Linux3.1之後的核心版本，引入了裝置樹的概念。同時，裝置樹還需要Bootloader的支援，如果使用Uboot，在1.1.3版本之後就可以支援裝置樹了。
裝置樹概念的提出其實有兩方面的原因。其一當然是程式碼冗餘，導致Linux核心臃腫不堪。在Linux核心原始碼中，只要是透過了基金會的認可，就可以把某個廠商的板級支援程式碼納入到Linux核心原始碼當中。比如國內曾經風靡一時的友善之臂Mini2440開發板，從Linux-2.6.31版本開始就被Linux官方核心所支援了，至今還可以在Linux核心原始碼中看到它的相關檔案（如：arch/arm/mach-s3c24xx/mach-mini2440.c以及arch/arm/configs/mini2440_defconfig等）。然而在Linux核心原始碼中，同樣還包含有很多其他廠商的板級支援檔案。其實這些開發板只要所選用的晶片相同，則很大一部分程式碼是相同的。這就造成了核心的冗餘和臃腫，並有愈演愈烈的趨勢（難怪Linus要發火了）。第二個原因，其實也是順理成章的。從Linux2.6版本之後核心就引入了platform匯流排平臺的概念，把驅動分成了裝置（platform_device）和驅動（platform_driver）兩個單獨的檔案，現在只要把裝置檔案從核心中提出來，單獨形成一個第三方檔案，不就不影響核心了嗎。這樣做還有一個好處，即這個提出來的裝置檔案，只要在驅動中統一相關介面和命名規則，該檔案的大部分工作還可以由廠商來完成（或由廠商提供的工具來完成），大大提高了開發的效率和可靠性。這個被單獨提出來的第三方檔案，後來就演化成了現在的裝置樹配置檔案。
下面就透過裝置樹方式來實現對LED的驅動。先給出裝置樹的配置內容，在核心原始碼（本例在/opt/ebf_linux_kernel_mp157_depth1/目錄下）的arch/arm/boot/dts目錄下找到一個名為“stm32mp157a-basic.dts”的檔案，該檔案就是開發板配套提供的裝置樹原始檔。開啟它，並在根節點的最後加入本例LED裝置的配置內容，如下。
/ { model = "Embedfire STM32MP157 Star LubanCat Robot S1 Board"; compatible = "st,stm32mp157a-dk1", "st,stm32mp157"; aliases { ethernet0 = ðernet0; serial0 = &uart4; serial1 = &usart1; serial2 = &usart2; serial3 = &usart3; }; chosen { stdout-path = "serial0:115200n8"; }; memory@c0000000 { reg = <0xc0000000 0x40000000>; }; reserved-memory { #address-cells = <1>; #size-cells = <1>; ranges; retram: retram@0x38000000 { compatible = "shared-dma-pool"; reg = <0x38000000 0x10000>; no-map; }; mcuram: mcuram@0x30000000 { compatible = "shared-dma-pool"; reg = <0x30000000 0x40000>; no-map; }; mcuram2: mcuram2@0x10000000 { compatible = "shared-dma-pool"; reg = <0x10000000 0x40000>; no-map; }; vdev0vring0: vdev0vring0@10040000 { compatible = "shared-dma-pool"; reg = <0x10040000 0x2000>; no-map; }; vdev0vring1: vdev0vring1@10042000 { compatible = "shared-dma-pool"; reg = <0x10042000 0x2000>; no-map; }; vdev0buffer: vdev0buffer@10044000 { compatible = "shared-dma-pool"; reg = <0x10044000 0x4000>; no-map; }; gpu_reserved: gpu@d4000000 { reg = <0xd4000000 0x4000000>; no-map; }; }; sram: sram@10050000 { compatible = "mmio-sram"; reg = <0x10050000 0x10000>; #address-cells = <1>; #size-cells = <1>; ranges = <0 0x10050000 0x10000>; dma_pool: dma_pool@0 { reg = <0x0 0x10000>; pool; }; }; leds { compatible = "gpio-leds"; status = "okay"; heartbeat { label = "heartbeat"; gpios = <&gpioa 14 GPIO_ACTIVE_HIGH>; linux,default-trigger = "heartbeat"; default-state = "off"; }; }; v3v3: regulator-3p3v { compatible = "regulator-fixed"; regulator-name = "v3v3"; regulator-min-microvolt = <3300000>; regulator-max-microvolt = <3300000>; regulator-always-on; regulator-boot-on; }; vdd: regulator-vdd { compatible = "regulator-fixed"; regulator-name = "vdd"; regulator-min-microvolt = <3300000>; regulator-max-microvolt = <3300000>; regulator-always-on; regulator-boot-on; }; vdd_usb: