[轉]基礎 Linux Device Driver 驅動程式#10 (select/poll)

備份好文章
http://csw-dawn.blogspot.tw/2012/01/linux-device-driver-10-selectpoll.html

基礎 Linux Device Driver 驅動程式#10 (select/poll)

相信各位都有在Linux上寫程式的經驗，
當您程式裡呼叫 open 時，Linux 預設會以 blocking mode的方式開啟，
block 是指當 process 為了等待某件事的發生，而進入 sleep 狀態的情形。

像 read 就是其中一種，當沒資料讀取時，process 就會被 block。
write 也是一樣，在寫入資料時，寫入對象還無法處理資料時，一樣會 block。

對某些程式來說，如果 read 系統呼叫被 block 的話，有時就會有設計上的問題，
所以為了避免這種問題發生，Linux 就準備了以下方式。

1. Non-blocking 模式

啟用 non-blocking 模式後，不管是 read 還是 write 就不會被 block住。但會傳回 errno 錯誤碼，
這時就必須自已再做讀寫的動作。
想使用 non-blocking 模式的話，可在 open() 開檔時指定 O_NONBLOCK。

2. 同時執行多個同步 I/O 工作

同時執行多個同步 I/O 工作指的是使用 select 系統呼叫的做法。
select 系統呼叫本身會被 block, 但可指定 timeout。

使用 select() 的時候，有幾個常用的函式巨集，可參考 man 2 select。

int select(int nfds, fd_set *readfds, fd_set *writefds,
    fd_set *exceptfds, struct timeval *timeout);

void FD_CLR(int fd, fd_set *set);
int  FD_ISSET(int fd, fd_set *set);
void FD_SET(int fd, fd_set *set);
void FD_ZERO(fd_set *set);

int select(int nfds, fd_set *readfds, fd_set *writefds,

fd_set *exceptfds, struct timeval *timeout);

void FD_CLR(int fd, fd_set *set);

int FD_ISSET(int fd, fd_set *set);

void FD_SET(int fd, fd_set *set);

void FD_ZERO(fd_set *set);

還有一個和 select()很類似的 poll()系統呼叫，以基本功能來說，select()與poll()都一樣，
但指定的 file handler的方式不一樣，且指定多個 file handler 的時候，poll()走訪所有
file handler的速度比較快。可參考 man 2 poll。

int poll(struct pollfd *fds, nfds_t nfds, int timeout);

1	int poll(struct pollfd *fds, nfds_t nfds, int timeout);

應用程式想同時執行多個同步 I/O 工作時，可使用的系統呼叫有好幾個，
但在驅動程式，只需要準備一個函式即可。
不管 user process 用了哪個系統呼叫，kernel 都只會呼叫驅動程式提供的這個函式。

Character 類型的裝置想支援同時執行多個同步 I/O 工作的話，只要在驅動程式準備 poll方法即可。
poll方法會收到 file_operations 結構。

unsigned int (*poll) (struct file *, struct poll_table_struct *);

1	unsigned int (poll) (struct file , struct poll_table_struct *);

poll方法會在kernel 處理 select 與 poll 之類的系統呼叫時用到。它必須的執行工作如下：
1. 在 wait queue 豋記。
2. 傳回目前可以操作的狀態。

在呼叫同時執行多個同步 I/O 工作的系統呼叫時，block 直到狀態變化的動作，指的是在 kernel裡面 sleep。
如果要 sleep的話，要先準備 wait queue(wait_queue_head_t)，這個由驅動程式負責提供。
豋記 wait queue 的工作可透過 poll_wait()完成，它定義在 include/linux/poll.h

void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p);
// filp:執行操作的設備文件指針
// wait_address:設備驅動的等待隊列頭
// p:內核傳遞的輪詢表指針
 調用poll_wait()函數將等待隊列添加到poll_table輪詢表 (<a href="http://blog.csdn.net/waldmer/article/details/18735517" target="_blank" rel="noopener noreferrer">出處</a>參考)

void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p);

// filp:執行操作的設備文件指針

// wait_address:設備驅動的等待隊列頭

// p:內核傳遞的輪詢表指針

調用poll_wait()函數將等待隊列添加到poll_table輪詢表 (<a href="http://blog.csdn.net/waldmer/article/details/18735517" target="_blank" rel="noopener noreferrer">出處</a>參考)

