【分析笔记】Linux 4.9 backlight 子系统分析

相关信息

内核版本：Linux version 4.9.56
驱动文件：lichee\linux-4.9\drivers\video\backlight\backlight.c

驱动作用

1. 对上，面对应用层提供统一的设备节点入口
2. 同级，面对驱动层提供设备驱动加载卸载通知事件，以及背光控制接口。
3. 对下，面对硬件层提供背光控制调节的回调接口
4. 监听 frambuffer 事件, 实现清屏联动背光控制
5. 监听系统休眠唤醒，实现休眠唤醒背光联动控制

控制背光的来源：应用访问、事件联动、休眠唤醒

源码分析

一、驱动初始化

1. 完成背光设备逻辑类的创建
2. 初始化用于通知背光设备驱动加载卸载的事件
3. 为该逻辑类设定五个背光控制相关的设备节点

/sys/class/backlight/xxx/type
/sys/class/backlight/xxx/bl_power
/sys/class/backlight/xxx/brightness
/sys/class/backlight/xxx/max_brightness
/sys/class/backlight/xxx/actual_brightness

代码解析

static int __init backlight_class_init(void)
{
	// 创建背光设备逻辑类: /sys/class/backlight
	backlight_class = class_create(THIS_MODULE, "backlight");
	if (IS_ERR(backlight_class)) {
		pr_warn("Unable to create backlight class; errno = %ld\n", PTR_ERR(backlight_class));
		return PTR_ERR(backlight_class);
	}

	// 面对应用层提供统一的设备节点入口(预先设置好节点名称和可读写权限)
	backlight_class->dev_groups = bl_device_groups;
	// 监听系统的休眠唤醒回调
	backlight_class->pm = &backlight_class_dev_pm_ops;

	// 初始化背光设备链表，用于被外部驱动查询获取
	INIT_LIST_HEAD(&backlight_dev_list);
	mutex_init(&backlight_dev_list_mutex);

	// 初始化内核通知链, 用于通知驱动层背光设备注册或卸载事件
	BLOCKING_INIT_NOTIFIER_HEAD(&backlight_notifier);

	return 0;
}

ATTRIBUTE_GROUPS 宏定义解析：

直接搜索源码是找不到 bl_device_groups 关键词的，实际该变量是通过宏进行定义

static struct attribute *bl_device_attrs[] = {
	&dev_attr_bl_power.attr,
	&dev_attr_brightness.attr,
	&dev_attr_actual_brightness.attr,
	&dev_attr_max_brightness.attr,
	&dev_attr_type.attr,
	NULL,
};
ATTRIBUTE_GROUPS(bl_device);

宏定义：include\linux\sysfs.h

#define __ATTRIBUTE_GROUPS(_name)				\
static const struct attribute_group *_name##_groups[] = {	\
	&_name##_group,						\
	NULL,							\
}

#define ATTRIBUTE_GROUPS(_name)					\
static const struct attribute_group _name##_group = {		\
	.attrs = _name##_attrs,					\
};								\
__ATTRIBUTE_GROUPS(_name)

宏定义展开：ATTRIBUTE_GROUPS(bl_device)

#define __ATTRIBUTE_GROUPS(bl_device)				\
static const struct attribute_group *bl_device_groups[] = {	\
	&bl_device_group,						\
	NULL,							\
}

#define ATTRIBUTE_GROUPS(bl_device)					\
static const struct attribute_group bl_device_group = {		\
	.attrs = bl_device_attrs,					\
};								\
__ATTRIBUTE_GROUPS(bl_device)

宏替换后的效果

static struct attribute *bl_device_attrs[] = {
	&dev_attr_bl_power.attr,
	&dev_attr_brightness.attr,
	&dev_attr_actual_brightness.attr,
	&dev_attr_max_brightness.attr,
	&dev_attr_type.attr,
	NULL,
};

static const struct attribute_group bl_device_group = {
	.attrs = bl_device_attrs,
};

static const struct attribute_group *bl_device_groups[] = {
	&bl_device_group,
	NULL,
};

