Linux驅動開發新手必讀|二、LED子系統——硬件驅動層

二、LED子系統——硬件驅動層

上篇文章我們了解了子系統的框架，下面我們來分析驅動框架中每層的實現以及作用。

在LED子系統中，硬件驅動層相關文件在包括：kernel/drivers/leds/ 目錄下，其主要的函數有：led-gpio.c、led-xxx.c，其中led-gpio.c為通用的平臺驅動程序，led-xxx.c為不同廠家提供的平臺驅動程序。

我們在這里主要分析led-gpio.c

1、gpio_led_probe分析

打開該文件，直接找到加載驅動的入口函數gpio_led_probe

1.1 相關數據結構

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;
};

結構體名稱：gpio_led_platform_data

文件位置：include/linux/leds.h

主要作用：LED的平臺數據，用于對LED硬件設備的統一管理

這個結構體用于父節點向子節點傳遞的數據時使用

1.1.2 gpio_leds_priv

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

結構體名稱：gpio_leds_priv

文件位置：drivers/leds/leds-gpio.c

主要作用：LED驅動的私有數據類型，管理全部的LED設備。

這里的num_leds通過解析設備樹的子節點的個數來獲取

leds[]根據獲取的num_leds個數，分配對應的空間，來初始化相關數據

1.2 實現流程

static?int?gpio_led_probe(struct?platform_device?*pdev)
{
????struct?gpio_led_platform_data?*pdata?=?dev_get_platdata(&pdev->dev);??//?檢索設備的平臺數據
????struct?gpio_leds_priv?*priv;
????int?i,?ret?=?0;

????if?(pdata?&&?pdata->num_leds)?{????????????//?判斷平臺數據LED數量
????????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);???????????//?創建LED設備?
????????if?(IS_ERR(priv))
????????????return?PTR_ERR(priv);
????}

????platform_set_drvdata(pdev,?priv);

????return?0;
}

函數介紹：gpio_led_probe是LED驅動的入口函數，也是LED子系統中，硬件設備和驅動程序匹配后，第一個執行的函數。

實現思路：

1. 通過dev_get_platdata檢索設備的平臺數據，如果平臺數據中的LED數量大于零，則使用devm_kzalloc為其分配內存空間，并且使用create_gpio_led進行初始化
2. 如果平臺數據不存在或LED的數量為零，則使用gpio_leds_create創建LED。
3. 最后，設置驅動程序數據，并返回0，表示操作成功。

數據結構：該函數主要包括了兩個數據結構gpio_led_platform_data和gpio_leds_priv

2、gpio_leds_create分析

2.1 相關數據結構

2.1.1 gpio_led

/*?For?the?leds-gpio?driver?*/
struct?gpio_led?{
????const?char?*name;?????//?LED名稱
????const?char?*default_trigger;??//?默認觸發類型?
????unsigned??gpio;?????//?GPIO編號
????unsigned?active_low?:?1;???//?低電平有效
????unsigned?retain_state_suspended?:?1;
????unsigned?panic_indicator?:?1;
????unsigned?default_state?:?2;??//?默認狀態
????unsigned?retain_state_shutdown?:?1;
????/*?default_state?should?be?one?of?LEDS_GPIO_DEFSTATE_(ON|OFF|KEEP)?*/
????struct?gpio_desc?*gpiod;???//?GPIO?Group
};

結構體名稱：gpio_led

文件位置：include/linux/leds.h

主要作用：LED的硬件描述結構，包括名稱，GPIO編號，有效電平等等信息。

該結構體的信息大多由解析設備樹獲得，將設備樹中label解析為name，gpios解析為gpiod，linux,default-trigger解析為default_trigger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;?????//?閃爍
????gpio_blink_set_t?platform_gpio_blink_set;?//?閃爍設置
};

結構體名稱：gpio_led_data

文件位置：drivers/leds/leds-gpio.c

主要作用：LED相關數據信息，主要在于led_classdev，用于注冊設備節點信息

由設備樹解析出來的gpio_led，然后將部分屬性賦值到gpio_led_data中，并且初始化led_classdev相關屬性，并且實現led_classdev結構體中的部分函數。

