驱动基本知识

1. 用户空间与内核空间##

用户空间：应用程序运行的空间，有独立的地址映射，虚拟地址中大栈空间，发生段错误无害，

内核空间：内核程序运行的空间，有独立的地址映射，虚拟地址中小栈空间，发生段错误可能到时系统无法正常运行，

用户空间和内核空间拥有独立的内存空间，用户空间数据与内核空间数据不能直接交互，不能直接相互引用，必须使用像copy_to_user 及 copy_from_user类似的函数，将数据从一个空间copy 到另一个空间。  对内核资源的直接操作可能导致系统崩溃。 内核与用户空间的操作级别不同，拥有的资源也不同。

2. 内核中并发

概念：同一时刻对同一资源的访问
原因：1、多进程并发； 2、SMP系统，多核同时调用同一个驱动；3、中断异步；4、定时异步
所以内核代码必须是可重入的（同时发生多次调用）

3、内核符号表

一个内核模块中的函数、变量可能被其他内核模块引用时，函数、变量需要使用模块导出宏来导出符号。
宏为
EXPORT_SYMBOL(name);
EXPORT_SYMBOL_GPL(name);  只能被GPL许可证下的模块使用；
导出的符号会被内核包含在内核符号表中，此表包含了所有的全局内核项的地址，可被其他模块使用，进而实现模块层叠技术

4、在用户空间编写驱动程序

通过mmap 函数将linux 系统的物理地址映射到用户空间，用户可以直接操作寄存器。系统不能直接使用物理地址，需要通过转换。

fd = open("/dev/mem",O_RDWR);

reg_addr = mmap（NULL,0x100,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0xE0080000);

可以直接使用 reg_addr地址。

5 hello 示例

/*
* $Id: hellop.c,v 1.4 2004/09/26 07:02:43 gregkh Exp $
*/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/moduleparam.h>

MODULE_LICENSE("Dual BSD/GPL");

/*
* These lines, although not shown in the book,
* are needed to make hello.c run properly even when
* your kernel has version support enabled
*/

/*
* A couple of parameters that can be passed in: how many times we say
* hello, and to whom.
*/
static char *whom = "world";
static int howmany = 1;
module_param(howmany, int, S_IRUGO);
module_param(whom, charp, S_IRUGO);

static int __init hello_init(void)
{
int i;
for (i = 0; i < howmany; i++)
printk(KERN_ALERT "(%d) Hello, %s\n", i, whom);
return 0;
}

static void __exit hello_exit(void)
{
printk(KERN_ALERT "Goodbye, cruel world\n");
}

module_init(hello_init);
module_exit(hello_exit);

参数加载 sudo insmod hellop.ko howmany=2 whom=ldd

module_param 为模块参数加载， 只能一次改写一个

对多个参数一起加载 module_param_array

/*
*  Custom GPIO-based I2C driver
*
*  Copyright (C) 2007-2008 Gabor Juhos <juhosg@openwrt.org>
*
*  This program is free software; you can redistribute it and/or modify
*  it under the terms of the GNU General Public License version 2 as
*  published by the Free Software Foundation.
*
* ---------------------------------------------------------------------------
*
*  The behaviour of this driver can be altered by setting some parameters
*  from the insmod command line.
*
*  The following parameters are adjustable:
*
*  bus0    These four arguments can be arrays of
*  bus1    1-8 unsigned integers as follows:
*  bus2
*  bus3    <id>,<sda>,<scl>,<udelay>,<timeout>,<sda_od>,<scl_od>,<scl_oo>
*
*  where:
*
*  <id>    ID to used as device_id for the corresponding bus (required)
*  <sda>   GPIO pin ID to used for SDA (required)
*  <scl>   GPIO pin ID to used for SCL (required)
*  <udelay>    signal toggle delay.
*  <timeout>   clock stretching timeout.
*  <sda_od>    SDA is configured as open drain.
*  <scl_od>    SCL is configured as open drain.
*  <scl_oo>    SCL output drivers cannot be turned off.
*
*  See include/i2c-gpio.h for more information about the parameters.
*
*  If this driver is built into the kernel, you can use the following kernel
*  command line parameters, with the same values as the corresponding module
*  parameters listed above:
*
*  i2c-gpio-custom.bus0
*  i2c-gpio-custom.bus1
*  i2c-gpio-custom.bus2
*  i2c-gpio-custom.bus3
*/

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/platform_device.h>

#include <linux/i2c-gpio.h>

#define DRV_NAME    "i2c-gpio-custom"
#define DRV_DESC    "Custom GPIO-based I2C driver"
#define DRV_VERSION "0.1.1"

#define PFX     DRV_NAME ": "