DEVICE_ATTR_RW 宏定义解析：

static ssize_t brightness_store(struct device *dev,
		struct device_attribute *attr, const char *buf, size_t count)
{
	int rc;
	struct backlight_device *bd = to_backlight_device(dev);
	unsigned long brightness;

	rc = kstrtoul(buf, 0, &brightness);
	if (rc)
		return rc;

	rc = backlight_device_set_brightness(bd, brightness);

	return rc ? rc : count;
}
static DEVICE_ATTR_RW(brightness);

宏定义：include\linux\device.h include\linux\sysfs.h

#define DEVICE_ATTR_RW(_name) \
	struct device_attribute dev_attr_##_name = __ATTR_RW(_name)

#define __ATTR_RW(_name) __ATTR(_name, (S_IWUSR | S_IRUGO),		\
			 _name##_show, _name##_store)

宏定义展开，取其中一个设备节点的例子：static DEVICE_ATTR_RW(brightness)

#define DEVICE_ATTR_RW(brightness) \
	struct device_attribute dev_attr_brightness = __ATTR_RW(brightness)

#define __ATTR_RW(brightness) __ATTR(brightness, (S_IWUSR | S_IRUGO),		\
			 brightness_show, brightness_store)

#define __ATTR(brightness, (S_IWUSR | S_IRUGO), brightness_show, brightness_store) { \
	.attr = {.name = __stringify(brightness),				\
		 .mode = VERIFY_OCTAL_PERMISSIONS((S_IWUSR | S_IRUGO)) },		\
	.show	= brightness_show,						\
	.store	= brightness_store,						\
}

宏替换后的效果

static struct device_attribute dev_attr_brightness = {
	.attr = {
		.name = __stringify(brightness),
		.mode = VERIFY_OCTAL_PERMISSIONS((S_IWUSR | S_IRUGO))
	},
	.show	= brightness_show,
	.store	= brightness_store,
}

二、背光设备驱动注册

假如 name=sunxi，那么调用此接口后将会产生如下几个设备节点：
背光设备类型：/sys/class/backlight/sunxi/type
背光电源控制：/sys/class/backlight/sunxi/bl_power
背光亮度设置：/sys/class/backlight/sunxi/brightness
最大背光亮度：/sys/class/backlight/sunxi/max_brightness
真实背光亮度：/sys/class/backlight/sunxi/actual_brightness

// name:背光设备名称	parent:父设备	devdata:私有数据	ops:背光控制调节的回调		props:默认的背光属性
struct backlight_device *backlight_device_register(const char *name, struct device *parent, void *devdata, const struct backlight_ops *ops, const struct backlight_properties *props)
{
	struct backlight_device *new_bd;
	int rc;

	// 创建背光设备
	new_bd = kzalloc(sizeof(struct backlight_device), GFP_KERNEL);
	if (!new_bd)
		return ERR_PTR(-ENOMEM);

	mutex_init(&new_bd->update_lock);
	mutex_init(&new_bd->ops_lock);

	// 设置设备逻辑类为背光设备逻辑类, 在驱动初始化时完成的创建
	// 这里要注意, struct device 是直接嵌入到 struct backlight_device 里面的
	new_bd->dev.class = backlight_class;
	new_bd->dev.parent = parent;
	new_bd->dev.release = bl_device_release;

	// 设置名称, 体现在: /sys/class/backlight/XXX
	dev_set_name(&new_bd->dev, "%s", name);
	dev_set_drvdata(&new_bd->dev, devdata);

	// 如果有指定的背光属性, 则拷贝并作为默认的背光属性
	if (props) {
		memcpy(&new_bd->props, props, sizeof(struct backlight_properties));
		if (props->type <= 0 || props->type >= BACKLIGHT_TYPE_MAX) {
			WARN(1, "%s: invalid backlight type", name);
			new_bd->props.type = BACKLIGHT_RAW;
		}
	} else {
		new_bd->props.type = BACKLIGHT_RAW;
	}

