上篇文章我们了解了子系统的框架，下面我们来分析驱动框架中每层的实现以及作用。
在LED子系统中，硬件驱动层相关文件在包括：kernel/drivers/leds/ 目录下，其主要的函数有：led-gpio.c、led-xxx.c，其中led-gpio.c为通用的平台驱动程序，led-xxx.c为不同厂家提供的平台驱动程序。
我们在这里主要分析
led-gpio.c
打开该文件，直接找到加载驱动的入口函数gpio_led_probe
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：LED的平台数据，用于对LED硬件设备的统一管理
这个结构体用于父节点向子节点传递的数据时使用
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
主要作用：LED驱动的私有数据类型，管理全部的LED设备。
这里的
num_leds通过解析设备树的子节点的个数来获取
leds[]根据获取的num_leds个数，分配对应的空间，来初始化相关数据
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 < priv->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); // 创建LED设备
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，硬件设备和驱动程序匹配后，第一个执行的函数。
实现思路：
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，并且使用create_gpio_led进行初始化LED的数量为零，则使用gpio_leds_create创建LED。数据结构：该函数主要包括了两个数据结构gpio_led_platform_data和gpio_leds_priv
/* 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：LED的硬件描述结构，包括名称，GPIO编号，有效电平等等信息。
该结构体的信息大多由解析设备树获得，将设备树中
label解析为name，gpios解析为gpiod，linux,default-trigger解析为default_trigger等
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：LED相关数据信息，主要在于led_classdev，用于注册设备节点信息
由设备树解析出来的
gpio_led，然后将部分属性赋值到gpio_led_data中，并且初始化led_classdev相关属性，并且实现led_classdev结构体中的部分函数。
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 < 0) {
fwnode_handle_put(child);
return ERR_PTR(ret);
}
led_dat->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：
device_get_child_node_count获取设备树中LED子节点的数量，根据获取到的子节点数量，分配LED设备对应的内存空间device_for_each_child_node遍历每个子节点，并为每个子节点创建对应的LED设备fwnode_property_read_string接口，读取设备树中相关的属性信息，如：label、linux,default-trigger等，将这些信息赋值给gpio_led结构体中LED，调用create_gpio_led进行设备的创建该数据结构属于核心层，在硬件驱动层需要与其进行关联，遂在此介绍。
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：该结构体所包括的内容较多，主要有以下几个功能
brightness当前亮度值，max_brightness最大亮度LED闪烁功能控制：blink_timer、blink_brightness、new_blink_brightness等attribute_group：创建sysfs文件节点，向上提供用户访问接口由上面可知，在创建
gpio_led_data时，顺便初始化led_classdev结构体，赋值相关属性以及部分回调函数，最终将led_classdev注册进入LED子系统框架中，在sysfs中创建对应的文件节点。
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 < 0)
return ret;
led_dat->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 < 0)
return state;
} else {
state = (template->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 < 0)
return ret;
return devm_of_led_classdev_register(parent, np, &led_dat->cdev); // 将LED设备注册到子系统中
}
函数介绍：create_gpio_led创建LED设备的核心函数
实现思路：
gpio_is_valid接口，判断GPIO是否合法gpio_led_data字段中，并且初始化部分字段，如：led_classdev、gpio_desc等gpio_led_set、gpio_led_set_blocking、gpio_blink_set等devm_of_led_classdev_register接口，将LED设备注册到LED框架之中。硬件驱动层，肯定包括最终操作硬件的部分，也就是上面提到的一些回调函数，属于我们驱动工程师开发的内容。
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
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 是不可睡眠的，gpio_led_set_blocking是可休眠的。
上面我们了解了硬件驱动层的实现流程以及相关数据结构，总结来看：
gpio_led_probe(drivers/leds/leds-gpio.c)
|--> gpio_leds_create
|--> create_gpio_led // 创建LED设备
|--> devm_of_led_classdev_register
LED相关属性信息，赋值给gpio_led结构体gpio_led、gpio_leds_priv、led_classdev等数据结构关联起来LED设备注册进入LED子系统中