regulator-vdd-usb { compatible = "regulator-fixed"; regulator-name = "vdd_usb"; regulator-min-microvolt = <3300000>; regulator-max-microvolt = <3300000>; regulator-always-on; regulator-boot-on; }; v2v8: v2v8 { compatible = "regulator-fixed"; regulator-name = "v2v8"; regulator-min-microvolt = <2800000>; regulator-max-microvolt = <2800000>; regulator-always-on; regulator-boot-on; }; vbus_otg: regulator-vbus-otg { compatible = "regulator-fixed"; regulator-name = "vbus_otg"; regulator-min-microvolt = <5000000>; regulator-max-microvolt = <5000000>; regulator-always-on; regulator-boot-on; }; usb_phy_tuning: usb-phy-tuning { st,hs-dc-level = <2>; st,fs-rftime-tuning; st,hs-rftime-reduction; st,hs-current-trim = <15>; st,hs-impedance-trim = <1>; st,squelch-level = <3>; st,hs-rx-offset = <2>; st,no-lsfs-sc; }; //以下為本次LED的追加內容 rgb_led{ #address-cells = <1>; #size-cells = <1>; compatible = "fire,rgb_led"; ranges; //紅色LED節點 led_red@0x50002000{ compatible = "fire,led_red"; reg = < 0x50002000 0x00000004 0x50002004 0x00000004 0x50002008 0x00000004 0x5000200C 0x00000004 0x50002018 0x00000004 0x50000A28 0x00000004 >; status = "okay"; }; //綠色LED節點 led_green@0x50000A28{ compatible = "fire,led_green"; reg = < 0x50008000 0x00000004 0x50008004 0x00000004 0x50008008 0x00000004 0x5000800C 0x00000004 0x50008018 0x00000004 >; status = "okay"; }; //藍色LED節點 led_blue@0x50000A28{ compatible = "fire,led_blue"; reg = < 0x50003000 0x00000004 0x50003004 0x00000004 0x50003008 0x00000004 0x5000300C 0x00000004 0x50003018 0x00000004 >; status = "okay"; }; }; };
在以上內容中，最末尾的部分才是本次追加的內容，其他部分內容是原裝置樹就有的，不要改動，完成後儲存並編譯它。編譯要在原始碼根目錄下進行（即/opt/ebf_linux_kernel_mp157_depth1/目錄下），先執行make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- stm32mp157_ebf_defconfig進行配置，然後執行make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- dtbs進行編譯。編譯完成後，會在裝置樹所在目錄下（arch/arm/boot/dts）生成名為stm32mp157a-basic.dtb的裝置樹檔案，把該檔案透過NFS複製到開發板的/boot/dts/目錄下並替換原有裝置樹檔案，然後執行reboot重啟開發板（不能按reset鍵重啟）。
以下是平臺驅動部分的程式碼，檔名為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> static dev_t devid; //裝置號 static struct cdev led_cdev; //定義字元型結構體 struct class *led_class; //類結構體 struct device_node *rgb_led_device_node; //rgb_led的裝置樹節點結構體 //以下定義led資源結構體，儲存獲取得到的節點資訊以及轉換後的虛擬暫存器地址 struct led_resource { struct device_node *device_node; void __iomem *MODER; void __iomem *OTYPER; void __iomem *OSPEEDR; void __iomem *PUPDR; void __iomem *BSRR; }; static void __iomem *clkaddr; //埠時鐘變數 //以下定義RGB三個燈的led_resource結構體，儲存獲取得到的節點資訊 struct led_resource led_red; struct led_resource led_green; struct led_resource led_blue; //實現open函式，為file_oprations結構體成員函式 static int led_open(struct inode *inode, struct file *filp) { unsigned int tmp; //以下使能GPIOA、GPIOB、GPIOG埠時鐘 tmp = ioread32(clkaddr); tmp |= 