	// 将此设备注册到系统里面，里面会创建 backlight_class 预设的那几个设备节点
	rc = device_register(&new_bd->dev);
	if (rc) {
		put_device(&new_bd->dev);
		return ERR_PTR(rc);
	}

	// 注册监听 frambuffer 的事件通知链
	rc = backlight_register_fb(new_bd);
	if (rc) {
		device_unregister(&new_bd->dev);
		return ERR_PTR(rc);
	}

	// 指定背光控制接口
	new_bd->ops = ops;

	// 将此设备加入到链表内, 便于外部查询获取
	mutex_lock(&backlight_dev_list_mutex);
	list_add(&new_bd->entry, &backlight_dev_list);
	mutex_unlock(&backlight_dev_list_mutex);

	// 发出事件通知, 有新的背光设备被注册到系统中
	blocking_notifier_call_chain(&backlight_notifier, BACKLIGHT_REGISTERED, new_bd);
	return new_bd;
}
EXPORT_SYMBOL(backlight_device_register);

三、应用层进行背光调节路径

代码解析

static ssize_t brightness_store(struct device *dev,
		struct device_attribute *attr, const char *buf, size_t count)
{
	int rc;
	// 通过 dev 计算出 backlight_device 地址, 取得对应背光设备对象
	struct backlight_device *bd = to_backlight_device(dev);
	unsigned long brightness;

	// 字符串转换为数值
	rc = kstrtoul(buf, 0, &brightness);
	if (rc)
		return rc;

	// 调用设置背光接口, 该接口也被开放给其它驱动程序调用
	rc = backlight_device_set_brightness(bd, brightness);

	return rc ? rc : count;
}
static DEVICE_ATTR_RW(brightness);

int backlight_device_set_brightness(struct backlight_device *bd,
				    unsigned long brightness)
{
	int rc = -ENXIO;

	// 有多个路径调用, 存在并发调用, 借助互斥锁保护
	mutex_lock(&bd->ops_lock);
	// 检查是否设置了回调
	if (bd->ops) {
		// 检查所设置的背光数值是否超出最大值
		if (brightness > bd->props.max_brightness)
			rc = -EINVAL;
		else {
			// 填充要设置的背光值并更新
			bd->props.brightness = brightness;
			backlight_update_status(bd);
			rc = 0;
		}
	}
	mutex_unlock(&bd->ops_lock);

	backlight_generate_event(bd, BACKLIGHT_UPDATE_SYSFS);

	return rc;
}
EXPORT_SYMBOL(backlight_device_set_brightness);

static inline int backlight_update_status(struct backlight_device *bd)
{
	int ret = -ENOENT;

	// 从这里可以看出, 最终调用的背光设备驱动的 ops->update_status() 接口
	mutex_lock(&bd->update_lock);
	if (bd->ops && bd->ops->update_status)
		ret = bd->ops->update_status(bd);
	mutex_unlock(&bd->update_lock);

	return ret;
}

关键解析

这里最主要的是搞清楚是如何区分应用层操作哪个背光设备（这部分实现很有意思，其原理会另写文章解析）。

1. Linux 最常用的是通过结构体嵌套的方式，实现以某个成员的内存地址计算出结构体的首地址，从而访问到其它的成员。
2. struct device 是直接嵌入到 struct backlight_device 里面，就可以通过 struct device 找到对应 struct backlight_device。
3. 应用层通过 /sys/class/backlight/xxx 来访问 xxx 背光设备，就可确定 device, 根据 device 找出 backlight_device，调用对应背光设备驱动的 ops 成员。

struct backlight_device {
	......
	struct device dev;
	......
};
#define to_backlight_device(obj) container_of(obj, struct backlight_device, dev)

