嵌入式Linux中的LED驱动控制（设备树方式）
Linux3.1之后的内核版本，引入了设备树的概念。同时，设备树还需要Bootloader的支持，如果使用Uboot，在1.1.3版本之后就可以支持设备树了。
设备树概念的提出其实有两方面的原因。其一当然是代码冗余，导致Linux内核臃肿不堪。在Linux内核源码中，只要是通过了基金会的认可，就可以把某个厂商的板级支持代码纳入到Linux内核源码当中。比如国内曾经风靡一时的友善之臂Mini2440开发板，从Linux-2.6.31版本开始就被Linux官方内核所支持了，至今还可以在Linux内核源码中看到它的相关文件（如：arch/arm/mach-s3c24xx/mach-mini2440.c以及arch/arm/configs/mini2440_defconfig等）。然而在Linux内核源码中，同样还包含有很多其他厂商的板级支持文件。其实这些开发板只要所选用的芯片相同，则很大一部分代码是相同的。这就造成了内核的冗余和臃肿，并有愈演愈烈的趋势（难怪Linus要发火了）。第二个原因，其实也是顺理成章的。从Linux2.6版本之后内核就引入了platform总线平台的概念，把驱动分成了设备（platform_device）和驱动（platform_driver）两个单独的文件，现在只要把设备文件从内核中提出来，单独形成一个第三方文件，不就不影响内核了吗。这样做还有一个好处，即这个提出来的设备文件，只要在驱动中统一相关接口和命名规则，该文件的大部分工作还可以由厂商来完成（或由厂商提供的工具来完成），大大提高了开发的效率和可靠性。这个被单独提出来的第三方文件，后来就演化成了现在的设备树配置文件。
下面就通过设备树方式来实现对LED的驱动。先给出设备树的配置内容，在内核源码（本例在/opt/ebf_linux_kernel/目录下）的arch/arm/boot/dts目录下找到一个名为“imx6ull-mmc-npi.dts”的文件，该文件就是开发板（EMMC版本）配套提供的设备树源文件。打开它，并在根节点的最后加入本例LED设备的配置内容，如下。
/ { model = "Embedfire i.MX6ULL Board"; compatible = "fsl,imx6ull-14x14-evk", "fsl,imx6ull"; aliases { pwm0 = &pwm1; pwm1 = &pwm2; pwm2 = &pwm3; pwm3 = &pwm4; }; chosen { stdout-path = &uart1; }; memory { reg = <0x80000000 0x20000000>; }; reserved-memory { #address-cells = <1>; #size-cells = <1>; ranges; linux,cma { compatible = "shared-dma-pool"; reusable; size = <0x14000000>; linux,cma-default; }; }; regulators { compatible = "simple-bus"; #address-cells = <1>; #size-cells = <0>; reg_sd1_vmmc: regulator@1 { compatible = "regulator-fixed"; regulator-name = "VSD_3V3"; regulator-min-microvolt = <3300000>; regulator-max-microvolt = <3300000>; gpio = <&gpio1 9 GPIO_ACTIVE_HIGH>; off-on-delay = <20000>; enable-active-high; }; }; leds { compatible = "gpio-leds"; pinctrl-names = "default"; pinctrl-0 = <&pinctrl_led>; led0: cpu { label = "cpu"; gpios = <&gpio5 3 GPIO_ACTIVE_LOW>; default-state = "on"; linux,default-trigger = "heartbeat"; }; }; sound: sound { status = "disabled"; }; spi4: 74hc595 { compatible = "spi-gpio"; pinctrl-names = "default"; pinctrl-0 = <&pinctrl_spi4>; pinctrl-assert-gpios = <&gpio5 8 GPIO_ACTIVE_LOW>; status = "disabled"; gpio-sck = <&gpio5 11 0>; gpio-mosi = <&gpio5 10 0>; cs-gpios = <&gpio5 7 0>; num-chipselects = <1>; #address-cells = <1>; #size-cells = <0>; gpio_spi: gpio_spi@0 { compatible = "fairchild,74hc595"; gpio-controller; #gpio-cells = <2>; reg = <0>; registers-number = <1>; registers-default = /bits/ 8 <0x57>; spi-max-frequency = <100000>; }; }; //以下为本次LED的追加内容 rgb_led{ #address-cells = <1>; #size-cells = <1>; compatible = "fire,rgb_led"; //红色LED节点 ranges; rgb_led_red@0x020C406C{ compatible = "fire,led_red"; reg = <0x020C406C 0x00000004 0x020E006C 0x00000004 0x020E02F8 0x00000004 0x0209C000 0x00000004 0x0209C004 0x00000004>; status = "okay"; }; //绿色LED节点 rgb_led_green@0x020C4074{ compatible = "fire,led_green"; reg = <0x020C4074 0x00000004 0x020E01E0 0x00000004 0x020E046C 0x00000004 0x020A8000 0x00000004 0x020A8004 0x00000004>; status = "okay"; }; //蓝色LED节点 rgb_led_blue@0x020C4074{ compatible = "fire,led_blue"; reg = <0x020C4074 0x00000004 0x020E01DC 0x00000004 0x020E0468 0x00000004 0x020A8000 0x00000004 0x020A8004 0x00000004>; status = "okay"; }; }; };
注意，以上内容只是设备树文件imx6ull-mmc-npi.dts中的一部分内容，并未全部给出。上面内容中最末尾的部分才是本次追加的内容，其他部分内容是原设备树就有的，不要改动（包括未给出的部分），完成后保存并编译它。编译要在源码根目录下进行（即/opt/ebf_linux_kernel/目录下），先执行make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- imx_v7_defconfig进行配置，然后执行make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- dtbs进行编译。编译完成后，会在设备树所在目录下（arch/arm/boot/dts）生成名为imx6ull-mmc-npi.dtb的设备树文件，把该文件通过NFS拷贝到开发板的/usr/lib/linux-image-4.19.35-imx6/目录下并替换原有设备树文件，然后执行reboot重启开发板（不能按reset键重启）。重启后，可查看目录/proc/device-tree，会看到新增的设备节点，如下图所示。
以下是平台驱动部分的代码，文件名为led.c。
#include <linux/init.h> #include <linux/module.h> #include <linux/fs.h> #include <linux/cdev.h> #include <linux/uaccess.h> #include <linux/types.h> #include <linux/kernel.h> #include <linux/delay.h> #include <linux/ide.h> #include <linux/errno.h> #include <linux/gpio.h> #include <asm/mach/map.h> #include <linux/of.h> #include <linux/of_address.h> #include <linux/of_gpio.h> #include <asm/io.h> #include <linux/device.h> #include <linux/platform_device.h> //以下定义led资源结构体，保存获取得到的节点信息以及转换后的虚拟寄存器地址 struct led_resource { struct device_node *device_node; void __iomem *CCM_CCGR; void __iomem *IOMUXC_SW_MUX_CTL_PAD; void __iomem *IOMUXC_SW_PAD_CTL_PAD; void __iomem *DR; void __iomem *GDIR; }; static struct class *led_class; //定义类结构体 struct led_dev { dev_t devid; //设备号 struct cdev led_cdev; //定义字符型结构体 struct device_node *rgb_led_device_node; //rgb_led的设备树节点结构体 struct led_resource led_red; struct led_resource led_green; struct led_resource led_blue; }; //实现open函数，为file_oprations结构体成员函数 static int led_open(struct inode *inode, struct file *filp) { unsigned int tmp; struct led_dev *led; if (!inode->i_cdev) { pr_err("btn_open: i_cdev is NULL\n"); return -ENODEV; } led = container_of(inode->i_cdev, struct led_dev, led_cdev); //把led结构体保存在file结构体的私有变量中 filp->private_data = led; tmp = ioread32(led->led_red.CCM_CCGR); tmp |= (0x03 << 26); iowrite32(tmp, led->led_red.CCM_CCGR); tmp = ioread32(led->led_green.CCM_CCGR); tmp |= (0x03 << 12); iowrite32(tmp, led->led_green.CCM_CCGR); //开启时钟 return 0; } //实现write函数，为file_oprations结构体成员函数 static ssize_t led_write(struct file *filp, const char __user *buf, size_t cnt, loff_t *offt) { unsigned char value; unsigned int tmp; unsigned long n; //以下把私有变量private_data中的值赋值给指针led（该值在上面open函数中存入） struct led_dev *led = filp->private_data; n = copy_from_user(&value, buf, cnt); //从应用空间获取值 switch (value) { case 0: /* 全部点亮 */ tmp = ioread32(led->led_red.DR); tmp &= ~(0x01 << 4); iowrite32(tmp, led->led_red.DR); tmp = ioread32(led->led_green.DR); tmp &= ~(0x01 << 20); iowrite32(tmp, led->led_green.DR); tmp = ioread32(led->led_blue.DR); tmp &= ~(0x01 << 19); iowrite32(tmp, led->led_blue.DR); break; case 1: /* 点亮红灯 */ tmp = ioread32(led->led_red.DR); tmp &= ~(0x01 << 4); iowrite32(tmp, led->led_red.DR); break; case 2: /* 点亮绿灯 */ tmp = ioread32(led->led_green.DR); tmp &= ~(0x01 << 20); iowrite32(tmp, led->led_green.DR); break; case 3: /* 点亮蓝灯 */ tmp = ioread32(led->led_blue.DR); tmp &= ~(0x01 << 19); iowrite32(tmp, led->led_blue.DR); break; case 4: /* 熄灭红灯 */ tmp = ioread32(led->led_red.DR); tmp |= (0x01 << 4); iowrite32(tmp, led->led_red.DR); break; case 5: /* 熄灭绿灯 */ tmp = ioread32(led->led_green.DR); tmp |= (0x01 << 20); iowrite32(tmp, led->led_green.DR); break; case 6: /* 熄灭蓝灯 */ tmp = ioread32(led->led_blue.DR); tmp |= (0x01 << 19); iowrite32(tmp, led->led_blue.DR); break; case 7: /* 全部熄灭 */ tmp = ioread32(led->led_red.DR); tmp |= (0x01 << 4); iowrite32(tmp, led->led_red.DR); tmp = ioread32(led->led_green.DR); tmp |= (0x01 << 20); iowrite32(tmp, led->led_green.DR); tmp = ioread32(led->led_blue.DR); tmp |= (0x01 << 19); iowrite32(tmp, led->led_blue.DR); break; default: /* 其他值全部熄灭 */ tmp = ioread32(led->led_red.DR); tmp |= (0x01 << 4); iowrite32(tmp, led->led_red.DR); tmp = ioread32(led->led_green.DR); tmp |= (0x01 << 20); iowrite32(tmp, led->led_green.DR); tmp = ioread32(led->led_blue.DR); tmp |= (0x01 << 19); iowrite32(tmp, led->led_blue.DR); break; } return cnt; } //实现release函数，为file_oprations结构体函数 static int led_release(struct inode *inode, struct file *filp) { unsigned int tmp; struct led_dev *led = filp->private_data; tmp = ioread32(led->led_red.CCM_CCGR); tmp &= ~(0x03 << 26); iowrite32(tmp, led->led_red.CCM_CCGR); tmp = ioread32(led->led_green.CCM_CCGR); tmp &= ~(0x03 << 12); iowrite32(tmp, led->led_green.CCM_CCGR); //关闭时钟 return 0; } //填充一个file_oprations类型的结构体，名为led_dev_fops，包含上述声明的成员函数 static struct file_operations led_dev_fops = { .owner = THIS_MODULE, .open = led_open, //指定open函数成员 .write = led_write, //指定write函数成员 .release = led_release, //指定release函数成员 }; //probe函数中,驱动提取设备树中的资源，并完成字符设备的注册 static int led_pdrv_probe(struct platform_device *pdev) { unsigned int tmp; struct led_dev *led; //以下动态申请led结构体大小的内存 led = devm_kzalloc(&pdev->dev, sizeof(struct led_dev), GFP_KERNEL); if(!led) return -ENOMEM; //获取rgb_led的设备树节点 led->rgb_led_device_node = of_find_node_by_path("/rgb_led"); if (led->rgb_led_device_node == NULL) { printk(KERN_ERR "\t get rgb_led failed! \n"); return -1; } //获取rgb_led节点的红灯子节点 