0x43; iowrite32(tmp, clkaddr); return 0; } //實現write函式，為file_oprations結構體成員函式 static ssize_t led_write(struct file *filp, const char __user *buf, size_t cnt, loff_t *offt) { unsigned char value; unsigned long n; n = copy_from_user(&value, buf, cnt); //從應用空間獲取值 switch(value) //根據應用空間的值判斷具體操作 { case 0: //全部點亮三個LED iowrite32(0x20000000, led_red.BSRR); iowrite32(0x200000, led_blue.BSRR); iowrite32(0x40000, led_green.BSRR); break; case 1: //點亮紅色LED iowrite32(0x20000000, led_red.BSRR); break; case 2: //點亮綠色LED iowrite32(0x40000, led_green.BSRR); break; case 3: //點亮藍色LED iowrite32(0x200000, led_blue.BSRR); break; case 4: //熄滅紅色LED iowrite32(0x2000, led_red.BSRR); break; case 5: //熄滅綠色LED iowrite32(0x04, led_green.BSRR); break; case 6: //熄滅藍色LED iowrite32(0x20, led_blue.BSRR); break; case 7: //全部熄滅三個LED iowrite32(0x2000, led_red.BSRR); iowrite32(0x20, led_blue.BSRR); iowrite32(0x04, led_green.BSRR); break; default: //全部熄滅 iowrite32(0x2000, led_red.BSRR); iowrite32(0x20, led_blue.BSRR); iowrite32(0x04, led_green.BSRR); break; } return cnt; } //實現release函式，為file_oprations結構體函式 static int led_release(struct inode *inode, struct file *filp) { unsigned int tmp; //以下禁能GPIOA、GPIOB、GPIOG埠時鐘 tmp = ioread32(clkaddr); tmp &= ~0x43; iowrite32(tmp, clkaddr); return 0; } //填充一個file_oprations型別的結構體，名為led_dev_fops，包含上述宣告的成員函式 static struct file_operations led_dev_fops = { .owner = THIS_MODULE, .open = led_open, //指定open函式成員 .write = led_write, //指定write函式成員 .release = led_release, //指定release函式成員 }; //probe函式中,驅動提取裝置樹中的資源，並完成字元裝置的註冊 static int led_pdrv_probe(struct platform_device *pdv) { unsigned int tmp; //獲取rgb_led的裝置樹節點 rgb_led_device_node = of_find_node_by_path("/rgb_led"); if (rgb_led_device_node == NULL) { printk(KERN_ERR "\t get rgb_led failed! \n"); return -1; } //獲取rgb_led節點的紅燈子節點 led_red.device_node = of_find_node_by_name(rgb_led_device_node,"led_red"); if (led_red.device_node == NULL) { printk(KERN_ERR "\n get rgb_led_red_device_node failed ! \n"); return -1; } //以下獲取裝置節點中紅燈子節點的reg屬性並轉化為虛擬地址 led_red.MODER = of_iomap(led_red.device_node, 0); led_red.OTYPER = of_iomap(led_red.device_node, 1); led_red.OSPEEDR = of_iomap(led_red.device_node, 2); led_red.PUPDR = of_iomap(led_red.device_node, 3); led_red.BSRR = of_iomap(led_red.device_node, 4); clkaddr = of_iomap(led_red.device_node, 5); //以下使能GPIOA、GPIOB、GPIOG埠時鐘 tmp = ioread32(clkaddr); tmp |= 0x43; iowrite32(tmp, clkaddr); //以下設定模式暫存器：輸出模式 tmp = ioread32(led_red.MODER); tmp &= ~(0x3 << (13 * 2)); tmp |= (0x1 << (13 * 2)); iowrite32(tmp, led_red.MODER); //以下設定輸出型別暫存器：推輓模式 tmp = ioread32(led_red.OTYPER); tmp &= ~(0x1 << 13); iowrite32(tmp, led_red.OTYPER); //以下設定輸出速度暫存器：高速 tmp = ioread32(led_red.OSPEEDR); tmp &= ~(0x3 << (13 * 2)); tmp |= (0x2 << (13 * 