驱动程序包含以下类型的功能函数：drv_open（打开设备），drv_read（读），drv_write（写），drv_ioctl（读/写）等。
驱动程序包括字符设备驱动和块设备驱动。
如何编写驱动程序：
- 确定主设备号，或者让系统分配
- 定义file_operations结构体
- 实现对应的drv_open/drv_read/drv_write等函数，填入file_operations结构体
- 在内核注册驱动（register_chrdev）
- 入口函数，在安装驱动（insmod xxx.ko）时调用
- 出口函数，在卸载驱动（rmmod xxx.ko）程序时调用
- 在内核解除注册
- 其他内容，比如设备信息，自动创建设备节点：class_create，device_create
驱动程序架构
- 用户层（APP层）：在APP里面使用驱动程序进行相关的驱动操作，使用open打开设备，使用write写值，比如LED驱动，我们需要在APP里面调用LED的on（亮）和off（灭）方法
- 驱动库层：编写某一个驱动，有时候需要适配很多机型或者芯片主板，我们需要建立一个公共的驱动库，上层提供给APP调用，底层基于不同单板的代码进行自动适配
- 硬件层：基于不同的单板编写针对性的代码，通过芯片手册找到需要操作的寄存器并编写相关操作功能
做驱动开发的准备工作
初识linux驱动，可以从linux内核源代码入手，源代码中提供可以参考的众多驱动程序代码。
前往下载linux源代码 https://www.kernel.org/
下载之后在windows上解压
下载源码阅读工具Source Insight https://www.sourceinsight.com/
安装Source Insight并且新建项目，添加linux内核源代码。具体步骤参考：https://blog.csdn.net/ceasadan/article/details/52120825
可参考的驱动源码misc.c内容如下：
/*
* linux/drivers/char/misc.c
*
* Generic misc open routine by Johan Myreen
*
* Based on code from Linus
*
* Teemu Rantanen's Microsoft Busmouse support and Derrick Cole's
* changes incorporated into 0.97pl4
* by Peter Cervasio (pete%q106fm.uucp@wupost.wustl.edu) (08SEP92)
* See busmouse.c for particulars.
*
* Made things a lot mode modular - easy to compile in just one or two
* of the misc drivers, as they are now completely independent. Linus.
*
* Support for loadable modules. 8-Sep-95 Philip Blundell <pjb27@cam.ac.uk>
*
* Fixed a failing symbol register to free the device registration
* Alan Cox <alan@lxorguk.ukuu.org.uk> 21-Jan-96
*
* Dynamic minors and /proc/mice by Alessandro Rubini. 26-Mar-96
*
* Renamed to misc and miscdevice to be more accurate. Alan Cox 26-Mar-96
*
* Handling of mouse minor numbers for kerneld:
* Idea by Jacques Gelinas <jack@solucorp.qc.ca>,
* adapted by Bjorn Ekwall <bj0rn@blox.se>
* corrected by Alan Cox <alan@lxorguk.ukuu.org.uk>
*
* Changes for kmod (from kerneld):
* Cyrus Durgin <cider@speakeasy.org>
*
* Added devfs support. Richard Gooch <rgooch@atnf.csiro.au> 10-Jan-1998
*/
#include <linux/module.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>
/*
* Head entry for the doubly linked miscdevice list
*/
static LIST_HEAD(misc_list);
static DEFINE_MUTEX(misc_mtx);
/*
* Assigned numbers, used for dynamic minors
*/
#define DYNAMIC_MINORS 64 /* like dynamic majors */
static DECLARE_BITMAP(misc_minors, DYNAMIC_MINORS);
#ifdef CONFIG_PROC_FS
static void *misc_seq_start(struct seq_file *seq, loff_t *pos)
{
mutex_lock(&misc_mtx);
return seq_list_start(&misc_list, *pos);
}
static void *misc_seq_next(struct seq_file *seq, void *v, loff_t *pos)
{
return seq_list_next(v, &misc_list, pos);
}
static void misc_seq_stop(struct seq_file *seq, void *v)
{
mutex_unlock(&misc_mtx);
}
static int misc_seq_show(struct seq_file *seq, void *v)
{
const struct miscdevice *p = list_entry(v, struct miscdevice, list);
seq_printf(seq, "%3i %s\n", p->minor, p->name ? p->name : "");
return 0;
}
static const struct seq_operations misc_seq_ops = {
.start = misc_seq_start,
.next = misc_seq_next,
.stop = misc_seq_stop,
.show = misc_seq_show,
};
static int misc_seq_open(struct inode *inode, struct file *file)
{