led->led_red.device_node = of_find_node_by_name(led->rgb_led_device_node,"rgb_led_red"); if (led->led_red.device_node == NULL) { printk(KERN_ERR "\n get rgb_led_red_device_node failed ! \n"); return -1; } //以下获取设备节点中红灯子节点的reg属性并转化为虚拟地址 led->led_red.CCM_CCGR = of_iomap(led->led_red.device_node, 0); led->led_red.IOMUXC_SW_MUX_CTL_PAD = of_iomap(led->led_red.device_node, 1); led->led_red.IOMUXC_SW_PAD_CTL_PAD = of_iomap(led->led_red.device_node, 2); led->led_red.DR = of_iomap(led->led_red.device_node, 3); led->led_red.GDIR = of_iomap(led->led_red.device_node, 4); // of_node_put(led->led_red.device_node); // 映射完毕，释放引用 // led->led_red.device_node = NULL; //初始化红灯 tmp = ioread32(led->led_red.CCM_CCGR); tmp |= (0x03 << 26); iowrite32(tmp, led->led_red.CCM_CCGR); //开启时钟 tmp = ioread32(led->led_red.IOMUXC_SW_MUX_CTL_PAD); tmp &= ~(0xf << 0); tmp |= (0x05 << 0); iowrite32(tmp, led->led_red.IOMUXC_SW_MUX_CTL_PAD); //设置复用功能 tmp = ioread32(led->led_red.IOMUXC_SW_PAD_CTL_PAD); tmp = 0x10B0; iowrite32(tmp, led->led_red.IOMUXC_SW_PAD_CTL_PAD); //设置PAD属性 tmp = ioread32(led->led_red.GDIR); tmp |= (0x01 << 4); iowrite32(tmp, led->led_red.GDIR); //设置GPIO1_04为输出模式 tmp = ioread32(led->led_red.DR); tmp |= (0x01 << 4); iowrite32(tmp, led->led_red.DR); //设置GPIO1_04默认输出高电平 //获取rgb_led节点的绿灯子节点 led->led_green.device_node = of_find_node_by_name(led->rgb_led_device_node,"rgb_led_green"); if (led->led_green.device_node == NULL) { printk(KERN_ERR "\n get rgb_led->led_green_device_node failed ! \n"); return -1; } //以下获取设备节点中绿灯子节点的reg属性并转化为虚拟地址 led->led_green.CCM_CCGR = of_iomap(led->led_green.device_node, 0); led->led_green.IOMUXC_SW_MUX_CTL_PAD = of_iomap(led->led_green.device_node, 1); led->led_green.IOMUXC_SW_PAD_CTL_PAD = of_iomap(led->led_green.device_node, 2); led->led_green.DR = of_iomap(led->led_green.device_node, 3); led->led_green.GDIR = of_iomap(led->led_green.device_node, 4); // of_node_put(led->led_green.device_node); // led->led_green.device_node = NULL; //初始化绿灯 tmp = ioread32(led->led_green.CCM_CCGR); tmp |= (0x03 << 12); iowrite32(tmp, led->led_green.CCM_CCGR); //开启时钟 tmp = ioread32(led->led_green.IOMUXC_SW_MUX_CTL_PAD); tmp &= ~(0xf << 0); tmp |= (0x05 << 0); iowrite32(tmp, led->led_green.IOMUXC_SW_MUX_CTL_PAD); //设置复用功能 tmp = ioread32(led->led_green.IOMUXC_SW_PAD_CTL_PAD); tmp = (0x10B0); iowrite32(tmp, led->led_green.IOMUXC_SW_PAD_CTL_PAD); //设置PAD属性 tmp = ioread32(led->led_green.GDIR); tmp |= (0x01 << 20); iowrite32(tmp, led->led_green.GDIR); //设置GPIO4_IO20为输出模式 tmp = ioread32(led->led_green.DR); tmp |= (0x01 << 20); iowrite32(tmp, led->led_green.DR); //设置GPIO4_IO20默认输出高电平 //获取rgb_led节点的蓝灯子节点 led->led_blue.device_node = of_find_node_by_name(led->rgb_led_device_node,"rgb_led_blue"); if (led->led_blue.device_node == NULL) { printk(KERN_ERR "\n get rgb_led->led_blue_device_node failed ! \n"); return -1; } //以下获取设备节点中蓝灯子节点的reg属性并转化为虚拟地址 led->led_blue.CCM_CCGR = of_iomap(led->led_blue.device_node, 0); led->led_blue.IOMUXC_SW_MUX_CTL_PAD = of_iomap(led->led_blue.device_node, 1); led->led_blue.IOMUXC_SW_PAD_CTL_PAD = of_iomap(led->led_blue.device_node, 2); led->led_blue.DR = of_iomap(led->led_blue.device_node, 3); led->led_blue.GDIR = of_iomap(led->led_blue.device_node, 4); // of_node_put(led->led_blue.device_node); // led->led_blue.device_node = NULL; //初始化蓝灯 tmp = ioread32(led->led_blue.CCM_CCGR); tmp |= (0x03 << 12); iowrite32(tmp, led->led_blue.CCM_CCGR); //开启时钟 tmp = ioread32(led->led_blue.IOMUXC_SW_MUX_CTL_PAD); tmp &= ~(0xf << 0); tmp |= (0x05 << 0); iowrite32(tmp, led->led_blue.IOMUXC_SW_MUX_CTL_PAD); //设置复用功能 tmp = ioread32(led->led_blue.IOMUXC_SW_PAD_CTL_PAD); tmp = (0x10B0); iowrite32(tmp, led->led_blue.IOMUXC_SW_PAD_CTL_PAD); //设置PAD属性 tmp = ioread32(led->led_blue.GDIR); tmp |= (0x01 << 19); iowrite32(tmp, led->led_blue.GDIR); //设置GPIO4_IO19为输出模式 tmp = ioread32(led->led_blue.DR); tmp |= (0x01 << 19); iowrite32(tmp, led->led_blue.DR); //设置GPIO4_IO19默认输出高电平 tmp = ioread32(led->led_red.CCM_CCGR); tmp &= ~(0x03 << 26); iowrite32(tmp, led->led_red.CCM_CCGR); tmp = ioread32(led->led_green.CCM_CCGR); tmp &= ~(0x03 << 12); iowrite32(tmp, led->led_green.CCM_CCGR); //关闭时钟 //申请主设备号 if (alloc_chrdev_region(&led->devid, 0, 1, "led") < 0) { printk("fail to alloc devid\n"); return -EFAULT; } led->led_cdev.owner = THIS_MODULE; //绑定前面声明的file_oprations类型的结构体到字符设备 cdev_init(&led->led_cdev, &led_dev_fops); //填充上面申请到的主设备号到字符设备 if ( cdev_add(&led->led_cdev, led->devid, 1) < 0) { printk("fail to add cdev\n"); return -EFAULT; } //创建一个设备节点 device_create(led_class, NULL, led->devid, NULL, "led"); //以下把LED数据信息存入在平台驱动结构体中pdev->dev->driver_data中，后面移除时会用到 platform_set_drvdata(pdev, led); /* 所有子节点处理完毕，释放根节点 */ // of_node_put(led->rgb_led_device_node); // led->rgb_led_device_node = NULL; printk("platform driver probed!\n"); return 0; } //remove函数中,删除设备并释放设备号 static int led_pdrv_remove(struct platform_device *pdev) { //platform_get_drvdata，获取当前LED灯对应的结构体 struct led_dev *led = platform_get_drvdata(pdev); //以下实现各个寄存器的解除映射 iounmap(led->led_green.CCM_CCGR); iounmap(led->led_green.IOMUXC_SW_MUX_CTL_PAD); iounmap(led->led_green.IOMUXC_SW_PAD_CTL_PAD); iounmap(led->led_green.DR); iounmap(led->led_green.GDIR); iounmap(led->led_red.CCM_CCGR); iounmap(led->led_red.IOMUXC_SW_MUX_CTL_PAD); iounmap(led->led_red.IOMUXC_SW_PAD_CTL_PAD); iounmap(led->led_red.DR); iounmap(led->led_red.GDIR); iounmap(led->led_blue.CCM_CCGR); iounmap(led->led_blue.IOMUXC_SW_MUX_CTL_PAD); iounmap(led->led_blue.IOMUXC_SW_PAD_CTL_PAD); iounmap(led->led_blue.DR); iounmap(led->led_blue.GDIR); unregister_chrdev_region(led->devid, 1); //释放主设备号 cdev_del(&led->led_cdev); //删除字符设备 device_destroy(led_class, led->devid); //销毁设备节点 printk("platform driver removed!