2)); iowrite32(tmp, led_red.OSPEEDR); //以下設定上下拉暫存器：上拉 tmp = ioread32(led_red.PUPDR); tmp &= ~(0x3 << (13 * 2)); tmp |= (0x1 << (13 * 2)); iowrite32(tmp,led_red.PUPDR); //以下設定置位暫存器：預設輸出高電平 tmp = ioread32(led_red.BSRR); tmp |= (0x1 << 13); iowrite32(tmp, led_red.BSRR); //獲取rgb_led節點的綠燈子節點 led_green.device_node = of_find_node_by_name(rgb_led_device_node,"led_green"); if (led_green.device_node == NULL) { printk(KERN_ERR "\n get rgb_led_green_device_node failed ! \n"); return -1; } //以下獲取裝置節點中綠燈子節點的reg屬性並轉化為虛擬地址 led_green.MODER = of_iomap(led_green.device_node, 0); led_green.OTYPER = of_iomap(led_green.device_node, 1); led_green.OSPEEDR = of_iomap(led_green.device_node, 2); led_green.PUPDR = of_iomap(led_green.device_node, 3); led_green.BSRR = of_iomap(led_green.device_node, 4); //以下設定模式暫存器：輸出模式 tmp = ioread32(led_green.MODER); tmp &= ~(0x3 << (2 * 2)); tmp |= (0x1 << (2 * 2)); iowrite32(tmp,led_green.MODER); //以下設定輸出型別暫存器：推輓模式 tmp = ioread32(led_green.OTYPER); tmp &= ~(0x1 << 2); iowrite32(tmp, led_green.OTYPER); //以下設定輸出速度暫存器：高速 tmp = ioread32(led_green.OSPEEDR); tmp &= ~(0x3 << (2 * 2)); tmp |= (0x2 << (2 * 2)); iowrite32(tmp, led_green.OSPEEDR); //以下設定上下拉暫存器：上拉 tmp = ioread32(led_green.PUPDR); tmp &= ~(0x3 << (2 * 2)); tmp |= (0x1 << (2 * 2)); iowrite32(tmp,led_green.PUPDR); //以下設定置位暫存器：預設輸出高電平 tmp = ioread32(led_green.BSRR); tmp |= (0x1 << 2); iowrite32(tmp, led_green.BSRR); //獲取rgb_led節點的藍燈子節點 led_blue.device_node = of_find_node_by_name(rgb_led_device_node,"led_blue"); if (led_blue.device_node == NULL) { printk(KERN_ERR "\n get rgb_led_blue_device_node failed ! \n"); return -1; } //以下獲取裝置節點中藍燈子節點的reg屬性並轉化為虛擬地址 led_blue.MODER = of_iomap(led_blue.device_node, 0); led_blue.OTYPER = of_iomap(led_blue.device_node, 1); led_blue.OSPEEDR = of_iomap(led_blue.device_node, 2); led_blue.PUPDR = of_iomap(led_blue.device_node, 3); led_blue.BSRR = of_iomap(led_blue.device_node, 4); //以下設定模式暫存器：輸出模式 tmp = ioread32(led_blue.MODER); tmp &= ~(0x3 << (5 * 2)); tmp |= (0x1 << (5 * 2)); iowrite32(tmp,led_blue.MODER); //以下設定輸出型別暫存器：推輓模式 tmp = ioread32(led_blue.OTYPER); tmp &= ~(0x1 << 5); iowrite32(tmp, led_blue.OTYPER); //以下設定輸出速度暫存器：高速 tmp = ioread32(led_blue.OSPEEDR); tmp &= ~(0x3 << (5 * 2)); tmp |= (0x2 << (5 * 2)); iowrite32(tmp, led_blue.OSPEEDR); //以下設定上下拉暫存器：上拉 tmp = ioread32(led_blue.PUPDR); tmp &= ~(0x3 << (5 * 2)); tmp |= (0x1 << (5 * 2)); iowrite32(tmp,led_blue.PUPDR); //以下設定置位暫存器：預設輸出高電平 tmp = ioread32(led_blue.BSRR); tmp |= (0x1 << 5); iowrite32(tmp, led_blue.BSRR); //申請主裝置號 if (alloc_chrdev_region(&devid, 0, 1, "led") < 0) { printk("fail to alloc devid\n"); return -EFAULT; } led_cdev.owner = THIS_MODULE; //繫結前面宣告的file_oprations型別的結構體到字元裝置 cdev_init(&led_cdev, &led_dev_fops); //填充上面申請到的主裝置號到字元裝置 if ( cdev_add(&led_cdev, devid, 1) < 0) { printk("fail to add cdev\n"); return -EFAULT; } //建立一個類 led_class = class_create(THIS_MODULE, "my_leds"); //建立一個裝置節點 device_create(led_class, NULL, devid, NULL, "led"); printk("platform driver probed!