四、事件联动进行背光调节路径

在背光设备驱动注册的时候，就调用 backlight_register_fb 来监听 fb 事件。

Xorg 就通过 FBIOBLANK 指令实现熄屏，但若没有对应的背光设备接口，就会出现显示屏全黑但是背光还亮着的问题。

代码解析

static int backlight_register_fb(struct backlight_device *bd)
{
	// 当有背光
	memset(&bd->fb_notif, 0, sizeof(bd->fb_notif));
	bd->fb_notif.notifier_call = fb_notifier_callback;

	return fb_register_client(&bd->fb_notif);
}

// drivers\video\fbdev\core\fb_notify.c
int fb_register_client(struct notifier_block *nb)
{
	return blocking_notifier_chain_register(&fb_notifier_list, nb);
}
EXPORT_SYMBOL(fb_register_client);

static int fb_notifier_callback(struct notifier_block *self,
				unsigned long event, void *data)
{
	struct backlight_device *bd;
	struct fb_event *evdata = data;
	int node = evdata->info->node;
	int fb_blank = 0;

	// 仅对显示空白事件感兴趣
	if (event != FB_EVENT_BLANK && event != FB_EVENT_CONBLANK)
		return 0;

	// 一样的操作, 通过 device 找到 backlight_device
	bd = container_of(self, struct backlight_device, fb_notif);
	mutex_lock(&bd->ops_lock);
	if (bd->ops)
		if (!bd->ops->check_fb ||
		    bd->ops->check_fb(bd, evdata->info)) {
			fb_blank = *(int *)evdata->data;
			if (fb_blank == FB_BLANK_UNBLANK && !bd->fb_bl_on[node]) {
				bd->fb_bl_on[node] = true;
				if (!bd->use_count++) {
					// 如果需要正常显示，则亮起背光
					bd->props.state &= ~BL_CORE_FBBLANK;
					bd->props.fb_blank = FB_BLANK_UNBLANK;
					backlight_update_status(bd);
				}
			} else if (fb_blank != FB_BLANK_UNBLANK && bd->fb_bl_on[node]) {
				bd->fb_bl_on[node] = false;
				if (!(--bd->use_count)) {
					// 如果显示空白画面，则熄灭背光
					bd->props.state |= BL_CORE_FBBLANK;
					bd->props.fb_blank = fb_blank;
					backlight_update_status(bd);
				}
			}
		}
	mutex_unlock(&bd->ops_lock);
	return 0;
}

五、最简单的背光设备驱动

#include <linux/kernel.h>
#include <linux/types.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/err.h>
#include <linux/device.h>
#include <linux/platform_device.h>
#include <linux/backlight.h>

static struct backlight_device *bdev = NULL;

// 更新背光亮度
static int mybl_bl_ops_update_bl(struct backlight_device *bdev)
{
	//......

	return 0;
}

// 获取背光亮度
static int mybl_bl_ops_get_brightness(struct backlight_device *bdev)
{
	//......

	return brightness;
}

static const struct backlight_ops mybl_bl_ops = {
	.update_status	= mybl_bl_ops_update_bl,
	.get_brightness	= mybl_bl_ops_get_brightness,
};

static int mybl_probe(struct platform_device *pdev)
{
	bdev = backlight_device_register("mydriver", &pdev->dev, NULL, &mybl_bl_ops, NULL);
	if(IS_ERR(bdev)){
		return -1;
	}
	return 0;
}

static int mybl_remove(struct platform_device *pdev)
{
	backlight_device_unregister(bdev);
	return 0;
}

static const struct of_device_id mybl_ids[] = {
	{ .compatible = "mybl"},{}
};

static struct platform_driver mybl_driver = {
	.probe	= mybl_probe,
	.remove	= mybl_remove,
	.driver	= {
		.owner	= THIS_MODULE,
		.name = "mybl",
		.of_match_table	= mybl_ids,
	},
};

module_platform_driver(mybl_driver);
MODULE_LICENSE("GPL");