Linux驱动开发新手必读 | 二、LED子系统——硬件驱动层

在led子系统中，硬件驱动层是关键的一部分，负责管理led设备的具体实现。本文将详细介绍led子系统硬件驱动层的实现流程及相关数据结构。

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LED子系统的硬件驱动层文件主要位于kernel/drivers/leds/目录下，包含的主要函数有led-gpio.c和led-xxx.c。其中，led-gpio.c是通用的平台驱动程序，而led-xxx.c则是不同厂商提供的特定平台驱动程序。

1、gpio_led_probe函数分析

打开led-gpio.c文件，直接找到加载驱动的入口函数gpio_led_probe。

1.1 相关数据结构

1.1.1 gpio_led_platform_data

struct gpio_led_platform_data {    int   num_leds;    const struct gpio_led *leds;    #define GPIO_LED_NO_BLINK_LOW 0 /* No blink GPIO state low */    #define GPIO_LED_NO_BLINK_HIGH 1 /* No blink GPIO state high */    #define GPIO_LED_BLINK  2 /* Please, blink */    gpio_blink_set_t gpio_blink_set;};

结构体名称：gpio_led_platform_data

文件位置：include/linux/leds.h

主要作用：用于LED的平台数据，统一管理LED硬件设备。

1.1.2 gpio_leds_priv

struct gpio_leds_priv {    int num_leds;    struct gpio_led_data leds[];};

结构体名称：gpio_leds_priv

文件位置：drivers/leds/leds-gpio.c

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主要作用：LED驱动的私有数据类型，管理所有LED设备。

1.2 实现流程

static int gpio_led_probe(struct platform_device *pdev){    struct gpio_led_platform_data *pdata = dev_get_platdata(&pdev->dev);  // 检索设备的平台数据    struct gpio_leds_priv *priv;    int i, ret = 0;    if (pdata && pdata->num_leds) {            // 判断平台数据LED数量        priv = devm_kzalloc(&pdev->dev,                sizeof_gpio_leds_priv(pdata->num_leds),                    GFP_KERNEL);        if (!priv)            return -ENOMEM;        priv->num_leds = pdata->num_leds;        for (i = 0; i num_leds; i++) {            ret = create_gpio_led(&pdata->leds[i], &priv->leds[i],                          &pdev->dev, NULL,                          pdata->gpio_blink_set);            if (ret < 0)                return ret;        }    } else {        priv = gpio_leds_create(pdev);        if (IS_ERR(priv))            return PTR_ERR(priv);    }    platform_set_drvdata(pdev, priv);    return 0;}

函数介绍：gpio_led_probe是LED驱动的入口函数，也是LED子系统中硬件设备与驱动程序匹配后执行的第一个函数。

实现思路：

使用dev_get_platdata检索设备的平台数据。如果平台数据中的LED数量大于零，则使用devm_kzalloc为其分配内存空间，并使用create_gpio_led进行初始化。如果平台数据不存在或LED数量为零，则使用gpio_leds_create创建LED。最后，设置驱动程序数据，并返回0表示操作成功。

数据结构：该函数主要涉及两个数据结构gpio_led_platform_data和gpio_leds_priv。

2、gpio_leds_create函数分析

2.1 相关数据结构

2.1.1 gpio_led

/* For the leds-gpio driver */struct gpio_led {    const char *name;     // LED名称    const char *default_trigger;  // 默认触发类型    unsigned  gpio;     // GPIO编号    unsigned active_low : 1;   // 低电平有效    unsigned retain_state_suspended : 1;    unsigned panic_indicator : 1;    unsigned default_state : 2;  // 默认状态    unsigned retain_state_shutdown : 1;    /* default_state should be one of LEDS_GPIO_DEFSTATE_(ON|OFF|KEEP) */    struct gpio_desc *gpiod;   // GPIO Group};

