本文详细介绍了Linux内核中的sysfs文件系统接口及DEVICE_ATTR宏的使用方法。通过具体示例，展示了如何创建sysfs接口并实现读写功能。
说道sysfs接口，就不得不提到函数宏 DEVICE_ATTR
原型是#define DEVICE_ATTR(_name, _mode, _show, _store)
struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store)
函数宏DEVICE_ATTR内封装的是__ATTR(_name,_mode,_show,_stroe)方法，_show表示的是读方法，_stroe表示的是写方法。
当然_ATTR不是独生子女，他还有一系列的姊妹__ATTR_RO宏只有读方法，__ATTR_NULL等等
如对设备的使用 DEVICE_ATTR ，对总线使用 BUS_ATTR ，对驱动使用 DRIVER_ATTR ，对类别 (class) 使用 CLASS_ATTR, 这四个高级的宏来自于<include/linux/device.h>
DEVICE_ATTR 宏声明有四个参数，分别是名称、权限位、读函数、写函数。其中读函数和写函数是读写功能函数的函数名。
1、如果你完成了DEVICE_ATTR函数宏的填充，下面就需要创建接口了
例如：
staticDEVICE_ATTR(polling,
S_IRUGO | S_IWUSR, show_polling, set_polling);
static struct attribute *dev_attrs[] = {
&dev_attr_polling.attr,
NULL,
};
eg:staticDEVICE_ATTR(proximity_poll_delay, S_IRUGO | S_IWUGO, proximity_poll_delay_show, proximity_poll_delay_store);
static struct device_attribute dev_attr_proximity_enable =
__ATTR(enable, S_IRUGO | S_IWUGO, proximity_enable_show, proximity_enable_store);
当你想要实现的接口名字是polling的时候，需要实现结构体struct attribute *dev_attrs[]
其中成员变量的名字必须是&dev_attr_polling.attr
eg: static struct attribute *proximity_sysfs_attrs[] = {
&dev_attr_proximity_enable.attr,
&dev_attr_proximity_poll_delay.attr,
2、然后再封装
static structattribute_group
dev_attr_grp = {
.attrs = dev_attrs,
};
eg:static struct attribute_group proximity_attribute_group = {
.attrs = proximity_sysfs_attrs,
通过以上简单的三个步骤，就可以在adb shell 终端查看到接口了。当我们将数据 echo 到接口中时，在上层实际上完成了一次 write 操作，对应到 kernel ，调用了驱动中的 “store”。同理，当我们cat 一个 接口时则会调用 “show” 。到这里，只是简单的建立了 android 层到 kernel 的桥梁，真正实现对硬件操作的，还是在 "show" 和 "store" 中完成的。
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以下来个例子：
/*
* Sample kobject implementation
*
* Copyright (C) 2004-2007 Greg Kroah-Hartman <greg@kroah.com>
* Copyright (C) 2007 Novell Inc.
*
* Released under the GPL version 2 only.
*
*/
#include <linux/kobject.h>
#include <linux/string.h>
#include <linux/sysfs.h>
#include <linux/module.h>
#include <linux/init.h>
#include <asm/gpio.h>
#include <linux/delay.h>
/*
* This module shows how to create a simple subdirectory in sysfs called
* /sys/kernel/kobject-example In that directory, 3 files are created:
* "foo", "baz", and "bar". If an integer is written to these files, it can be
* later read out of it.
*/
static int foo;
/*
* The "foo" file where a static variable is read from and written to.
