linux下am335x點亮LED
與裸機程序不同，linux下的應用開發並不直接訪問硬件，而是由應用程序調用驅動來訪問硬件，這是linux的系統結構，具體可參考《嵌入式linux應用開發完全手冊》。
回到am335x，已經在uboot實現LED的操作，基本原理一致，設置GPIO輸出，低電平點亮LED。如果想通過內核空間操作文件的方式訪問GPIO，可參考[BeagleBone的GPIO控制 ](http://blog.sina.com.cn/s/blog_9e16dc4d01012x7z.html)
linux3.2中driver/leds目錄下存放有led相關文件，在此編寫led驅動模塊。代碼參照[Am335x 下GPIO控制實例](https://blog.csdn.net/hudaweikevin/article/details/16826995)
添加文件leds-run.c
```
#include <linux/gpio.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/moduleparam.h>
#include <linux/delay.h>
#include <linux/types.h>
#include <linux/miscdevice.h>
#include <linux/device.h>
#include <linux/fs.h>
#include <linux/init.h>
#define TEST_IO_NUM 89
#define NAME_MISC "GpioTest"
#define NAME_MOUDULE "GpioTest1"
#define USE_MISC_MODE 1
static int major = 251;
void GpioTest(void);
static long GpioIOctl(struct file *filp, unsigned cmd, unsigned long arg)
{
GpioTest();
return 1;
}
void GpioTest(void)
{
int iCount = 0;
for(iCount = 0; iCount <=20; iCount++ )
{
if(iCount%2 == 0)
{
gpio_direction_output(TEST_IO_NUM, 1);
printk(KERN_INFO"#######LED statu is high.\r\n");
}
else
{
gpio_direction_output(TEST_IO_NUM, 0);
printk(KERN_INFO"#######LED statu is low.\r\n");
}
mdelay(3000);
}
printk(KERN_INFO"#######App run over!");
}
static int GpioOpen(struct inode *inode, struct file *file)
{
int iRen = -1;
iRen = gpio_request(TEST_IO_NUM, "LED");
if(iRen < 0)
{
printk(KERN_INFO"#######Failed to request the LED!");
}else
{
printk(KERN_INFO"#######Success to request the LED");
}
return iRen;
}
static int GpioClose(struct inode *inode, struct file *file)
{
printk(KERN_INFO"#######Free the LED");
gpio_free(TEST_IO_NUM);
return 1;
}
//****entry point for TEST GPIO module
static const struct file_operations gpio_test_driver = {
.owner = THIS_MODULE,
.unlocked_ioctl= GpioIOctl,
.llseek = no_llseek,
.open = GpioOpen,
.release = GpioClose,
};
#if USE_MISC_MODE
static struct miscdevice gpiotest_misc_device = {
.minor = MISC_DYNAMIC_MINOR,
.name = NAME_MISC,
.fops = &gpio_test_driver,
};
#endif
static int __init GpioTestInit(void)
{
int iRet;
printk(KERN_INFO"#######GpioTest modules is install!\r\n");
#if USE_MISC_MODE
iRet = misc_register(&gpiotest_misc_device);
if (iRet) {
printk(KERN_INFO"#######unable to register a misc device\r\n");
return iRet;
}
#else
iRet = register_chrdev(major, NAME_MOUDULE, &gpio_test_driver);
if (iRet < 0) {
printk(KERN_INFO"#######unable to register a chr device\r\n");
return iRet;
}
#endif
return iRet;
}
static void __exit GpioTestExit(void)
{
#if USE_MISC_MODE
misc_deregister(&gpiotest_misc_device);
#else
unregister_chrdev(major, NAME_MOUDULE);
#endif
printk(KERN_INFO"#######GpioTest modules is exit!\r\n");
}
module_init(GpioTestInit);
module_exit(GpioTestExit);
MODULE_AUTHOR("XXXXXXXXXXXX");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("System status led");```
driver/leds目錄下修改Kconfig，添加一項：
config GPIO_LED bool "Gpio LED test" help Just test the Gpio LED status
同目錄下Makefile文件修改：
obj-$(CONFIG_INPUT_obj-$(CONFIG_GPIO_LED)+=leds-run.o
通過make menuconfig，將驅動模塊GPIO_LED編譯進內核。
應用程序app.c
#include <stdio.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <linux/input.h>
#include <fcntl.h>
int main(int argc, char *argv)
{
int fd;
fd = open("/dev/GpioTest", O_RDWR);
if(fd < 0)
{
printf("***Can't open the gpiotest!\r\n");
return -1;
}
ioctl(fd, 0, 0);
close(fd);
printf("***App run over!\r\n");
return 1;
}燒寫內核，內核啟動後打印“#######GpioTest modules is install!”，運行app，可以正常運行，可led沒有按照程序進行亮滅操作。
細想整個過程發現，am335x的IO口是復用的，需要先完成mux設置。
再回到linux內核，需要明確linux的運行流程，此次參考了 linux內核代碼分析1 TI am335x：
Board-am335xevm.c(./arch/arm/mach-omap2)中開始執行，
MACHINE_START(AM335XEVM, "am335xevm") /* Maintainer: Texas Instruments */ .atag_offset= 0x100, .map_io= am335x_evm_map_io, .init_early= am33xx_init_early, .init_irq= ti81xx_init_irq, .handle_irq = omap3_intc_handle_irq, .timer= &omap3_am33xx_timer, .init_machine= am335x_evm_init, MACHINE_END
轉到啟動程序am335x_evm_init：
linux下am335x點亮LED