结构体名称：gpio_led

文件位置：include/linux/leds.h

主要作用：描述LED的硬件信息，包括名称、GPIO编号、有效电平等。

2.1.2 gpio_led_data

struct gpio_led_data {    struct led_classdev cdev;  // LED Class    struct gpio_desc *gpiod;  // GPIO description    u8 can_sleep;    u8 blinking;     // 闪烁    gpio_blink_set_t platform_gpio_blink_set; // 闪烁设置};

结构体名称：gpio_led_data

文件位置：drivers/leds/leds-gpio.c

主要作用：存储LED相关数据信息，主要包括led_classdev，用于注册设备节点信息。

2.2 实现流程

static struct gpio_leds_priv *gpio_leds_create(struct platform_device *pdev){    struct device *dev = &pdev->dev;    struct fwnode_handle *child;    struct gpio_leds_priv *priv;    int count, ret;    count = device_get_child_node_count(dev);  // 获取子节点数量    if (!count)        return ERR_PTR(-ENODEV);    priv = devm_kzalloc(dev, sizeof_gpio_leds_priv(count), GFP_KERNEL);    if (!priv)        return ERR_PTR(-ENOMEM);    device_for_each_child_node(dev, child) {        struct gpio_led_data *led_dat = &priv->leds[priv->num_leds]; // 与gpio_leds_priv结构体关联        struct gpio_led led = {};        const char *state = NULL;        struct device_node *np = to_of_node(child);        ret = fwnode_property_read_string(child, "label", &led.name); // 读设备树属性，赋值gpio_led结构体        if (ret && IS_ENABLED(CONFIG_OF) && np)            led.name = np->name;        if (!led.name) {            fwnode_handle_put(child);            return ERR_PTR(-EINVAL);        }        led.gpiod = devm_fwnode_get_gpiod_from_child(dev, NULL, child,                                 GPIOD_ASIS,                                 led.name);        if (IS_ERR(led.gpiod)) {            fwnode_handle_put(child);            return ERR_CAST(led.gpiod);        }        fwnode_property_read_string(child, "linux,default-trigger",                        &led.default_trigger);        if (!fwnode_property_read_string(child, "default-state",                         &state)) {            if (!strcmp(state, "keep"))                led.default_state = LEDS_GPIO_DEFSTATE_KEEP;            else if (!strcmp(state, "on"))                led.default_state = LEDS_GPIO_DEFSTATE_ON;            else                led.default_state = LEDS_GPIO_DEFSTATE_OFF;        }        if (fwnode_property_present(child, "retain-state-suspended"))            led.retain_state_suspended = 1;        if (fwnode_property_present(child, "retain-state-shutdown"))            led.retain_state_shutdown = 1;        if (fwnode_property_present(child, "panic-indicator"))            led.panic_indicator = 1;        ret = create_gpio_led(&led, led_dat, dev, np, NULL); // 将gpio_led结构体、gpio_led_data关联起来        if (ret cdev.dev->of_node = np;        priv->num_leds++;    }    return priv;}

函数介绍：gpio_leds_create主要用于创建LED设备。

实现思路：

使用device_get_child_node_count获取设备树中LED子节点的数量，并根据数量分配LED设备对应的内存空间。通过device_for_each_child_node遍历每个子节点，为每个子节点创建对应的LED设备。对于每个子节点，使用fwnode_property_read_string接口读取设备树中的相关属性信息，如label、linux,default-trigger等，并将这些信息赋值给gpio_led结构体。最后，调用create_gpio_led进行设备的创建。

