android 休眠唤醒机制分析(二) — early_suspend

early_suspend是Android休眠流程的第一阶段即浅度休眠，不会受到wake_lock的阻止，一般用于关闭lcd、tp等设备为运行的应用节约电能。Android的PowerManagerService会根据用户的操作情况调整电源状态，如果需要休眠则会调用到HAL层的set_screen_state()接口，在set_screen_state()中会向/sys/power/state节点写入"mem"值让驱动层开始进入休眠流程。

一、休眠唤醒机制及其用户空间接口

Linux系统支持如下休眠唤醒等级

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const char *const pm_states[PM_SUSPEND_MAX] = {

#ifdef CONFIG_EARLYSUSPEND

[PM_SUSPEND_ON] = "on",

#endif

[PM_SUSPEND_STANDBY] = "standby",

[PM_SUSPEND_MEM] = "mem",

};

但在Android中一般只支持"on"和"mem"，其中"on"为唤醒设备，"mem"为休眠设备。/sys/power/state节点的读写操作如下：

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static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,

char *buf)

{

char *s = buf;

#ifdef CONFIG_SUSPEND

int i;

for (i = 0; i < PM_SUSPEND_MAX; i++) {

if (pm_states[i] && valid_state(i))

s += sprintf(s,"%s ", pm_states[i]); // 打印系统支持的休眠等级

}

#endif

#ifdef CONFIG_HIBERNATION

s += sprintf(s, "%s\n", "disk");

#else

if (s != buf)

/* convert the last space to a newline */

*(s-1) = '\n';

#endif

return (s - buf);

}

static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr,

const char *buf, size_t n)

{

#ifdef CONFIG_SUSPEND

#ifdef CONFIG_EARLYSUSPEND

suspend_state_t state = PM_SUSPEND_ON;

#else

suspend_state_t state = PM_SUSPEND_STANDBY;

#endif

const char * const *s;

#endif

char *p;

int len;

int error = -EINVAL;

p = memchr(buf, '\n', n);

len = p ? p - buf : n;

/* First, check if we are requested to hibernate */

if (len == 4 && !strncmp(buf, "disk", len)) {

error = hibernate();

goto Exit;

}

#ifdef CONFIG_SUSPEND

for (s = &pm_states[state]; state < PM_SUSPEND_MAX; s++, state++) {

if (*s && len == strlen(*s) && !strncmp(buf, *s, len))

break;

}

if (state < PM_SUSPEND_MAX && *s)

#ifdef CONFIG_EARLYSUSPEND

if (state == PM_SUSPEND_ON || valid_state(state)) {

error = 0;

request_suspend_state(state); // 请求进入android的休眠流程

}

#else

error = enter_state(state); // linux的标准休眠流程

#endif

#endif

Exit:

return error ? error : n;

}

power_attr(state);

其中state_show()为节点的读函数，主要打印出系统支持的休眠等级；state_store()为节点的写函数，根据参数请求休眠或者唤醒流程。节点的创建代码如下：

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static struct attribute * g[] = {

&state_attr.attr, // state节点

#ifdef CONFIG_PM_TRACE

&pm_trace_attr.attr,

#endif

#if defined(CONFIG_PM_SLEEP) && defined(CONFIG_PM_DEBUG)

&pm_test_attr.attr, // pm_test节点

#endif

#ifdef CONFIG_USER_WAKELOCK

&wake_lock_attr.attr, // wake_lock节点

&wake_unlock_attr.attr, // wake_unlock节点

#endif

NULL,

};

static struct attribute_group attr_group = {

.attrs = g,

};

static int __init pm_init(void)

{

int error = pm_start_workqueue();

if (error)

return error;

power_kobj = kobject_create_and_add("power", NULL); // 创建power节点

if (!power_kobj)

return -ENOMEM;

return sysfs_create_group(power_kobj, &attr_group); // 创建一组属性节点

}

core_initcall(pm_init);

二、early_suspend 实现

1、early_suspend 定义、接口及其用法

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enum {

EARLY_SUSPEND_LEVEL_BLANK_SCREEN = 50,

EARLY_SUSPEND_LEVEL_STOP_DRAWING = 100,

EARLY_SUSPEND_LEVEL_DISABLE_FB = 150,

};

struct early_suspend {

#ifdef CONFIG_HAS_EARLYSUSPEND

struct list_head link; // 链表节点

int level; // 优先等级

void (*suspend)(struct early_suspend *h);

void (*resume)(struct early_suspend *h);

#endif

};

可以看到early_suspend由两个函数指针、链表节点、优先等级组成；内核默认定义了3个优先等级，在suspend的时候先执行优先等级低的handler，在resume的时候则先执行等级高的handler，用户可以定义自己的优先等级；early_suspend向内核空间提供了2个接口用于注册和注销handler：