\n"); return 0; } //填充of_device_id结构体，名为rgb_led，用于指明匹配表 static const struct of_device_id rgb_led[] = { {.compatible = "fire,rgb_led"}, //匹配内容 {/* sentinel */} }; //以下填充一个platform_driver结构体 struct platform_driver led_platform_driver = { .probe = led_pdrv_probe, //指定probe函数成员 .remove = led_pdrv_remove, //指定remove函数成员 .driver = { .name = "rgb-leds-platform", //指定平台总线名称 .owner = THIS_MODULE, .of_match_table = rgb_led, //指定匹配表名称 } }; //以下定义模块的入口函数 static int __init led_pdrv_init(void) { led_class = class_create(THIS_MODULE, "my_leds"); //创建一个类 platform_driver_register(&led_platform_driver); //注册一个platform驱动 printk("led platform driver initted!\n"); return 0; } //以下定义模块的出口函数 static void __exit led_pdrv_exit(void) { platform_driver_unregister(&led_platform_driver); //释放一个platform驱动 class_destroy(led_class); //销毁类 printk("led platform driver exited!\n"); } module_init(led_pdrv_init); module_exit(led_pdrv_exit); MODULE_LICENSE("GPL"); MODULE_INFO(intree,"Y");
在上述驱动程序中，在填充的of_device_id结构体中，最后一个成员必须是一对空的花括号，它用来用来标记数组的结束（即哨兵），以避免越界。配套的Makefile文件内容如下。
KERNEL_DIR=/opt/ebf_linux_kernel/build_image/build ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- export ARCH CROSS_COMPILE obj-m := led.o all: $(MAKE) -C $(KERNEL_DIR) M=$(CURDIR) modules modules clean: $(MAKE) -C $(KERNEL_DIR) M=$(CURDIR) clean
以下是测试用的应用程序代码，文件名为app.c。
#include <stdio.h> #include <fcntl.h> #include <string.h> #include <unistd.h> int main(int argc, char *argv[]) { int fd; unsigned char val = 7; fd = open("/dev/led", O_RDWR); //打开设备节点 if( fd < 0 ) printf("can`t open\n"); if( argc != 3 ) //命令参数不对时提示 { printf("Usage :\n"); printf("%s <all|red|green|blue> <on|off>\n", argv[0]); return 0; } if(strcmp(argv[1], "all") == 0) { if(strcmp(argv[2], "on") == 0) val = 0; //值为0时全部点亮 else val = 7; //值为7时全部熄灭 } else if(strcmp(argv[1], "red") == 0) { if(strcmp(argv[2], "on") == 0) val = 1; //值为1时红色点亮 else val = 4; //值为4时红色熄灭 } else if(strcmp(argv[1], "green") == 0) { if(strcmp(argv[2], "on") == 0) val = 2; //值为2时绿色点亮 else val = 5; //值为5时绿色熄灭 } else if(strcmp(argv[1], "blue") == 0) { if(strcmp(argv[2], "on") == 0) val = 3; //值为3时蓝色点亮 else val = 6; //值为6时蓝色熄灭 } write(fd, &val, 1); //把值写入设备节点 close(fd); //关闭设备节点 return 0; }
完成后，先执行make命令编译驱动程序，若成功会生成名为led.ko的驱动模块文件。然后对应用程序进行交叉编译，执行“arm-linux-gnueabihf-gcc app.c -o app”即可。实验结果与“嵌入式Linux中的LED驱动控制”一文中的完全一样，这里就不给出了。
关于设备树的介绍，可以参考STM32MP157部分的”嵌入式Linux中的LED驱动控制（设备树方式）（续）“一文。
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