在呼叫同時執行多個同步 I/O 工作的系統呼叫時，解除 block 的時機，是驅動程式透過傳給 poll_wait()
的 wait queue 被喚醒的時候。Kernel 在被 wait queue 喚醒之後，會再次呼叫驅動程式的 poll()
確認是否成為等待中的狀態(可寫入或可讀出)，如果是的話，FD_ISSET 巨集就會成立。
想判斷是不是這種狀態的話，還是需要驅動程式提供資訊才行，這個資訊就是透過poll()方法的傳回值來表示。
傳回值要透過 include/linux/poll.h 定義的巨集 OR 起來表示，下面是常用到的組合。
數值定義於 android/bionic/libc/kernel/arch-arm/asm/poll.h

POLLIN|POLLRDNORM                     可讀取
POLLOUT|POLLWRNORM                    可寫入
POLLIN|POLLRDNORM|POLLOUT|POLLWRNORM  可讀寫
POLLERR                               發生錯誤
POLLHUP                               裝置離線(EOF)

POLLIN|POLLRDNORM 可讀取

POLLOUT|POLLWRNORM 可寫入

POLLIN|POLLRDNORM|POLLOUT|POLLWRNORM 可讀寫

POLLERR 發生錯誤

POLLHUP 裝置離線(EOF)

說了那麼多，還倒不如寫個程式比較好了解。

poll操作一般結構:

xxx_poll() 
{ 
  unsigned int mask = 0;//定義返回值 
  ... 
  //調用poll_wait()把進程添加到輪詢表 
  ... 
  if(device is ready for read){ 
    mask = POLLIN | POLLRDNORM; 
  } 
  ... 
  return mask; 
}

xxx_poll()

{

unsigned int mask = 0;//定義返回值

...

//調用poll_wait()把進程添加到輪詢表

...

if(device is ready for read){

mask = POLLIN | POLLRDNORM;

}

...

return mask;

}

剛試了一下，果然新舊版核心還是有差，幾個問題，稍為提出來討論一下：
1. 要用 kmalloc 及 kfree 的話，要 include 註：其實在上一個示範程式就已有這問題了。
請參考：基礎 Linux Device Driver 驅動程式#9 (IOCTL)
2. void init_MUTEX (struct semaphore *sem); /* 新版 kernel 已不適用 */
改用
void sema_init (struct semaphore *sem, int val);

test_select.c 原始碼如下：

/*****************************************************************************/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/device.h>
#include <linux/slab.h>
#include <linux/poll.h>
#include <linux/sched.h>
#include <asm/uaccess.h>

#define DRIVER_NAME "test_select"

static unsigned int test_select_major = 0;
static unsigned int num_of_dev = 1;
static struct cdev test_select_cdev;
static unsigned int timeout_value = 10;

struct test_select_data {
 struct timer_list timeout;
 spinlock_t lock;
 wait_queue_head_t read_wait;
 int timeout_done;
 struct semaphore sem;
};

unsigned int test_select_poll(struct file *filp, poll_table *wait)
{
 struct test_select_data *data = filp->private_data;
 unsigned int mask = POLLOUT|POLLWRNORM;
 printk(KERN_ALERT "Call test_select_poll.\n");

 if (data == NULL)
  return -EBADFD;
 down(&data->sem);
 poll_wait(filp, &data->read_wait, wait);

 if (data->timeout_done == 1) {    /* readable */
  mask |= POLLIN|POLLRDNORM;
 }
 up(&data->sem);
 printk(KERN_ALERT "%s returned (mask 0x%x)\n", __func__,  mask);
}

static void test_select_timeout(unsigned long arg)
{
 struct test_select_data *data = (struct test_select_data*)arg;
 unsigned long flags;
 printk(KERN_ALERT "Call test_select_timeout.\n");

 spin_lock_irqsave(&data->lock, flags);

 data->timeout_done = 1;
 wake_up_interruptible(&data->read_wait);

 spin_unlock_irqrestore(&data->lock, flags);
}

ssize_t test_select_write (struct file *filp, const char __user *buf, size_t count, loff_t *pos)
{
 return -EFAULT;
}

ssize_t test_select_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
{
 struct test_select_data *data = filp->private_data;
 int i;
 unsigned char val;
 int retval;

 if (down_interruptible(&data->sem))
  return -ERESTARTSYS;
 if (data->timeout_done == 0) {    /* no read */
  up(&data->sem);
  if (filp->f_flags & O_NONBLOCK)    /* non-blocking mode */
   return -EAGAIN;
  do {
   retval = wait_event_interruptible_timeout(
       data->read_wait,
       data->timeout_done == 1,
       1*HZ);
   if (retval == -ERESTARTSYS)
    return -ERESTARTSYS;
  } while (retval == 0);    /* timeout elapsed */

  if (down_interruptible(&data->sem))
   return -ERESTARTSYS;
 }
 val = 0xff;
 for (i = 0; i < count; i++) {
  if (copy_to_user(&buf[i], &val, 1)) {
   retval = -EFAULT;
   goto out;
  }
 }
 retval = count;

out:
 data->timeout_done = 0;
 
