前言
上一篇的學習中介紹了如何在用戶空間直接操作GPIO，並寫了一個腳本可以產生PWM。本篇的學習會將寫一個驅動操作GPIO，同樣的也可以發生PWM，因此這里還需要部分的硬件配合，需要一塊開發板，當然可能還需要一台示波器。
原理圖
和上一篇相同，引腳依然是GPIO3_D0，具體硬件肯定會不同，注意參考soc的datasheet和硬件原理圖，先定位正確需要操作的GPIO。
IO模擬輸出PWM
這里驅動實現的方式是先創建一個內核線程，如何創建內核線程可以參考Linux內核驅動學習（五）KThread學習總結，然后在線程函數一直循環反轉IO口的輸出。這里的目的單純是為了學習操作GPIO，不建議項目中通過這種IO口模擬的方式去實現PWM的輸出，而應該直接使用自帶PWM功能的引腳。
設備樹
gpio-demo {
compatible = "gpio-demo";
gpios = <&gpio3 0 GPIO_ACTIVE_LOW>;
};
驅動源碼中通過of_get_gpio接口去解析gpio。
驅動端
驅動源碼中of_device_id結構體變量中的成員.compatible的值必須和設備樹的設備節點兼容屬性compatible的值相同；
static struct of_device_id gpio_demo_of_match[] = {
{ .compatible = "gpio-demo"},
{},
}
MODULE_DEVICE_TABLE(of,gpio_demo_of_match);
static struct platform_driver gpio_demo_driver = {
.probe = gpio_demo_probe,
.driver = {
.name = "gpio-demo-device",
.owner = THIS_MODULE,
.of_match_table = of_match_ptr(gpio_demo_of_match),
}
};
在probe函數實現對設備樹節點的解析，of_get_gpio對應gpio-demo節點下的gpios屬性；
然后ret = devm_gpio_request_one(dev, gpio, GPIOF_DIR_OUT, pdev->name)語句初始化GPIO為輸出引腳；
static int gpio_demo_probe(struct platform_device *pdev){
int ret,i;
struct device *dev = &pdev->dev;
struct device_node *node = dev->of_node;
if (!node)
return -EINVAL;
ret = of_gpio_count(node);
if (ret == 0){
return -EINVAL;
}
priv = devm_kzalloc(dev, sizeof(*priv) + sizeof(int) * ret, GFP_KERNEL);
if (!priv){
return -ENOMEM;
}
priv->count = ret;
mutex_init(&priv->mtx);
for (i = 0; i < priv->count; i++) {
unsigned int gpio;
gpio = of_get_gpio(node, i);
if (gpio < 0) {
dev_warn(dev, "Unable to get gpio #%d\n", i);
continue;
}
ret = devm_gpio_request_one(dev, gpio, GPIOF_DIR_OUT, pdev->name);
priv->gpio[i] = gpio;
if (ret < 0) {
dev_warn(dev, "Unable to re quest GPIO %d: %d\n",
gpio, ret);
continue;
}
printk(KERN_INFO "success request gpio %d\n",gpio);
gpio_direction_output(gpio, 1); //設置輸出的電平
}
return 0;
}
線程執行函數中通過gpio_set_value設置GPIO的輸出值，然后休眠50毫秒，最終PWM的周期應該是100毫秒左右。
static int thread_func(void *data) {
int i, count;
while (1){
count++;
mutex_lock(&priv->mtx);
for ( i = 0; i < priv->count; i++){
gpio_set_value(priv->gpio[i], count%2);
}
mutex_unlock(&priv->mtx);
msleep(50);
printk(KERN_INFO "thread count %d\n", count);
}
return 0;
}
gpio_set_value和gpio_direction_output的區別
如果使用該GPIO時，不會動態地切換輸入輸出，建議在開始時就設置好GPIO 輸出方向，后面拉高拉低時使用gpio_set_value()接口，而不建議使用gpio_direction_output(), 因為gpio_direction_output接口里面有mutex鎖，對中斷上下文調用會有錯誤異常，且相比gpio_set_value，gpio_direction_output所做事情更多，浪費。
調試信息
先通過debugfs查看相應的GPIO已經成功加載到內核了；但是我們目前沒有留用戶層調用的接口，這個有悖於我們的初衷，但是目前為止已經實現了自己想要的效果。
實驗結果
附錄
#include <linux/module.h>
#include <linux/init.h>
#include <linux/platform_device.h>
//API for libgpio
#include <linux/gpio.h>
//API for malloc
#include <linux/slab.h>
//API for device tree
#include <linux/of_platform.h>
#include <linux/of_gpio.h>
#include <linux/of_device.h>
//API for thread
#include <linux/kthread.h>
#include <linux/delay.h>
#include <linux/mutex.h>
static struct task_struct *thread_body;
struct gpio_demo_priv{
int count;
int gpio[0];
struct mutex mtx;
int mode;
};
struct gpio_demo_priv *priv;
static int thread_func(void *data) {
int i, count;
while (1){
count++;
mutex_lock(&priv->mtx);
for ( i = 0; i < priv->count; i++){
gpio_set_value(priv->gpio[i], count%2);
}
mutex_unlock(&priv->mtx);
msleep(50);
printk(KERN_INFO "thread count %d\n", count);
}
return 0;
}
static int gpio_demo_probe(struct platform_device *pdev){
int ret,i;
struct device *dev = &pdev->dev;
struct device_node *node = dev->of_node;
if (!node)
return -EINVAL;
ret = of_gpio_count(node);
if (ret == 0){
return -EINVAL;
}
priv = devm_kzalloc(dev, sizeof(*priv) + sizeof(int) * ret, GFP_KERNEL);
if (!priv){
return -ENOMEM;
}
priv->count = ret;
mutex_init(&priv->mtx);
for (i = 0; i < priv->count; i++) {
unsigned int gpio;
gpio = of_get_gpio(node, i);
if (gpio < 0) {
dev_warn(dev, "Unable to get gpio #%d\n", i);
continue;
}
ret = devm_gpio_request_one(dev, gpio, GPIOF_DIR_OUT, pdev->name);
priv->gpio[i] = gpio;
if (ret < 0) {
dev_warn(dev, "Unable to re quest GPIO %d: %d\n",
gpio, ret);
continue;
}
printk(KERN_INFO "success request gpio %d\n",gpio);
gpio_direction_output(gpio, 1); //設置輸出的電平
}
platform_set_drvdata(pdev,priv);
thread_body = kthread_create(thread_func, NULL, "thread_pwm");
if((thread_body))
{
wake_up_process(thread_body);
}
return 0;
}
static struct of_device_id gpio_demo_of_match[] = {
{ .compatible = "gpio-demo"},
{},
}
MODULE_DEVICE_TABLE(of,gpio_demo_of_match);
static struct platform_driver gpio_demo_driver = {
.probe = gpio_demo_probe,
.driver = {
.name = "gpio-demo-device",
.owner = THIS_MODULE,
.of_match_table = of_match_ptr(gpio_demo_of_match),
}
};
static int __init gpio_demo_init(void){
return platform_driver_register(&gpio_demo_driver);
}
static void __exit gpio_demo_exit(void){
platform_driver_unregister(&gpio_demo_driver);
}
late_initcall(gpio_demo_init);
module_exit(gpio_demo_exit);
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Gpio demo Driver");
MODULE_ALIAS("platform:gpio-demo");