return seq_open(file, &misc_seq_ops);
}
static const struct file_operations misc_proc_fops = {
.owner = THIS_MODULE,
.open = misc_seq_open,
.read = seq_read,
.llseek = seq_lseek,
.release = seq_release,
};
#endif
static int misc_open(struct inode * inode, struct file * file)
{
int minor = iminor(inode);
struct miscdevice *c;
int err = -ENODEV;
const struct file_operations *new_fops = NULL;
mutex_lock(&misc_mtx);
list_for_each_entry(c, &misc_list, list) {
if (c->minor == minor) {
new_fops = fops_get(c->fops);
break;
}
}
if (!new_fops) {
mutex_unlock(&misc_mtx);
request_module("char-major-%d-%d", MISC_MAJOR, minor);
mutex_lock(&misc_mtx);
list_for_each_entry(c, &misc_list, list) {
if (c->minor == minor) {
new_fops = fops_get(c->fops);
break;
}
}
if (!new_fops)
goto fail;
}
/*
* Place the miscdevice in the file's
* private_data so it can be used by the
* file operations, including f_op->open below
*/
file->private_data = c;
err = 0;
replace_fops(file, new_fops);
if (file->f_op->open)
err = file->f_op->open(inode,file);
fail:
mutex_unlock(&misc_mtx);
return err;
}
static struct class *misc_class;
static const struct file_operations misc_fops = {
.owner = THIS_MODULE,
.open = misc_open,
.llseek = noop_llseek,
};
/**
* misc_register - register a miscellaneous device
* @misc: device structure
*
* Register a miscellaneous device with the kernel. If the minor
* number is set to %MISC_DYNAMIC_MINOR a minor number is assigned
* and placed in the minor field of the structure. For other cases
* the minor number requested is used.
*
* The structure passed is linked into the kernel and may not be
* destroyed until it has been unregistered. By default, an open()
* syscall to the device sets file->private_data to point to the
* structure. Drivers don't need open in fops for this.
*
* A zero is returned on success and a negative errno code for
* failure.
*/
int misc_register(struct miscdevice * misc)
{
dev_t dev;
int err = 0;
bool is_dynamic = (misc->minor == MISC_DYNAMIC_MINOR);
INIT_LIST_HEAD(&misc->list);
mutex_lock(&misc_mtx);
if (is_dynamic) {
int i = find_first_zero_bit(misc_minors, DYNAMIC_MINORS);
if (i >= DYNAMIC_MINORS) {
err = -EBUSY;
goto out;
}
misc->minor = DYNAMIC_MINORS - i - 1;
set_bit(i, misc_minors);
} else {
struct miscdevice *c;
list_for_each_entry(c, &misc_list, list) {
if (c->minor == misc->minor) {
err = -EBUSY;
goto out;
}
}
}
dev = MKDEV(MISC_MAJOR, misc->minor);
misc->this_device =
device_create_with_groups(misc_class, misc->parent, dev,
misc, misc->groups, "%s", misc->name);
if (IS_ERR(misc->this_device)) {
if (is_dynamic) {
int i = DYNAMIC_MINORS - misc->minor - 1;
if (i < DYNAMIC_MINORS && i >= 0)
clear_bit(i, misc_minors);
misc->minor = MISC_DYNAMIC_MINOR;
}
err = PTR_ERR(misc->this_device);
goto out;
}
/*
* Add it to the front, so that later devices can "override"
* earlier defaults
*/
list_add(&misc->list, &misc_list);
out:
mutex_unlock(&misc_mtx);
return err;
}
/**
* misc_deregister - unregister a miscellaneous device
* @misc: device to unregister
*
* Unregister a miscellaneous device that was previously
* successfully registered with misc_register().
