linux内核开发第6讲：应用层的write怎么样调用到驱动里的write

1.应用层code

#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/select.h>

#define DATA_NUM (64)

int main() {
    int fd, i;
    int r_len, w_len;
    fd_set fdset;

    char buf[DATA_NUM] = "hello world";
    memset(buf, 0, DATA_NUM);
    fd = open("/dev/hello", O_RDWR);
    printf("%d\n", fd);
    if (fd == -1) {
        printf("open file error\n");
        return -1;
    }
    else {
        printf("open success\n");
    }

    w_len = write(fd, buf, DATA_NUM);
    r_len = read(fd, buf, DATA_NUM);
    printf("w_len %d r_len %d\n", w_len, r_len);
    return 0;
}

2.设备文件

代码中的open的“/dev/hello”是个设备文件

zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ ls -lh /dev/hello
ls: 无法访问'/dev/hello': 没有那个文件或目录

现在还没有创建这个设备文件
大部分驱动可以自动创建设备文件
也可以手动的创建设备文件

可以使用mknod命令创建设备文件

mknod [OPTION]... NAME TYPE [MAJOR MINOR]

zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ sudo mknod /dev/hello c 232 0
[sudo] zhaoxr 的密码：
zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ ls -lh /dev/hello
crw-r--r-- 1 root root 232, 0 4月  24 22:39 /dev/hello

查看驱动 lsmod 
lsmod查看的是insmod的.ko文件
这个.ko文件的名字来自makefile文件的驱动名称

cat /proc/devices
查看列出字符和块设备的主设备号，以及分配到这些设备号的设备名称。
这个设备名称来自register_chrdev_region的第三个参数

3.应用层程序调用驱动过程

3.1 编写驱动helloDev.c

#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/cdev.h>
#include <linux/fs.h>
#include <linux/wait.h>
#include <linux/poll.h>
#include <linux/sched.h>
#include <linux/slab.h>

#define BUFFER_MAX (10)
#define OK (0)
#define ERROR (-1)

struct cdev *gDev = NULL;
struct file_operations *gFile = NULL;
dev_t devNum;
unsigned int subDevNum = 1;
int reg_major= 232;
int reg_minor = 0;
char *buffer = NULL;
int flag = 0;

int hello_open(struct inode *p, struct file *f) {
    printk(KERN_EMERG "hello_open\r\n");
    return 0;
}

ssize_t hello_write(struct file *f, const char __user *u, size_t s, loff_t *l) {
    printk(KERN_EMERG "hello_write\r\n");
    return 0;
}

ssize_t hello_read(struct file *f, char __user *u, size_t s, loff_t *l) {
    printk(KERN_EMERG "hello_read\r\n");
    return 0;
}

int hello_init(void) {
    devNum = MKDEV(reg_major, reg_minor);
    if (OK == register_chrdev_region(devNum, subDevNum, "helloworld")) {
        printk(KERN_EMERG "register_chrdev_region ok\n");
    }
    else {
        printk(KERN_EMERG "register_chrdev_region error\n");
        return ERROR;
    }
    printk(KERN_EMERG "hello driver init\n");
    gDev = kzalloc(sizeof(struct cdev), GFP_KERNEL);
    gFile = kzalloc(sizeof(struct file_operations), GFP_KERNEL);
    gFile->open = hello_open;
    gFile->read = hello_read;
    gFile->write = hello_write;
    gFile->owner = THIS_MODULE;
    cdev_init(gDev, gFile);
    cdev_add(gDev, devNum, 3);
    return 0;
}

void __exit hello_exit(void) {
    cdev_del(gDev);
    unregister_chrdev_region(devNum, subDevNum);
    return;
}

module_init(hello_init);
module_exit(hello_exit);
MODULE_LICENSE("GPL");

3.2 编写驱动的makefile

ifneq ($(KERNELRELEASE),)
obj-m := helloDev.o
else

PWD := $(shell pwd)
$(info "$(PWD)")
KDIR := /lib/modules/$(shell uname -r)/build
$(info "$(KDIR)")
all:
	make -C $(KDIR) M=$(PWD)
clean:
	rm -rf *.o *.ko *.mod.c *.symvers *.c~ *~ *.mod *.order
endif

3.3 编写应用层程序main.c

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


#define DATA_NUM (64)

int main() {
    int fd, i;
    int r_len, w_len;
    fd_set fdset;

    char buf[DATA_NUM] = "hello world";
    memset(buf, 0, DATA_NUM);
    fd = open("/dev/hello", O_RDWR);
    printf("%d\n", fd);
    if (fd == -1) {
        printf("open file error\n");
        return -1;
    }
    else {
        printf("open success\n");
    }

    w_len = write(fd, buf, DATA_NUM);
    r_len = read(fd, buf, DATA_NUM);
    printf("w_len %d r_len %d\n", w_len, r_len);
    return 0;
}