\n"); return 0; } //remove函式中,刪除裝置並釋放裝置號 static int led_pdrv_remove(struct platform_device *pdev) { //以下實現各個暫存器的解除對映 iounmap(clkaddr); iounmap(led_green.MODER); iounmap(led_green.OTYPER); iounmap(led_green.OSPEEDR); iounmap(led_green.PUPDR); iounmap(led_green.BSRR); iounmap(led_red.MODER); iounmap(led_red.OTYPER); iounmap(led_red.OSPEEDR); iounmap(led_red.PUPDR); iounmap(led_red.BSRR); iounmap(led_blue.MODER); iounmap(led_blue.OTYPER); iounmap(led_blue.OSPEEDR); iounmap(led_blue.PUPDR); iounmap(led_blue.BSRR); unregister_chrdev_region(devid, 1); //釋放主裝置號 cdev_del(&led_cdev); //刪除字元裝置 device_destroy(led_class, devid); //銷燬裝置節點 class_destroy(led_class); //銷燬類 printk("platform driver removed!\n"); return 0; } //填充of_device_id結構體，名為rgb_led，用於指明匹配表 static const struct of_device_id rgb_led[] = { {.compatible = "fire,rgb_led"}, //匹配內容 {/* sentinel */} }; //以下填充一個platform_driver結構體 struct platform_driver led_platform_driver = { .probe = led_pdrv_probe, //指定probe函式成員 .remove = led_pdrv_remove, //指定remove函式成員 .driver = { .name = "rgb-leds-platform", //指定裝置名稱 .owner = THIS_MODULE, .of_match_table = rgb_led, //指定匹配表名稱 } }; //以下定義模組的入口函式 static int __init led_pdrv_init(void) { platform_driver_register(&led_platform_driver);//註冊一個platform驅動 printk("led platform driver initted!\n"); return 0; } //以下定義模組的出口函式 static void __exit led_pdrv_exit(void) { platform_driver_unregister(&led_platform_driver); //釋放一個platform驅動 printk("led platform driver exited!\n"); } module_init(led_pdrv_init); module_exit(led_pdrv_exit); MODULE_LICENSE("GPL");
配套的Makefile檔案內容如下。
KERNEL_DIR=/opt/ebf_linux_kernel_mp157_depth1/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 clean: $(MAKE) -C $(KERNEL_DIR) M=$(CURDIR) clean
以下是測試用的應用程式程式碼，檔名為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 = 0; fd = open("/dev/led", O_RDWR); //開啟裝置節點 if( fd < 0 ) printf("can`t open\n"); if( argc != 3 ) //命令引數不對時提示 { 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; //值為0時全部點亮 else val = 7; //值為7時全部熄滅 } else if(strcmp(argv[1], "red") == 0) { if(strcmp(argv[2], "on") == 0) val = 1; //值為1時紅色點亮 else val = 4; //值為4時紅色熄滅 } else if(strcmp(argv[1], "green") == 0) { if(strcmp(argv[2], "on") == 0) val = 2; //值為2時綠色點亮 else val = 5; //值為5時綠色熄滅 } else if(strcmp(argv[1], "blue") == 0) { if(strcmp(argv[2], "on") == 0) val = 3; //值為3時藍色點亮 else val = 6; //值為6時藍色熄滅 } write(fd, &val, 1); //把值寫入裝置節點 close(fd); //關閉裝置節點 return 0; }
完成後，先執行make命令編譯驅動程式，若成功會生成名為led.ko的驅動模組檔案。然後對應用程式進行交叉編譯，執行“arm-linux-gnueabihf-gcc app.c -o app”即可。實驗結果與“嵌入式Linux中的LED驅動控制”一文中的完全一樣，這裡就不給出了。
--待續--