首先借用一张图说明linux应用程序和内核的关系
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与裸机程序不同，linux下的应用开发并不直接访问硬件，而是由应用程序调用驱动来访问硬件，这是linux的系统结构，具体可参考《嵌入式linux应用开发完全手册》。
回到am335x，已经在uboot实现LED的操作，基本原理一致，设置GPIO输出，低电平点亮LED。如果想通过内核空间操作文件的方式访问GPIO，可参考 BeagleBone的GPIO控制
linux3.2中driver/leds目录下存放有led相关文件，在此编写led驱动模块。代码参照 Am335x 下GPIO控制实例
添加文件leds-run.c
```
#include#include #include #include #include #include #include #include #include #include #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目录下修改Kconfig，添加一项：
config GPIO_LED bool "Gpio LED test" help Just test the Gpio LED status
同目录下Makefile文件修改：
obj-$(CONFIG_GPIO_LED)+=leds-run.o
通过make menuconfig，将驱动模块GPIO_LED编译进内核。
应用程序app.c
#include#include #include #include #include #include #include #include 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; }
烧写内核，内核启动后打印“#######GpioTest modules is install!”，运行app，可以正常运行，可led没有按照程序进行亮灭操作。
细想整个过程发现，am335x的IO口是复用的，需要先完成mux设置。
再回到linux内核，需要明确linux的运行流程，此次参考了 linux内核代码分析1 TI am335x：
Board-am335xevm.c(./arch/arm/mach-omap2)中开始执行，
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
转到启动程序am335x_evm_init：
am33xx_cpuidle_init(); am33xx_mux_init(board_mux); omap_serial_init(); am335x_evm_i2c_init(); am335x_evm_setup(); omap_sdrc_init(NULL, NULL); usb_musb_init(&musb_board_data);
static void am335x_evm_setup()
{
setup_general_purpose_evm();
}static void setup_general_purpose_evm(void)
{
_configure_device(boardid,invt_evm_dev_cfg, -1);
}所以初始化开发板的设置，重点就是invt_evm_dev_cfg
static struct evm_dev_cfginvt_evm_dev_cfg[] = {
{enable_ecap0, DEV_ON_BASEBOARD, PROFILE_NONE},
{lcdc_init, DEV_ON_BASEBOARD, PROFILE_NONE},
{mfd_tscadc_init, DEV_ON_BASEBOARD, PROFILE_NONE},
{rmii1_init, DEV_ON_BASEBOARD, PROFILE_NONE},
{rmii2_init, DEV_ON_BASEBOARD, PROFILE_NONE},
{usb0_init, DEV_ON_BASEBOARD, PROFILE_NONE},
{usb1_init, DEV_ON_BASEBOARD, PROFILE_NONE},
{evm_nand_init,DEV_ON_BASEBOARD,PROFILE_NONE},
{mmc0_init, DEV_ON_BASEBOARD, PROFILE_NONE},
{spi0_init, DEV_ON_BASEBOARD, PROFILE_NONE},
{led_init, DEV_ON_BASEBOARD, PROFILE_NONE}, //添加led操作
};static void led_init(int evm_id, int profile)
{
int err;
setup_pin_mux(gpio_led_mux);
err = platform_device_register(&leds_gpio);
if (err)
pr_err("failed to register gpio led device\n");
}static struct pinmux_config gpio_led_mux[] = {
{"lcd_ac_bias_en.gpio2_25", OMAP_MUX_MODE7 | AM33XX_PIN_OUTPUT}, //LED GPIO 配置
{NULL, 0},
};修改完成后，重新编译内核，烧写内核，运行程序，成功实现LED的操作。