*/
static struct msm_gpio qup_i2c_gpios_io[] = {
{ GPIO_CFG(60, 0, GPIO_CFG_OUTPUT, GPIO_CFG_NO_PULL, GPIO_CFG_8MA),
"qup_scl" },
{ GPIO_CFG(61, 0, GPIO_CFG_INPUT, GPIO_CFG_NO_PULL, GPIO_CFG_8MA),
"qup_sda" },
{ GPIO_CFG(131, 0, GPIO_CFG_OUTPUT, GPIO_CFG_NO_PULL, GPIO_CFG_8MA),
"qup_scl" },
{ GPIO_CFG(132, 0, GPIO_CFG_INPUT, GPIO_CFG_NO_PULL, GPIO_CFG_8MA),
"qup_sda" },
};
static struct msm_gpio qup_i2c_gpios_hw[] = {
{ GPIO_CFG(60, 1, GPIO_CFG_INPUT, GPIO_CFG_NO_PULL, GPIO_CFG_8MA),
"qup_scl" },
{ GPIO_CFG(61, 1, GPIO_CFG_INPUT, GPIO_CFG_NO_PULL, GPIO_CFG_8MA),
"qup_sda" },
{ GPIO_CFG(131, 2, GPIO_CFG_INPUT, GPIO_CFG_NO_PULL, GPIO_CFG_8MA),
"qup_scl" },
{ GPIO_CFG(132, 2, GPIO_CFG_INPUT, GPIO_CFG_NO_PULL, GPIO_CFG_8MA),
"qup_sda" },
};
static void gsbi_qup_i2c_gpio_config(int adap_id, int config_type)
{
int rc;
if (adap_id < 0 || adap_id > 1)
return;
/* Each adapter gets 2 lines from the table */
if (config_type)
rc = msm_gpios_enable(&qup_i2c_gpios_hw[adap_id*2], 2);
else
rc = msm_gpios_enable(&qup_i2c_gpios_io[adap_id*2], 2);
if (rc < 0)
pr_err("QUP GPIO request/enable failed: %d\n", rc);
}
static ssize_t foo_show(struct kobject *kobj, struct kobj_attribute *attr,
char *buf)
{
return sprintf(buf, "%d\n", foo);
}
static ssize_t foo_store(struct kobject *kobj, struct kobj_attribute *attr,
const char *buf, size_t count)
{
int sda,scl;
int i;
sscanf(buf, "%du", &foo);
printk("foo = %d.\n",foo);
foo = foo -1;
if (foo < 0 || foo > 1)
{
printk("input foo error. foo=%d.\n",foo);
return 0;
}
sda = GPIO_PIN((&qup_i2c_gpios_hw[foo*2+1])->gpio_cfg);
scl = GPIO_PIN((&qup_i2c_gpios_hw[foo*2])->gpio_cfg);
printk("sda = %d.\n",sda);
printk("scl = %d.\n",scl);
gsbi_qup_i2c_gpio_config(foo,0);
for(i=0;i<9;i++)
{
if(gpio_get_value(sda))
{
printk("sda of i2c%d is high when %d pulse is output on scl.\n",foo,i);
break;
}
gpio_set_value(scl,0);
udelay(5);
gpio_set_value(scl,1);
udelay(5);
}
gsbi_qup_i2c_gpio_config(foo,1);
printk("finish.\n");
return count;
}
static struct kobj_attribute foo_attribute =
__ATTR(i2c_unlock, 0666, foo_show, foo_store);
/*
* Create a group of attributes so that we can create and destroy them all
* at once.
*/
static struct attribute *attrs[] = {
&foo_attribute.attr,
NULL,» /* need to NULL terminate the list of attributes */
};
/*
* An unnamed attribute group will put all of the attributes directly in
* the kobject directory. If we specify a name, a subdirectory will be
* created for the attributes with the directory being the name of the
* attribute group.
*/
static struct attribute_group attr_group = {
.attrs = attrs,
};
static struct kobject *example_kobj;
static int __init example_init(void)
{
int retval;
/*
* Create a simple kobject with the name of "kobject_example",
* located under /sys/kernel/
*
* As this is a simple directory, no uevent will be sent to
* userspace. That is why this function should not be used for
* any type of dynamic kobjects, where the name and number are
* not known ahead of time.
*/
example_kobj = kobject_create_and_add("kobject_example", kernel_kobj);
if (!example_kobj)
return -ENOMEM;
/* Create the files associated with this kobject */
retval = sysfs_create_group(example_kobj, &attr_group);
if (retval)
kobject_put(example_kobj);
return retval;
}
static void __exit example_exit(void)
{
kobject_put(example_kobj);
}
module_init(example_init);
module_exit(example_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Wupeng");
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