Linux设备驱动学习(6) 时间、延迟、延缓操作 jit类设备驱动
current是查看当前时间的设备，
jitbusy、jitqueue、jitsche、jitscheto是延迟一段时间的设备
jitimer是使用了定时器类的设备，定时器到期时执行一个过程。
jitasklet、jitasklethi是使用了小任务tasklet类的设备，tasklet是为了尽快执行一个过程而设计。
/*
* jit.c -- the just-in-time module
*
* Copyright (C) 2001,2003 Alessandro Rubini and Jonathan Corbet
* Copyright (C) 2001,2003 O'Reilly & Associates
*
* The source code in this file can be freely used, adapted,
* and redistributed in source or binary form, so long as an
* acknowledgment appears in derived source files. The citation
* should list that the code comes from the book "Linux Device
* Drivers" by Alessandro Rubini and Jonathan Corbet, published
* by O'Reilly & Associates. No warranty is attached;
* we cannot take responsibility for errors or fitness for use.
*
* $Id: jit.c,v 1.16 2004/09/26 07:02:43 gregkh Exp $
*/
#include <linux/config.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/init.h>
#include <linux/time.h>
#include <linux/timer.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include <linux/types.h>
#include <linux/spinlock.h>
#include <linux/interrupt.h>
#include <asm/hardirq.h>
/*
* This module is a silly one: it only embeds short code fragments
* that show how time delays can be handled in the kernel.
*/
int delay = HZ; /* the default delay, expressed in jiffies */
module_param(delay, int, 0);
MODULE_AUTHOR("Alessandro Rubini");
MODULE_LICENSE("Dual BSD/GPL");
/* use these as data pointers, to implement four files in one function */
enum jit_files {
JIT_BUSY,
JIT_SCHED,
JIT_QUEUE,
JIT_SCHEDTO
};
/*
* This function prints one line of data, after sleeping one second.
* It can sleep in different ways, according to the data pointer
*/
int jit_fn(char *buf, char **start, off_t offset,
int len, int *eof, void *data)
{
unsigned long j0, j1; /* jiffies */
wait_queue_head_t wait;
init_waitqueue_head (&wait);
j0 = jiffies;
j1 = j0 + delay;//等待(delay/HZ)秒
switch((long)data) {
case JIT_BUSY:
while (time_before(jiffies, j1))
cpu_relax();//cpu_relax函数不做任何事，却不让出CPU，忙等待，此种循环等待方式极其浪费系统
break;
case JIT_SCHED:
while (time_before(jiffies, j1)) {
schedule();//时间没到则主动释放CPU，但进程仍然在运行队列中
}
break;
case JIT_QUEUE://以上time_before函数是采取了监视内核变量jiffies计数器的方式
wait_event_interruptible_timeout(wait, 0, delay);//这里采取了让进程在等待队列wait上休眠，时间到期时结束休眠
break; //此函数在内部依赖于schedule_timeout()
case JIT_SCHEDTO:
set_current_state(TASK_INTERRUPTIBLE);
schedule_timeout (delay);//这是相对于采取等待队列延时的改进，不等待特定事件而延迟
break;
}
j1 = jiffies; /* actual value after we delayed */
len = sprintf(buf, "%9li %9li\n", j0, j1);//填充20个字节到buf
*start = buf;
return len;
}
/*
* This file, on the other hand, returns the current time forever
*///获取当前的时间
int jit_currentime(char *buf, char **start, off_t offset,
int len, int *eof, void *data)
{
struct timeval tv1;
struct timespec tv2;
unsigned long j1;
u64 j2;
/* get them four */