 /* restart timer */
 mod_timer(&data->timeout, jiffies + timeout_value*HZ);
 up(&data->sem);

 return retval;
}

static int test_select_close(struct inode *inode, struct file *filp)
{
 struct test_select_data *data = filp->private_data;
 printk(KERN_ALERT "Call test_select_close.\n");

 if (data) {
  del_timer_sync(&data->timeout);
  kfree(data);
 }

 return 0;
}

static int test_select_open(struct inode *inode, struct file *filp)
{
 struct test_select_data *data;
 printk(KERN_ALERT "Call test_select_open.\n");

 data = kmalloc(sizeof(struct test_select_data), GFP_KERNEL);
 if (data == NULL)
  return -ENOMEM;

 /* initialize members */
 spin_lock_init(&data->lock);
 init_waitqueue_head(&data->read_wait);
 // init_MUTEX(&data->sem); /* 新版 kernel 已不適用 */
 sema_init(&data->sem, 1);  /* 改用 sema_init */
 init_timer(&data->timeout);

 data->timeout.function = test_select_timeout;
 data->timeout.data = (unsigned long)data;

 filp->private_data = data;

 /* start timer */
 data->timeout_done = 0;
 mod_timer(&data->timeout, jiffies + timeout_value*HZ);

 return 0;
}

struct file_operations fops = {
 .owner = THIS_MODULE,
 .open = test_select_open,
 .release = test_select_close,
 .read = test_select_read,
 .write = test_select_write,
 .poll = test_select_poll,
};

static int test_select_init(void)
{
 dev_t dev = MKDEV(test_select_major, 0);
 int alloc_ret = 0;
 int cdev_ret = 0;

 alloc_ret = alloc_chrdev_region(&dev, 0, num_of_dev, DRIVER_NAME);
 if (alloc_ret)
  goto error;

 test_select_major = MAJOR(dev);
 cdev_init(&test_select_cdev, &fops);
 cdev_ret = cdev_add(&test_select_cdev, dev, num_of_dev);
 if (cdev_ret)
  goto error;
 printk(KERN_ALERT "%s driver(major: %d) installed.\n", DRIVER_NAME, test_select_major);
 
 return 0;
error:
 if (cdev_ret == 0)
  cdev_del(&test_select_cdev);
 if (alloc_ret == 0)
  unregister_chrdev_region(dev, num_of_dev);
 
 return -1;
}

static void test_select_exit(void)
{
 dev_t dev = MKDEV(test_select_major, 0);

 cdev_del(&test_select_cdev);
 unregister_chrdev_region(dev, num_of_dev);
 
 printk(KERN_ALERT "%s driver removed\n", DRIVER_NAME);
}

module_init(test_select_init);
module_exit(test_select_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Wang Chen Shu");
MODULE_DESCRIPTION("This is test_select module.");

/*****************************************************************************/

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/*****************************************************************************/

#include <linux/init.h>

#include <linux/module.h>

#include <linux/types.h>

#include <linux/kernel.h>

#include <linux/fs.h>

#include <linux/cdev.h>

#include <linux/device.h>

#include <linux/slab.h>

#include <linux/poll.h>

#include <linux/sched.h>

#include <asm/uaccess.h>

#define DRIVER_NAME "test_select"

static unsigned int test_select_major = 0;

static unsigned int num_of_dev = 1;

static struct cdev test_select_cdev;

static unsigned int timeout_value = 10;

struct test_select_data {

struct timer_list timeout;

spinlock_t lock;

wait_queue_head_t read_wait;

int timeout_done;

struct semaphore sem;

};

unsigned int test_select_poll(struct file *filp, poll_table *wait)

{

struct test_select_data *data = filp->private_data;

unsigned int mask = POLLOUT|POLLWRNORM;

printk(KERN_ALERT "Call test_select_poll.\n");

if (data == NULL)

return -EBADFD;

down(&data->sem);

poll_wait(filp, &data->read_wait, wait);

if (data->timeout_done == 1) { /* readable */

mask |= POLLIN|POLLRDNORM;

}

up(&data->sem);

printk(KERN_ALERT "%s returned (mask 0x%x)\n", __func__, mask);