3、create_gpio_led函数分析

3.1 相关数据结构

3.1.1 led_classdev

struct led_classdev {    const char  *name;    enum led_brightness  brightness;    enum led_brightness  max_brightness;    int    flags;    /* Lower 16 bits reflect status */    #define LED_SUSPENDED  BIT(0)    #define LED_UNREGISTERING BIT(1)    /* Upper 16 bits reflect control information */    #define LED_CORE_SUSPENDRESUME BIT(16)    #define LED_SYSFS_DISABLE BIT(17)    #define LED_DEV_CAP_FLASH BIT(18)    #define LED_HW_PLUGGABLE BIT(19)    #define LED_PANIC_INDICATOR BIT(20)    #define LED_BRIGHT_HW_CHANGED BIT(21)    #define LED_RETAIN_AT_SHUTDOWN BIT(22)    /* set_brightness_work / blink_timer flags, atomic, private. */    unsigned long  work_flags;    #define LED_BLINK_SW   0    #define LED_BLINK_ONESHOT  1    #define LED_BLINK_ONESHOT_STOP  2    #define LED_BLINK_INVERT  3    #define LED_BLINK_BRIGHTNESS_CHANGE  4    #define LED_BLINK_DISABLE  5    /* Set LED brightness level     * Must not sleep. Use brightness_set_blocking for drivers     * that can sleep while setting brightness.     */    void  (*brightness_set)(struct led_classdev *led_cdev,                      enum led_brightness brightness);    /*      * Set LED brightness level immediately - it can block the caller for     * the time required for accessing a LED device register.     */    int (*brightness_set_blocking)(struct led_classdev *led_cdev,                       enum led_brightness brightness);    /* Get LED brightness level */    enum led_brightness (*brightness_get)(struct led_classdev *led_cdev);    /*     * Activate hardware accelerated blink, delays are in milliseconds     * and if both are zero then a sensible default should be chosen.     * The call should adjust the timings in that case and if it can't     * match the values specified exactly.     * Deactivate blinking again when the brightness is set to LED_OFF     * via the brightness_set() callback.     */    int  (*blink_set)(struct led_classdev *led_cdev,                     unsigned long *delay_on,                     unsigned long *delay_off);    struct device  *dev;    const struct attribute_group **groups;    struct list_head  node;   /* LED Device list */    const char  *default_trigger; /* Trigger to use */    unsigned long   blink_delay_on, blink_delay_off;    struct timer_list  blink_timer;    int    blink_brightness;    int    new_blink_brightness;    void   (*flash_resume)(struct led_classdev *led_cdev);    struct work_struct set_brightness_work;    int   delayed_set_value;    #ifdef CONFIG_LEDS_TRIGGERS    /* Protects the trigger data below */    struct rw_semaphore  trigger_lock;    struct led_trigger *trigger;    struct list_head  trig_list;    void   *trigger_data;    /* true if activated - deactivate routine uses it to do cleanup */    bool   activated;    #endif    #ifdef CONFIG_LEDS_BRIGHTNESS_HW_CHANGED    int    brightness_hw_changed;    struct kernfs_node *brightness_hw_changed_kn;    #endif    /* Ensures consistent access to the LED Flash Class device */    struct mutex  led_access;};

结构体名称：led_classdev

文件位置：include/linux/leds.h

主要作用：该结构体包含多个功能，如：

brightness：当前亮度值max_brightness：最大亮度值LED闪烁功能控制：blink_timer、blink_brightness、new_blink_brightness等attribute_group：创建sysfs文件节点，提供用户访问接口