j1 = jiffies;
j2 = get_jiffies_64();
do_gettimeofday(&tv1);//有接近微秒级的精度
tv2 = current_kernel_time();//以纳秒精度显示，但是只有时钟滴答的分辨率HZ=1000时即精度为0.001
/* print */
len=0;
len += sprintf(buf,"0x%08lx 0x%016Lx %10i.%06i\n"
"%40i.%09i\n",
j1, j2,
(int) tv1.tv_sec, (int) tv1.tv_usec,
(int) tv2.tv_sec, (int) tv2.tv_nsec);
*start = buf;
return len;
}
/*
* The timer example follows
*/
int tdelay = 10;
module_param(tdelay, int, 0);
/* This data structure used as "data" for the timer and tasklet functions */
struct jit_data { //使用定时器的"设备"
struct timer_list timer;
struct tasklet_struct tlet;
int hi; /* tasklet or tasklet_hi */
wait_queue_head_t wait;
unsigned long prevjiffies;
unsigned char *buf;//缓冲区指针
int loops;
};
#define JIT_ASYNC_LOOPS 5
void jit_timer_fn(unsigned long arg)
{
struct jit_data *data = (struct jit_data *)arg;
unsigned long j = jiffies;
data->buf += sprintf(data->buf, "%9li %3li %i %6i %i %s\n",
j, j - data->prevjiffies, in_interrupt() ? 1 : 0,
current->pid, smp_processor_id(), current->comm);
if (--data->loops) {
data->timer.expires += tdelay;
data->prevjiffies = j;
add_timer(&data->timer);//再次注册一个延期tdelay的定时器，到期后再调用自己。
} else {//循环结束，唤醒等待队列
wake_up_interruptible(&data->wait);
}
}
/* the /proc function: allocate everything to allow concurrency */
int jit_timer(char *buf, char **start, off_t offset,
int len, int *eof, void *unused_data)
{
struct jit_data *data;
char *buf2 = buf;
unsigned long j = jiffies;
data = kmalloc(sizeof(*data), GFP_KERNEL);//对每一个读取"设备"的进程都分配一个单独的设备内存。
if (!data)
return -ENOMEM;
init_timer(&data->timer);//初始化定时器
init_waitqueue_head (&data->wait);//初始化等待队列
/* write the first lines in the buffer */
buf2 += sprintf(buf2, " time delta inirq pid cpu command\n");
buf2 += sprintf(buf2, "%9li %3li %i %6i %i %s\n",
j, 0L, in_interrupt() ? 1 : 0,
current->pid, smp_processor_id(), current->comm);
/* fill the data for our timer function */
data->prevjiffies = j;
data->buf = buf2;
data->loops = JIT_ASYNC_LOOPS;
/* register the timer */
data->timer.data = (unsigned long)data;//将指针强制转换成无符号长整型，来作为定时器到期执行函数的参数传入
data->timer.function = jit_timer_fn;//设置定时器到期时执行的函数
data->timer.expires = j + tdelay; /* parameter *///定时器到期的时间
add_timer(&data->timer);//向内核注册定时器
/* wait for the buffer to fill */
wait_event_interruptible(data->wait, !data->loops);//等待定时器中的循环结束
if (signal_pending(current))
return -ERESTARTSYS;
buf2 = data->buf;
kfree(data);
*eof = 1;
return buf2 - buf;
}
void jit_tasklet_fn(unsigned long arg)
{
struct jit_data *data = (struct jit_data *)arg;
unsigned long j = jiffies;
data->buf += sprintf(data->buf, "%9li %3li %i %6i %i %s\n",
j, j - data->prevjiffies, in_interrupt() ? 1 : 0,
current->pid, smp_processor_id(), current->comm);
if (--data->loops) {
data->prevjiffies = j;
if (data->hi)
tasklet_hi_schedule(&data->tlet);//继续调用自身
else
tasklet_schedule(&data->tlet);