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void register_early_suspend(struct early_suspend *handler);

void unregister_early_suspend(struct early_suspend *handler);

其中register_early_suspend()用于注册，unregister_early_suspend用于注销；一般early_suspend的使用方式如下：

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ts->earlysuspend.suspend = sitronix_i2c_suspend_early;

ts->earlysuspend.resume = sitronix_i2c_resume_late;

ts->earlysuspend.level = EARLY_SUSPEND_LEVEL_BLANK_SCREEN;

register_early_suspend(&ts->earlysuspend);

设置好suspend和resume接口，定义优先等级，然后注册结构即可。

2、初始化信息

我们看一下early_suspend需要用到的一些数据：

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static DEFINE_MUTEX(early_suspend_lock);

static LIST_HEAD(early_suspend_handlers); // 初始化浅度休眠链表

// 声明3个工作队列用于同步、浅度休眠和唤醒

static void early_sys_sync(struct work_struct *work);

static void early_suspend(struct work_struct *work);

static void late_resume(struct work_struct *work);

static DECLARE_WORK(early_sys_sync_work,early_sys_sync);

static DECLARE_WORK(early_suspend_work, early_suspend);

static DECLARE_WORK(late_resume_work, late_resume);

static DEFINE_SPINLOCK(state_lock);

enum {

SUSPEND_REQUESTED = 0x1, // 当前正在请求浅度休眠

SUSPENDED = 0x2, // 浅度休眠完成

SUSPEND_REQUESTED_AND_SUSPENDED = SUSPEND_REQUESTED | SUSPENDED,

};

static int state;

初始化了一个链表early_suspend_handlers用于管理early_suspend，还定义读写链表用到的互斥体；另外还声明了3个工作队列，分别用于缓存同步、浅度休眠和唤醒；还声明了early_suspend操作的3个状态。

3、register_early_suspend 和 unregister_early_suspend

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void register_early_suspend(struct early_suspend *handler)

{

struct list_head *pos;

mutex_lock(&early_suspend_lock);

// 遍历浅度休眠链表

list_for_each(pos, &early_suspend_handlers) {

struct early_suspend *e;

e = list_entry(pos, struct early_suspend, link);

// 判断当前节点的优先等级是否大于handler的优先等级

// 以此决定handler在链表中的顺序

if (e->level > handler->level)

break;

}

// 将handler加入当前节点之前,优先等级越低越靠前

list_add_tail(&handler->link, pos);

if ((state & SUSPENDED) && handler->suspend)

handler->suspend(handler);

mutex_unlock(&early_suspend_lock);

}

EXPORT_SYMBOL(register_early_suspend);

注册的流程比较简单，首先遍历链表，依次比较每个节点的优先等级，如果遇到优先等级比新节点优先等级高则跳出，然后将新节点加入优先等级较高的节点前面，这样就确保了链表是优先等级低在前高在后的顺序；在将节点加入链表后查看当前状态是否为浅度休眠完成状态，如果是则执行handler的suspend函数。

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void unregister_early_suspend(struct early_suspend *handler)

{

mutex_lock(&early_suspend_lock);

list_del(&handler->link);

mutex_unlock(&early_suspend_lock);

}

EXPORT_SYMBOL(unregister_early_suspend);

注销流程则只是将节点从链表中移除。

4、request_suspend_state

前面我们看到用户空间在写/sys/power/state节点的时候会执行request_suspend_state()函数，该函数代码如下：

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void request_suspend_state(suspend_state_t new_state)

{

unsigned long irqflags;

int old_sleep;

spin_lock_irqsave(&state_lock, irqflags);

old_sleep = state & SUSPEND_REQUESTED;

// 打印当前状态

if (debug_mask & DEBUG_USER_STATE) {

struct timespec ts;

struct rtc_time tm;

getnstimeofday(&ts);

rtc_time_to_tm(ts.tv_sec, &tm);

pr_info("request_suspend_state: %s (%d->%d) at %lld "

"(%d-%02d-%02d %02d:%02d:%02d.%09lu UTC)\n",

new_state != PM_SUSPEND_ON ? "sleep" : "wakeup",

requested_suspend_state, new_state,

ktime_to_ns(ktime_get()),

tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,

tm.tm_hour, tm.tm_min, tm.tm_sec, ts.tv_nsec);

}

// 如果新状态是休眠状态

if (!old_sleep && new_state != PM_SUSPEND_ON) {

state |= SUSPEND_REQUESTED;

pr_info("sys_sync_work_queue early_sys_sync_work.\n");

// 执行缓存同步与浅度休眠的工作队列

queue_work(sys_sync_work_queue, &early_sys_sync_work);

queue_work(suspend_work_queue, &early_suspend_work);