3.4 编写应用层makefile

out : main.o
	gcc -o out main.o
main.o : main.c
	gcc -c main.c

clean :
	rm -rf *.o out

3.5编译驱动和加载驱动

zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ make
"/home/zhaoxr/xiangrui.zhao/linux_kernel"
"/lib/modules/5.15.0-69-generic/build"
make -C /lib/modules/5.15.0-69-generic/build M=/home/zhaoxr/xiangrui.zhao/linux_kernel
make[1]: 进入目录“/usr/src/linux-headers-5.15.0-69-generic”
  CC [M]  /home/zhaoxr/xiangrui.zhao/linux_kernel/helloDev.o
  MODPOST /home/zhaoxr/xiangrui.zhao/linux_kernel/Module.symvers
  CC [M]  /home/zhaoxr/xiangrui.zhao/linux_kernel/helloDev.mod.o
  LD [M]  /home/zhaoxr/xiangrui.zhao/linux_kernel/helloDev.ko
  BTF [M] /home/zhaoxr/xiangrui.zhao/linux_kernel/helloDev.ko
Skipping BTF generation for /home/zhaoxr/xiangrui.zhao/linux_kernel/helloDev.ko due to unavailability of vmlinux
make[1]: 离开目录“/usr/src/linux-headers-5.15.0-69-generic”
zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ ls
helloDev.c  helloDev.ko  helloDev.mod  helloDev.mod.c  helloDev.mod.o  helloDev.o  Makefile  modules.order  Module.symvers  user
zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ sudo insmod helloDev.ko

Message from syslogd@zhaoxr-ThinkPad-E450 at Apr 25 22:47:40 ...
 kernel:[169201.833551] register_chrdev_region ok

Message from syslogd@zhaoxr-ThinkPad-E450 at Apr 25 22:47:40 ...
 kernel:[169201.833556] hello driver init
zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ lsmod
Module                  Size  Used by
helloDev               16384  0
rfcomm                 81920  4
ccm                    20480  6

3.6 创建驱动对应的设备文件

利用主次设备号，创建驱动对应的设备文件，并设置权限，以方便应用层访问

zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ sudo mknod /dev/hello c 232 0
zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ ls -lh /dev/hello
crw-r--r-- 1 root root 232, 0 4月  25 22:52 /dev/hello
zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ sudo chmod 777 /dev/hello
zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel$ ls -lh /dev/hello
crwxrwxrwx 1 root root 232, 0 4月  25 22:52 /dev/hello

3.7 编译应用层程序并执行

zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel/user$ make
gcc -c main.c
main.c: In function ‘main’:
main.c:28:13: warning: implicit declaration of function ‘write’; did you mean ‘fwrite’? [-Wimplicit-function-declaration]
   28 |     w_len = write(fd, buf, DATA_NUM);
      |             ^~~~~
      |             fwrite
main.c:29:13: warning: implicit declaration of function ‘read’; did you mean ‘fread’? [-Wimplicit-function-declaration]
   29 |     r_len = read(fd, buf, DATA_NUM);
      |             ^~~~
      |             fread
gcc -o out main.o
zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel/user$ ls
main.c  main.o  Makefile  out
zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel/user$ ./out
3
open success
w_len 0 r_len 0
zhaoxr@zhaoxr-ThinkPad-E450:~/xiangrui.zhao/linux_kernel/user$
Message from syslogd@zhaoxr-ThinkPad-E450 at Apr 25 22:54:55 ...
 kernel:[169637.294848] hello_open

Message from syslogd@zhaoxr-ThinkPad-E450 at Apr 25 22:54:55 ...
 kernel:[169637.295049] hello_write

Message from syslogd@zhaoxr-ThinkPad-E450 at Apr 25 22:54:55 ...
 kernel:[169637.295056] hello_read

4.write的调用过程

应用层调用write，首先会调用C库函数，然后通过系统调用进入到内核里面。
内核中关于write的系统调用
在linux源代码的fs目录下有read_write.c文件

首先是SYSCALL_DEFINE

SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
		size_t, count)
{
	struct fd f = fdget_pos(fd);
	ssize_t ret = -EBADF;

	if (f.file) {
		loff_t pos = file_pos_read(f.file);
		ret = vfs_write(f.file, buf, count, &pos);
		if (ret >= 0)
			file_pos_write(f.file, pos);
		fdput_pos(f);
	}

	return ret;
}

然后是vfs_write

ssize_t vfs_write(struct file *file, const char __user *buf, size_t count, loff_t *pos)
{
	ssize_t ret;

	if (!(file->f_mode & FMODE_WRITE))
		return -EBADF;
	if (!(file->f_mode & FMODE_CAN_WRITE))
		return -EINVAL;
	if (unlikely(!access_ok(VERIFY_READ, buf, count)))
		return -EFAULT;

	ret = rw_verify_area(WRITE, file, pos, count);
	if (!ret) {
		if (count > MAX_RW_COUNT)
			count =  MAX_RW_COUNT;
		file_start_write(file);
		ret = __vfs_write(file, buf, count, pos);
		if (ret > 0) {
			fsnotify_modify(file);
			add_wchar(current, ret);
		}
		inc_syscw(current);
		file_end_write(file);
	}

	return ret;
}

然后是__vfs_write

ssize_t __vfs_write(struct file *file, const char __user *p, size_t count,
		    loff_t *pos)
{
	if (file->f_op->write)
		return file->f_op->write(file, p, count, pos);
	else if (file->f_op->write_iter)
		return new_sync_write(file, p, count, pos);
	else
		return -EINVAL;
}

由于fd对应的file->f_op->write存在，因此会调用到我们自己驱动编写的hello_write

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