【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
- 对上，面对应用层提供统一的设备节点入口
- 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。
- 对下，面对硬件层提供背光控制调节的回调接口
- 监听 frambuffer 事件, 实现清屏联动背光控制
- 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
- 完成背光设备逻辑类的创建
- 初始化用于通知背光设备驱动加载卸载的事件
- 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）。
- 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。
- 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。
- 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");