} else if (old_sleep && new_state == PM_SUSPEND_ON) {

// 如果新状态是唤醒状态

state &= ~SUSPEND_REQUESTED;

// 激活内核锁

wake_lock(&main_wake_lock);

// 执行浅度唤醒的工作队列

queue_work(suspend_work_queue, &late_resume_work);

}

// 更新全局状态

requested_suspend_state = new_state;

spin_unlock_irqrestore(&state_lock, irqflags);

}

函数首先打印出当前状态变化的log，然后判断新状态，如果是休眠状态则置位SUSPEND_REQUESTED标志，然后将同步缓存、浅度休眠工作队列加入相应的内核线程执行；如果新状态是唤醒则首先将main_wake_lock激活，然后再将浅度唤醒工作队列加入内核线程执行；最后更新全局状态变量，因为提供了一个内核空间接口用于获取当前休眠唤醒状态：

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// 返回系统状态值

suspend_state_t get_suspend_state(void)

{

return requested_suspend_state;

}

5、early_suspend_work、late_resume_work 和 early_sys_sync

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static void early_suspend(struct work_struct *work)

{

struct early_suspend *pos;

unsigned long irqflags;

int abort = 0;

mutex_lock(&early_suspend_lock);

spin_lock_irqsave(&state_lock, irqflags);

if (state == SUSPEND_REQUESTED) // 判断当前状态是否在请求浅度休眠

state |= SUSPENDED; // 如果是则置位SUSPENDED

else

abort = 1;

spin_unlock_irqrestore(&state_lock, irqflags);

if (abort) { // 取消early_suspend

if (debug_mask & DEBUG_SUSPEND)

pr_info("early_suspend: abort, state %d\n", state);

mutex_unlock(&early_suspend_lock);

goto abort;

}

if (debug_mask & DEBUG_SUSPEND)

pr_info("early_suspend: call handlers\n");

// 遍历浅度休眠链表并执行其中所有suspend函数

// 执行顺序根据优先等级而定,等级越低越先执行

list_for_each_entry(pos, &early_suspend_handlers, link) {

if (pos->suspend != NULL)

pos->suspend(pos);

}

mutex_unlock(&early_suspend_lock);

if (debug_mask & DEBUG_SUSPEND)

pr_info("early_suspend: sync\n");

/* Remove sys_sync from early_suspend, and use work queue to complete sys_sync */

//sys_sync();

abort:

spin_lock_irqsave(&state_lock, irqflags);

if (state == SUSPEND_REQUESTED_AND_SUSPENDED)

wake_unlock(&main_wake_lock);

spin_unlock_irqrestore(&state_lock, irqflags);

}

在suspend流程中首先判断当前状态是否为SUSPEND_REQUESTED，如果是则置位SUSPENDED标志，如果不是则取消suspend流程；然后遍历浅度休眠链表，从链表头部到尾部依次调用各节点的suspend()函数，执行完后判断当前状态是否为SUSPEND_REQUESTED_AND_SUSPENDED，如果是则释放main_wake_lock，当前系统中如果只存在main_wake_lock这个有效锁，则会在wake_unlock()里面启动深度休眠线程，如果还有其他其他wake_lock则保持当前状态。

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static void late_resume(struct work_struct *work)

{

struct early_suspend *pos;

unsigned long irqflags;

int abort = 0;

mutex_lock(&early_suspend_lock);

spin_lock_irqsave(&state_lock, irqflags);

if (state == SUSPENDED) // 清除浅度休眠完成标志

state &= ~SUSPENDED;

else

abort = 1;

spin_unlock_irqrestore(&state_lock, irqflags);

if (abort) {

if (debug_mask & DEBUG_SUSPEND)

pr_info("late_resume: abort, state %d\n", state);

goto abort;

}

if (debug_mask & DEBUG_SUSPEND)

pr_info("late_resume: call handlers\n");

// 反向遍历浅度休眠链表并执行其中所有resume函数

// 执行顺序根据优先等级而定,等级越高越先执行

list_for_each_entry_reverse(pos, &early_suspend_handlers, link)

if (pos->resume != NULL)

pos->resume(pos);

if (debug_mask & DEBUG_SUSPEND)

pr_info("late_resume: done\n");

abort:

mutex_unlock(&early_suspend_lock);

}

在resume流程中同样首先判断当前状态是否为SUSPENDED，如果是则清除SUSPENDED标志，然后反向遍历浅度休眠链表，按照优先等级从高到低的顺序执行节点的resume()函数。

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static void early_sys_sync(struct work_struct *work)

{

wake_lock(&sys_sync_wake_lock);

sys_sync();

wake_unlock(&sys_sync_wake_lock);

}

内核专门为缓存同步建立了一个线程，同时还创建了sys_sync_wake_lock防止在同步缓存时系统进入深度休眠。