}

static void test_select_timeout(unsigned long arg)

{

struct test_select_data *data = (struct test_select_data*)arg;

unsigned long flags;

printk(KERN_ALERT "Call test_select_timeout.\n");

spin_lock_irqsave(&data->lock, flags);

data->timeout_done = 1;

wake_up_interruptible(&data->read_wait);

spin_unlock_irqrestore(&data->lock, flags);

}

ssize_t test_select_write (struct file *filp, const char __user *buf, size_t count, loff_t *pos)

{

return -EFAULT;

}

ssize_t test_select_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)

{

struct test_select_data *data = filp->private_data;

int i;

unsigned char val;

int retval;

if (down_interruptible(&data->sem))

return -ERESTARTSYS;

if (data->timeout_done == 0) { /* no read */

up(&data->sem);

if (filp->f_flags & O_NONBLOCK) /* non-blocking mode */

return -EAGAIN;

do {

retval = wait_event_interruptible_timeout(

data->read_wait,

data->timeout_done == 1,

1*HZ);

if (retval == -ERESTARTSYS)

return -ERESTARTSYS;

} while (retval == 0); /* timeout elapsed */

if (down_interruptible(&data->sem))

return -ERESTARTSYS;

}

val = 0xff;

for (i = 0; i < count; i++) {

if (copy_to_user(&buf[i], &val, 1)) {

retval = -EFAULT;

goto out;

}

retval = count;

out:

data->timeout_done = 0;

/* restart timer */

mod_timer(&data->timeout, jiffies + timeout_value*HZ);

up(&data->sem);

return retval;

}

static int test_select_close(struct inode *inode, struct file *filp)

{

struct test_select_data *data = filp->private_data;

printk(KERN_ALERT "Call test_select_close.\n");

if (data) {

del_timer_sync(&data->timeout);

kfree(data);

}

return 0;

}

static int test_select_open(struct inode *inode, struct file *filp)

{

struct test_select_data *data;

printk(KERN_ALERT "Call test_select_open.\n");

data = kmalloc(sizeof(struct test_select_data), GFP_KERNEL);

if (data == NULL)

return -ENOMEM;

/* initialize members */

spin_lock_init(&data->lock);

init_waitqueue_head(&data->read_wait);

// init_MUTEX(&data->sem); /* 新版 kernel 已不適用 */

sema_init(&data->sem, 1); /* 改用 sema_init */

init_timer(&data->timeout);

data->timeout.function = test_select_timeout;

data->timeout.data = (unsigned long)data;

filp->private_data = data;

/* start timer */

data->timeout_done = 0;

mod_timer(&data->timeout, jiffies + timeout_value*HZ);

return 0;

}

struct file_operations fops = {

.owner = THIS_MODULE,

.open = test_select_open,

.release = test_select_close,

.read = test_select_read,

.write = test_select_write,

.poll = test_select_poll,

};

static int test_select_init(void)

{

dev_t dev = MKDEV(test_select_major, 0);

int alloc_ret = 0;

int cdev_ret = 0;

alloc_ret = alloc_chrdev_region(&dev, 0, num_of_dev, DRIVER_NAME);

if (alloc_ret)

goto error;

test_select_major = MAJOR(dev);

cdev_init(&test_select_cdev, &fops);

cdev_ret = cdev_add(&test_select_cdev, dev, num_of_dev);

if (cdev_ret)

goto error;

printk(KERN_ALERT "%s driver(major: %d) installed.\n", DRIVER_NAME, test_select_major);

return 0;

error:

if (cdev_ret == 0)

cdev_del(&test_select_cdev);

if (alloc_ret == 0)

unregister_chrdev_region(dev, num_of_dev);

return -1;

}

static void test_select_exit(void)

{

dev_t dev = MKDEV(test_select_major, 0);

cdev_del(&test_select_cdev);

unregister_chrdev_region(dev, num_of_dev);

printk(KERN_ALERT "%s driver removed\n", DRIVER_NAME);

}

module_init(test_select_init);

module_exit(test_select_exit);

MODULE_LICENSE("GPL");

MODULE_AUTHOR("Wang Chen Shu");

MODULE_DESCRIPTION("This is test_select module.");

/*****************************************************************************/

Makefile 如下：

/*****************************************************************************/
KDIR="/opt/linux-source-2.6.38"
PWD=$(shell pwd)

obj-m := test_select.o

all:
 $(MAKE) -C ${KDIR} M=${PWD} modules
clean:
 $(MAKE) -C ${KDIR} M=${PWD} clean

