio_keys_100ask[i].flag = flag & OF_GPIO_ACTIVE_LOW;
gpio_keys_100ask[i].irq = gpio_to_irq(gpio_keys_100ask[i].gpio);
}
for (i = 0; i < count; i++)
{
err = request_irq(gpio_keys_100ask[i].irq, gpio_key_isr, IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING, "100ask_gpio_key", &gpio_keys_100ask[i]);
}
major = register_chrdev(0, "100ask_gpio_key", &gpio_key_drv);
gpio_key_class = class_create(THIS_MODULE, "100ask_gpio_key_class");
if (IS_ERR(gpio_key_class)) {
printk("%s %s line %d\n", __FILE__, __FUNCTION__, __LINE__);
unregister_chrdev(major, "100ask_gpio_key");
return PTR_ERR(gpio_key_class);
}
device_create(gpio_key_class, NULL, MKDEV(major, 0), NULL, "100ask_gpio_key");
return 0;
}
static int gpio_key_remove(struct platform_device *pdev)
{
struct device_node *node = pdev->dev.of_node;
int count;
int i;
device_destroy(gpio_key_class, MKDEV(major, 0));
class_destroy(gpio_key_class);
unregister_chrdev(major, "100ask_gpio_key");
count = of_gpio_count(node);
for (i = 0; i < count; i++)
{
free_irq(gpio_keys_100ask[i].irq, &gpio_keys_100ask[i]);
}
kfree(gpio_keys_100ask);
return 0;
}
static const struct of_device_id ask100_keys[] = {
{ .compatible = "100ask,gpio_key" },
{ },
};
static struct platform_driver gpio_keys_driver = {
.probe = gpio_key_probe,
.remove = gpio_key_remove,
.driver = {
.name = "100ask_gpio_key",
.of_match_table = ask100_keys,
},
};
static int __init gpio_key_init(void)
{
int err;
printk("%s %s line %d\n", __FILE__, __FUNCTION__, __LINE__);
err = platform_driver_register(&gpio_keys_driver);
return err;
}
static void __exit gpio_key_exit(void)
{
printk("%s %s line %d\n", __FILE__, __FUNCTION__, __LINE__);
platform_driver_unregister(&gpio_keys_driver);
}
module_init(gpio_key_init);
module_exit(gpio_key_exit);
MODULE_LICENSE("GPL");