#define BUS_PARAM_ID        0
#define BUS_PARAM_SDA       1
#define BUS_PARAM_SCL       2
#define BUS_PARAM_UDELAY    3
#define BUS_PARAM_TIMEOUT   4
#define BUS_PARAM_SDA_OD    5
#define BUS_PARAM_SCL_OD    6
#define BUS_PARAM_SCL_OO    7

#define BUS_PARAM_REQUIRED  3
#define BUS_PARAM_COUNT     8
#define BUS_COUNT_MAX       4

static unsigned int bus0[BUS_PARAM_COUNT] __initdata;
static unsigned int bus1[BUS_PARAM_COUNT] __initdata;
static unsigned int bus2[BUS_PARAM_COUNT] __initdata;
static unsigned int bus3[BUS_PARAM_COUNT] __initdata;

static unsigned int bus_nump[BUS_COUNT_MAX] __initdata;

#define BUS_PARM_DESC \
" config -> id,sda,scl[,udelay,timeout,sda_od,scl_od,scl_oo]"

**module_param_array**(bus0, uint, &bus_nump[0], 0);
MODULE_PARM_DESC(bus0, "bus0" BUS_PARM_DESC);
module_param_array(bus1, uint, &bus_nump[1], 0);
MODULE_PARM_DESC(bus1, "bus1" BUS_PARM_DESC);
module_param_array(bus2, uint, &bus_nump[2], 0);
MODULE_PARM_DESC(bus2, "bus2" BUS_PARM_DESC);
module_param_array(bus3, uint, &bus_nump[3], 0);
MODULE_PARM_DESC(bus3, "bus3" BUS_PARM_DESC);

static struct platform_device *devices[BUS_COUNT_MAX];
static unsigned int nr_devices;

static void i2c_gpio_custom_cleanup(void)
{
int i;

for (i = 0; i < nr_devices; i++)
if (devices[i])
platform_device_unregister(devices[i]);
}

static int __init i2c_gpio_custom_add_one(unsigned int id, unsigned int *params)
{
struct platform_device *pdev;
struct i2c_gpio_platform_data pdata;
int err;

if (!bus_nump[id])
return 0;

if (bus_nump[id] < BUS_PARAM_REQUIRED) {
printk(KERN_ERR PFX "not enough parameters for bus%d\n", id);
err = -EINVAL;
goto err;
}

pdev = platform_device_alloc("i2c-gpio", params[BUS_PARAM_ID]);
if (!pdev) {
err = -ENOMEM;
goto err;
}

pdata.sda_pin = params[BUS_PARAM_SDA];
pdata.scl_pin = params[BUS_PARAM_SCL];
pdata.udelay = params[BUS_PARAM_UDELAY];
pdata.timeout = params[BUS_PARAM_TIMEOUT];
pdata.sda_is_open_drain = params[BUS_PARAM_SDA_OD] != 0;
pdata.scl_is_open_drain = params[BUS_PARAM_SCL_OD] != 0;
pdata.scl_is_output_only = params[BUS_PARAM_SCL_OO] != 0;

err = platform_device_add_data(pdev, &pdata, sizeof(pdata));
if (err)
goto err_put;

err = platform_device_add(pdev);
if (err)
goto err_put;

devices[nr_devices++] = pdev;
return 0;

err_put:
platform_device_put(pdev);
err:
return err;
}

static int __init i2c_gpio_custom_probe(void)
{
int err;

printk(KERN_INFO DRV_DESC " version " DRV_VERSION "\n");

err = i2c_gpio_custom_add_one(0, bus0);
if (err)
goto err;

err = i2c_gpio_custom_add_one(1, bus1);
if (err)
goto err;

err = i2c_gpio_custom_add_one(2, bus2);
if (err)
goto err;

err = i2c_gpio_custom_add_one(3, bus3);
if (err)
goto err;

if (!nr_devices) {
printk(KERN_ERR PFX "no bus parameter(s) specified\n");
err = -ENODEV;
goto err;
}

return 0;

err:
i2c_gpio_custom_cleanup();
return err;
}

#ifdef MODULE
static int __init i2c_gpio_custom_init(void)
{
return i2c_gpio_custom_probe();
}
module_init(i2c_gpio_custom_init);

static void __exit i2c_gpio_custom_exit(void)
{
i2c_gpio_custom_cleanup();
}
module_exit(i2c_gpio_custom_exit);
#else
subsys_initcall(i2c_gpio_custom_probe);
#endif /* MODULE*/

MODULE_LICENSE("GPL v2");
MODULE_AUTHOR("Gabor Juhos <juhosg@openwrt.org >");
MODULE_DESCRIPTION(DRV_DESC);
MODULE_VERSION(DRV_VERSION);

参数加载方式：sudo insmod i2c-gpio-custom.ko bus0=0,1,2