*/
void misc_deregister(struct miscdevice *misc)
{
int i = DYNAMIC_MINORS - misc->minor - 1;
if (WARN_ON(list_empty(&misc->list)))
return;
mutex_lock(&misc_mtx);
list_del(&misc->list);
device_destroy(misc_class, MKDEV(MISC_MAJOR, misc->minor));
if (i < DYNAMIC_MINORS && i >= 0)
clear_bit(i, misc_minors);
mutex_unlock(&misc_mtx);
}
EXPORT_SYMBOL(misc_register);
EXPORT_SYMBOL(misc_deregister);
static char *misc_devnode(struct device *dev, umode_t *mode)
{
struct miscdevice *c = dev_get_drvdata(dev);
if (mode && c->mode)
*mode = c->mode;
if (c->nodename)
return kstrdup(c->nodename, GFP_KERNEL);
return NULL;
}
static int __init misc_init(void)
{
int err;
struct proc_dir_entry *ret;
ret = proc_create("misc", 0, NULL, &misc_proc_fops);
misc_class = class_create(THIS_MODULE, "misc");
err = PTR_ERR(misc_class);
if (IS_ERR(misc_class))
goto fail_remove;
err = -EIO;
if (register_chrdev(MISC_MAJOR,"misc",&misc_fops))
goto fail_printk;
misc_class->devnode = misc_devnode;
return 0;
fail_printk:
printk("unable to get major %d for misc devices\n", MISC_MAJOR);
class_destroy(misc_class);
fail_remove:
if (ret)
remove_proc_entry("misc", NULL);
return err;
}
subsys_initcall(misc_init);
下面创建一个字符设备驱动hello_drv为例
hello_drv.c:
#include <linux/module.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>
/*1. 确定主设备号，或者让系统分配*/
static int major = 0;
static char kernel_buf[1024];
static struct class *hello_class;
#define MIN(a,b) (a < b ? a : b)
/*3. 实现对应的drv_open/drv_read/drv_write等函数，填入file_operations结构体*/
static ssize_t hello_drv_read(struct file *file, char __user *buf, size_t size, loff_t *offset)
{
int err;
printk("%s %s line %d\n", __FILE__, __FUNCTION__, __LINE__);
err = copy_to_user(buf, kernel_buf, MIN(1024, size));
return MIN(1024, size);
}
static ssize_t hello_drv_write(struct file *file, const char __user *buf, size_t size, loff_t *offset)
{
int err;
printk("%s %s line %d\n", __FILE__, __FUNCTION__, __LINE__);
err = copy_from_user(kernel_buf, buf, MIN(1024, size));
return MIN(1024, size);
}
static int hello_drv_open(struct inode *node, struct file *file)
{
printk("%s %s line %d", __FILE__, __FUNCTION__, __LINE__);
return 0;
}
static int hello_drv_close(struct inode *node, struct file *file)
{
printk("%s %s line %d", __FILE__, __FUNCTION__, __LINE__);
return 0;
}
/*2. 定义file_operations结构体*/
static struct file_operations hello_drv =
{
.owner = THIS_MODULE,
.open = hello_drv_open,
.write = hello_drv_write,
.read = hello_drv_read,
.release = hello_drv_close,
};
/*4. 在内核注册驱动（register_chrdev）*/
/*5. 入口函数，在安装驱动（insmod xxx.ko）时调用*/