} else {//最后tasklet执行结束
wake_up_interruptible(&data->wait);
}
}
/* the /proc function: allocate everything to allow concurrency */
int jit_tasklet(char *buf, char **start, off_t offset,
int len, int *eof, void *arg)//使用小任务机制来延迟处理
{
struct jit_data *data;
char *buf2 = buf;
unsigned long j = jiffies;
long hi = (long)arg;
data = kmalloc(sizeof(*data), GFP_KERNEL);
if (!data)
return -ENOMEM;
init_waitqueue_head (&data->wait);
/* write the first lines in the buffer */
buf2 += sprintf(buf2, " time delta inirq pid cpu command\n");
buf2 += sprintf(buf2, "%9li %3li %i %6i %i %s\n",
j, 0L, in_interrupt() ? 1 : 0,
current->pid, smp_processor_id(), current->comm);
/* fill the data for our tasklet function */
data->prevjiffies = j;
data->buf = buf2;
data->loops = JIT_ASYNC_LOOPS;
/* register the tasklet *///注册tasklet
tasklet_init(&data->tlet, jit_tasklet_fn, (unsigned long)data);
data->hi = hi;
if (hi)
tasklet_hi_schedule(&data->tlet);//高优先级的tasklet
else
tasklet_schedule(&data->tlet);
/* wait for the buffer to fill */
wait_event_interruptible(data->wait, !data->loops);//等待循环结束
if (signal_pending(current))
return -ERESTARTSYS;
buf2 = data->buf;
kfree(data);
*eof = 1;
return buf2 - buf;
}
int __init jit_init(void)
{
create_proc_read_entry("currentime", 0, NULL, jit_currentime, NULL);//创建/proc目录下的文件并将文件和读文件的函数相关联
create_proc_read_entry("jitbusy", 0, NULL, jit_fn, (void *)JIT_BUSY);//函数最后一个参数是私有数据
create_proc_read_entry("jitsched",0, NULL, jit_fn, (void *)JIT_SCHED);
create_proc_read_entry("jitqueue",0, NULL, jit_fn, (void *)JIT_QUEUE);
create_proc_read_entry("jitschedto", 0, NULL, jit_fn, (void *)JIT_SCHEDTO);
create_proc_read_entry("jitimer", 0, NULL, jit_timer, NULL);
create_proc_read_entry("jitasklet", 0, NULL, jit_tasklet, NULL);
create_proc_read_entry("jitasklethi", 0, NULL, jit_tasklet, (void *)1);//最后的1表示使用高优先级的tasklet
return 0; /* success */
}
void __exit jit_cleanup(void)
{
remove_proc_entry("currentime", NULL);
remove_proc_entry("jitbusy", NULL);
remove_proc_entry("jitsched", NULL);
remove_proc_entry("jitqueue", NULL);
remove_proc_entry("jitschedto", NULL);
remove_proc_entry("jitimer", NULL);
remove_proc_entry("jitasklet", NULL);
remove_proc_entry("jitasklethi", NULL);
}
module_init(jit_init);
module_exit(jit_cleanup);
测试设备
#dd bs=20 count=5 < /proc/jitsched || jitqueue || jitbusy || jitschedto
内核会反复调用读设备函数，来填充用户要求的数据缓冲区。
书上说不能用cat命令读，cat每次读取4KB才返回，会使计算机冻结205s，因为内核每次调用读函数都会延迟1s，每次只读20B，那么4K则会延迟（4K/20B）=204.8s没错。
在我虚拟机上cpu空闲的情况下运行结果是：
jitbusy输出两次数据有1000+的差，jitsched则都是整整1000的差。
jitqueue和jitschedto只是偶尔有1000+i的差(0<i<5)
（驱动默认延迟1s，而1s内有1000次时钟中断，每次中断jiffies自加1）
测试其他设备
#cat /proc/jitimer || jitasklet || jitasklethi
具体细节书上有写，看书就知道了。
关于这两个函数我有点疑问
wake_up_interruptible(&data->wait);
wait_event_interruptible(data->wait, !data->loops);//等待循环结束
是先唤醒等待队列，然后判断条件是否满足，满足为真则退出等待队列，否则继续等待，是吗？