/*****************************************************************************/

KDIR="/opt/linux-source-2.6.38"

PWD=$(shell pwd)

obj-m := test_select.o

all:

$(MAKE) -C ${KDIR} M=${PWD} modules

clean:

$(MAKE) -C ${KDIR} M=${PWD} clean

/*****************************************************************************/

test.c 如下:
/*****************************************************************************/

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

#define DEVFILE "/dev/test_select0"

int main()
{
 int fd;
        fd_set rfds;
        struct timeval tv;
        int retval;
        unsigned char buf;
        ssize_t sz;
        int i;

        fd = open(DEVFILE, O_RDWR);
        if (fd == -1) {
                perror("open");
                return -1;
        }

        do {
                FD_ZERO(&rfds);
                FD_SET(fd, &rfds);
                tv.tv_sec = 5;
                tv.tv_usec = 0;

                printf("select() ...\n");
                retval = select(fd + 1, &rfds, NULL, NULL, &tv);
                if (retval == -1) {
                        perror("select");
                        break;
                }

                if (retval) {
                        break;
                }
        } while (retval == 0);   /* timeout elapsed */

        if (FD_ISSET(fd, &rfds)) {
                printf("read() ...\n");
                sz = read(fd, &buf, 1);
                printf("read() %d\n", sz);
                printf("%02x ", buf);
                printf("\n");
        }

        close(fd);

        return 0;

}

#include <stdio.h>

#include <fcntl.h>

#include <errno.h>

#define DEVFILE "/dev/test_select0"

int main()

{

int fd;

fd_set rfds;

struct timeval tv;

int retval;

unsigned char buf;

ssize_t sz;

int i;

fd = open(DEVFILE, O_RDWR);

if (fd == -1) {

perror("open");

return -1;

}

do {

FD_ZERO(&rfds);

FD_SET(fd, &rfds);

tv.tv_sec = 5;

tv.tv_usec = 0;

printf("select() ...\n");

retval = select(fd + 1, &rfds, NULL, NULL, &tv);

if (retval == -1) {

perror("select");

break;

}

if (retval) {

break;

}

} while (retval == 0); /* timeout elapsed */

if (FD_ISSET(fd, &rfds)) {

printf("read() ...\n");

sz = read(fd, &buf, 1);

printf("read() %d\n", sz);

printf("%02x ", buf);

printf("\n");

}

close(fd);

return 0;

}

/*****************************************************************************/

執行結果如下：

# ls
Makefile test_code test_select.c
# make
make -C “/opt/linux-source-2.6.38” M=/opt/test_driver/my_driver/test_select modules
make[1]: Entering directory /opt/linux-source-2.6.38' CC [M] /opt/test_driver/my_driver/test_select/test_select.o /opt/test_driver/my_driver/test_select/test_select.c: In function ‘test_select_poll’: /opt/test_driver/my_driver/test_select/test_select.c:44:1: warning: control reaches end of non-void function Building modules, stage 2. MODPOST 1 modules CC /opt/test_driver/my_driver/test_select/test_select.mod.o LD [M] /opt/test_driver/my_driver/test_select/test_select.ko make[1]: Leaving directory/opt/linux-source-2.6.38′
# ls
Makefile Module.symvers test_select.c test_select.mod.c test_select.o
modules.order test_code test_select.ko test_select.mod.o
# cd test_code/
# ls
test.c
# gcc test.c -o test
# ls
test test.c
＃ cd ..
# insmod ./test_select.ko
test_select driver(major: 250) installed.
# mknod /dev/test_select0 c 250 0
# ./test_code/test
select() …
read() …
… 經過 10秒 …
read() 1
ff
# dmesg | tail
… 以上略過 …
Call test_select_open.
Call test_select_poll.
test_select_poll returned (mask 0x104)
Call test_select_timeout.
Call test_select_close.

# rm -rf test_code/test
# rm /dev/test_select0
# rmmod test_select
# make clean
make -C “/opt/linux-source-2.6.38” M=/opt/test_driver/my_driver/test_select clean
make[1]: Entering directory /opt/linux-source-2.6.38' CLEAN /opt/test_driver/my_driver/test_select/.tmp_versions CLEAN /opt/test_driver/my_driver/test_select/Module.symvers make[1]: Leaving directory/opt/linux-source-2.6.38′

OK, 一切就是如此的順利，世界就是如此的美好 ^^

註記及聲明：
本教學，是參考Linux Device Driver Programming驅動程式設計由平田豐著的這本書。

基礎 Linux Device Driver 驅動程式#10 (select/poll)

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