3.2 实现流程

static int create_gpio_led(const struct gpio_led *template,    struct gpio_led_data *led_dat, struct device *parent,    struct device_node *np, gpio_blink_set_t blink_set){    int ret, state;    led_dat->gpiod = template->gpiod;    if (!led_dat->gpiod) {        /*          * This is the legacy code path for platform code that          * still uses GPIO numbers. Ultimately we would like to get          * rid of this block completely.         */        unsigned long flags = GPIOF_OUT_INIT_LOW;        /* skip leds that aren't available */        if (!gpio_is_valid(template->gpio)) {        // 判断是否gpio合法            dev_info(parent, "Skipping unavailable LED gpio %d (%s)n",                    template->gpio, template->name);            return 0;        }        if (template->active_low)            flags |= GPIOF_ACTIVE_LOW;        ret = devm_gpio_request_one(parent, template->gpio, flags,                        template->name);        if (ret gpiod = gpio_to_desc(template->gpio);      // 获取gpio组        if (!led_dat->gpiod)            return -EINVAL;    }    led_dat->cdev.name = template->name;         // 赋值一些属性信息    led_dat->cdev.default_trigger = template->default_trigger;    led_dat->can_sleep = gpiod_cansleep(led_dat->gpiod);    if (!led_dat->can_sleep)        led_dat->cdev.brightness_set = gpio_led_set;      // 设置LED    else        led_dat->cdev.brightness_set_blocking = gpio_led_set_blocking;    led_dat->blinking = 0;    if (blink_set) {        led_dat->platform_gpio_blink_set = blink_set;        led_dat->cdev.blink_set = gpio_blink_set;    }    if (template->default_state == LEDS_GPIO_DEFSTATE_KEEP) {        state = gpiod_get_value_cansleep(led_dat->gpiod);        if (state default_state == LEDS_GPIO_DEFSTATE_ON);    }    led_dat->cdev.brightness = state ? LED_FULL : LED_OFF;    if (!template->retain_state_suspended)        led_dat->cdev.flags |= LED_CORE_SUSPENDRESUME;    if (template->panic_indicator)        led_dat->cdev.flags |= LED_PANIC_INDICATOR;    if (template->retain_state_shutdown)        led_dat->cdev.flags |= LED_RETAIN_AT_SHUTDOWN;    ret = gpiod_direction_output(led_dat->gpiod, state);    if (ret cdev);  // 将LED设备注册到子系统中}

函数介绍：create_gpio_led是创建LED设备的核心函数。

实现思路：

通过gpio_is_valid接口判断GPIO是否合法。将从设备树解析出来的信息填充到gpio_led_data字段中，并初始化部分字段，如led_classdev、gpio_desc等。填充回调函数，实现相应的动作，如gpio_led_set、gpio_led_set_blocking、gpio_blink_set等。最后调用devm_of_led_classdev_register接口，将LED设备注册到LED框架中。

4、回调函数分析

4.1 gpio_blink_set

static int gpio_blink_set(struct led_classdev *led_cdev,    unsigned long *delay_on, unsigned long *delay_off){    struct gpio_led_data *led_dat = cdev_to_gpio_led_data(led_cdev);    led_dat->blinking = 1;    return led_dat->platform_gpio_blink_set(led_dat->gpiod, GPIO_LED_BLINK,                        delay_on, delay_off);}

函数介绍：gpio_blink_set主要用于设置闪烁的时延。

4.2 gpio_led_set 和 gpio_led_set_blocking

static inline struct gpio_led_data *            cdev_to_gpio_led_data(struct led_classdev *led_cdev){    return container_of(led_cdev, struct gpio_led_data, cdev);}static void gpio_led_set(struct led_classdev *led_cdev,    enum led_brightness value){    struct gpio_led_data *led_dat = cdev_to_gpio_led_data(led_cdev);    int level;    if (value == LED_OFF)        level = 0;    else        level = 1;    if (led_dat->blinking) {        led_dat->platform_gpio_blink_set(led_dat->gpiod, level,                         NULL, NULL);        led_dat->blinking = 0;    } else {        if (led_dat->can_sleep)            gpiod_set_value_cansleep(led_dat->gpiod, level);        else            gpiod_set_value(led_dat->gpiod, level);    }}static int gpio_led_set_blocking(struct led_classdev *led_cdev,    enum led_brightness value){    gpio_led_set(led_cdev, value);    return 0;}

函数介绍：gpio_led_set 和 gpio_led_set_blocking主要用于设置亮度，区别在于gpio_led_set不可睡眠，而gpio_led_set_blocking可休眠。

5、总结

以上我们了解了硬件驱动层的实现流程以及相关数据结构，总结如下：

5.1 数据结构之间的关系如下

LED子系统-LED数据结构.drawio

5.2 函数实现流程如下

gpio_led_probe(drivers/leds/leds-gpio.c)    |--> gpio_leds_create        |--> create_gpio_led            //  创建LED设备            |--> devm_of_led_classdev_register      

5.3 主要作用如下

从设备树获取LED相关属性信息，赋值给gpio_led结构体。将gpio_led、gpio_leds_priv、led_classdev等数据结构关联起来。将LED设备注册进入LED子系统中。

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