static int __init hello_init(void)
{
/*6. 出口函数，在卸载驱动（rmmod xxx.ko）程序时调用*/
int err;
printk("%s %s line %d", __FILE__, __FUNCTION__, __LINE__);
major = register_chrdev(0, "huanghai_hello", &hello_drv);
hello_class = class_create(THIS_MODULE, "hello_class");
err = PTR_ERR(hello_class);
if (IS_ERR(hello_class))
{
printk("%s %s line %d", __FILE__, __FUNCTION__, __LINE__);
unregister_chrdev(0, "huanghai_hello");
return -1;
}
device_create(hello_class, NULL, MKDEV(major, 0), NULL, "huanghai_hello");
return 0;
}
static void __exit hello_exit(void)
{
/*7. 在内核解除注册*/
printk("%s %s line %d", __FILE__, __FUNCTION__, __LINE__);
device_destroy(hello_class, MKDEV(major, 0));
class_destroy(hello_class);
unregister_chrdev(0, "huanghai_hello");
}
/*8. 其他内容，比如设备信息，自动创建设备节点：class_create，device_create*/
module_init(hello_init);
module_exit(hello_exit);
MODULE_LICENSE("GPL");
hello_drv_test.c:
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
/*
./hello_drv_test -w abc
./hello_drv_test -r
*/
int main(int argc, char **argv)
{
int fd;
char buf[1024];
int len;
/*1.判断参数*/
if(argc < 2)
{
printf("Usage: %s -w <string>\n",argv[0]);
printf(" %s -r\n",argv[0]);
return -1;
}
/*2.打开文件*/
fd = open("/dev/huanghai_hello", O_RDWR);
if(fd == -1)
{
printf("can not open file /dev/huanghai_hello\n");
}
/*3.写文件或读文件*/
if((0 == strcmp(argv[1], "-w")) && (argc == 3))
{
len = strlen(argv[2]) + 1;
len = len < 1024 ? len : 1024;
write(fd, argv[2], len);
}
else
{
len = read(fd,buf,1024);
buf[1023] = '\0';
printf("APP READ : %s\n", buf);
}
close(fd);
}
Makefile:
# 1. 使用不同的开发板内核时，一定要修改KERN_DIR
# 2. KERN_DIR中的内核要事先配置、编译，为了能编译内核，要先设置下列环境变量
# 2.1 ARCH， 比如：export ARCH=arm64
# 2.2 CROSS_COMPILE, 比如： export CROSS_COMPILE=aarch64-linux-gnu-
# 2.3 PATH,比如：export PATH=$PATH:/home/book/100ask_rocr-k3399-pc/ToolChain-6.3.1/gcc-linaro-6.3.1-2017.05-x86_64_aarch64-linux-gnu/bin
# 注意：不同的开发板不同的编译器上述3个环境变量不一定相同，请参考各开发板的高级用户使用手册
#KERN_DIR = /home/book/work/linux-4.4.210
KERN_DIR = /usr/src/linux-headers-4.18.0-15
all:
make -C $(KERN_DIR) M=`pwd` modules
$(CROSS_COMPILE)gcc -o hello_drv_test hello_drv_test.c
clean:
make -C $(KERN_DIR) M=`pwd` modules clean
rm -rf modules.order
rm -f hello_drv_test
obj-m += hello_drv.o
配置Makefile环境变量，确认linux内核目录并且编译过内核。
将以上3个文件拷贝到ubuntu中进行编译
编译完成使用insmod hello_drv.ko进行安装驱动
使用./hello_drv_test -w "hello my first driver" 进行写入测试
使用./hello_drv_test -r 进行读测试
参考链接
Firefly论坛-韦东山老师嵌入式Linux教学视频汇总：http://dev.t-firefly.com/thread-100207-1-1.html
百问网-嵌入式Linux应用开发完全手册第 2 版：https://book.100ask.net/
百问网-嵌入式初学者学习路线：http://wiki.100ask.org/BeginnerLearningRoute