2.2 實現流程

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);??//?獲取子節點數量
????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];?//?與gpio_leds_priv結構體關聯
????????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);?//?讀設備樹屬性，賦值gpio_led結構體
????????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);?//?將gpio_led結構體、gpio_led_data關聯起來
????????if?(ret?0)?{
????????????fwnode_handle_put(child);
????????????return?ERR_PTR(ret);
????????}
????????led_dat->cdev.dev->of_node?=?np;
????????priv->num_leds++;
????}

????return?priv;
}

函數介紹：gpio_leds_create主要用于創建LED設備。

實現思路：

1. 通過device_get_child_node_count獲取設備樹中LED子節點的數量，根據獲取到的子節點數量，分配LED設備對應的內存空間
2. 通過device_for_each_child_node遍歷每個子節點，并為每個子節點創建對應的LED設備
3. 對于每個子節點，使用fwnode_property_read_string接口，讀取設備樹中相關的屬性信息，如：label、linux,default-trigger等，將這些信息賦值給gpio_led結構體中
4. 最后將遍歷的每個LED，調用create_gpio_led進行設備的創建

3、create_gpio_led分析

3.1 相關數據結構

3.1.1 led_classdev

該數據結構屬于核心層，在硬件驅動層需要與其進行關聯，遂在此介紹。

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;
};

結構體名稱：led_classdev

文件位置：include/linux/leds.h

主要作用：該結構體所包括的內容較多，主要有以下幾個功能

• brightness當前亮度值，max_brightness最大亮度
• LED閃爍功能控制：blink_timer、blink_brightness、new_blink_brightness等
• attribute_group：創建sysfs文件節點，向上提供用戶訪問接口

由上面可知，在創建gpio_led_data時，順便初始化 led_classdev結構體，賦值相關屬性以及部分回調函數，最終將led_classdev注冊進入LED子系統框架中，在sysfs中創建對應的文件節點。

3.2 實現流程

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))?{????????//?判斷是否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);??????//?獲取gpio組
????????if?(!led_dat->gpiod)
????????????return?-EINVAL;
????}

????led_dat->cdev.name?=?template->name;?????????//?賦值一些屬性信息
????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;??????//?設置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);??//?將LED設備注冊到子系統中
}

函數介紹：create_gpio_led創建LED設備的核心函數

實現思路：

1. 先通過gpio_is_valid接口，判斷GPIO是否合法
2. 將上層從設備樹解析出來的信息，填充到gpio_led_data字段中，并且初始化部分字段，如：led_classdev、gpio_desc等
3. 填充回調函數，實現相應的動作，如：gpio_led_set、gpio_led_set_blocking、gpio_blink_set等
4. 最后調用devm_of_led_classdev_register接口，將LED設備注冊到LED框架之中。

4、回調函數分析

硬件驅動層，肯定包括最終操作硬件的部分，也就是上面提到的一些回調函數，屬于我們驅動工程師開發的內容。

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);
}

函數介紹：gpio_blink_set主要用于設置閃爍的時延

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;
}

函數介紹：gpio_led_set 和gpio_led_set_blocking主要用于設置亮度，區別在于gpio_led_set 是不可睡眠的，gpio_led_set_blocking是可休眠的。

5、總結

上面我們了解了硬件驅動層的實現流程以及相關數據結構，總結來看：

5.1 數據結構之間的關系如下

5.2 函數實現流程如下

gpio_led_probe(drivers/leds/leds-gpio.c)
????|-->?gpio_leds_create
????????|-->?create_gpio_led????????????//??創建LED設備
????????????|-->?devm_of_led_classdev_register??????

5.3 主要作用如下

1. 從設備樹獲取LED相關屬性信息，賦值給gpio_led結構體
2. 將gpio_led、gpio_leds_priv、led_classdev等數據結構關聯起來
3. 將LED設備注冊進入LED子系統中