说明：
1. Based on linux 2.6.32 and android 2.2，only support SDR(mem).
2. 参考文章：
http://2695477.blog.51cto.com/blog/2685477/484751
http://www.docin.com/p-115475680.html
http://blogold.chinaunix.net/u3/113927/showart_2447111.html
http://www.cnmsdn.com/html/201003/1269407632ID2530.html
一、新增特性介绍
实际上，android仍然是利用了标准linux的休眠唤醒系统，只不过添加了一些使用上的新特性，early suspend、late resume、wake lock。
Early suspend - 这个机制定义了在suspend的早期，关闭显示屏的时候，一些和显示屏相关的设备，比如背光、重力感应器和触摸屏等设备都应该被关掉，但是此时系统可能还有持有wake lock的任务在运行，如音乐播放，电话，或者扫描sd卡上的文件等，这个时候整个系统还不能进入真正睡眠，直到所有的wake lock都没释放。在嵌入式设备中，悲观是一个很大的电源消耗，所有android加入了这种机制。
Late resume - 这个机制定义了在resume的后期，也就是唤醒源已经将处理器唤醒，标准linux的唤醒流程已经走完了，在android上层系统识别出这个物理上的唤醒源是上层定义的，那么上层将会发出late resume的命令给下层，这个时候将会调用相关设备注册的late resume回调函数。
Wake lock - wakelock在android的电源管理系统中扮演一个核心的角色，wakelock是一种锁的机制, 只要有task拿着这个锁, 系统就无法进入休眠, 可以被用户态进程和内核线程获得。这个锁可以是有超时的或者是没有超时的, 超时的锁会在时间过去以后自动解锁。如果没有锁了或者超时了, 内核就会启动标准linux的那套休眠机制机制来进入休眠。
二、kernel层源码解析 - early suspend 和 late resume实现
相关源码：
kernel/kernel/power/main.c
kernel/kernel/power/earlysuspend.c
kernel/kernel/power/wakelock.c
kernel/kernel/power/userwakelock.c
kernel/kernel/power/suspend.c
之前标准的linux的sysfs的接口只需要一个state就够了，现在至少需要3个接口文件：state、wake_lock、wake_unlock。现在为了配合android为休眠唤醒添加的几种新特性，可以填入文件state的模式又多了一种：on, 标准android系统中只支持state的on和mem模式，其余的暂不支持。wake_lock和wake_unlock接口对应的读写函数在文件userwakelock.c中，对wakelock.c中的create wakelock或者release wakelock进行了封装，供用户空间来使用。
如果上层用户执行:echo xxx(on or mem) > sys/power/state的话，将会调用到如下函数：
static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr,
const char *buf, size_t n)
{
#ifdef CONFIG_SUSPEND // set
#ifdef CONFIG_EARLYSUSPEND //set
suspend_state_t state = PM_SUSPEND_ON; // for early suspend and late resume
#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(); // 检查是否要求进入disk省电模式，暂时不支持
goto Exit;
}
#ifdef CONFIG_SUSPEND // def
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)) {
// 需要经过平台pm.c文件定义的模式支持检查函数，mtk只支持mem，同时如果是android发送出来的late resume命令(on)，这里也会放行，往下执行
error = 0;
request_suspend_state(state); // android休眠唤醒的路线
}
#else
error = enter_state(state);// 标准linux休眠唤醒的路线
#endif
#endif
Exit:
return error ? error : n;
}
@ kernel/kernel/power/earlysuspend.c
enum {
DEBUG_USER_STATE = 1U << 0,
DEBUG_SUSPEND = 1U << 2,
};
int Earlysuspend_debug_mask = DEBUG_USER_STATE;
module_param_named(Earlysuspend_debug_mask, Earlysuspend_debug_mask, int, S_IRUGO | S_IWUSR | S_IWGRP);
static DEFINE_MUTEX(early_suspend_lock);
static LIST_HEAD(early_suspend_handlers);
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,
SUSPENDED = 0x2,
SUSPEND_REQUESTED_AND_SUSPENDED = SUSPEND_REQUESTED | SUSPENDED,
};
static int state; // 初始化为0
static DECLARE_COMPLETION(fb_drv_ready);
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; // state = 1 or 3
// state的值会在0->1->3->2->0循环变化，后面分析代码都可以看出这些值代表系统目前处于什么阶段，简单得说就是：正常->准备进early suspend->开始early suspend并且对名为mian的wakelock解锁，如果此时没有其余wakelock处于lock状态，那么系统就走linux的休眠唤醒路线让整个系统真正休眠，直到唤醒源发生，然后将处理器和linux层唤醒。之后android层判断本次底层醒来是由于我所定义的唤醒源引起的吗？如果不是，android将不予理会，过段时间没有wakelock锁，系统会再次走linux的休眠路线进入休眠。如果是，那么android上层就会写一个on的指令到state接口中，同样是会调用到函数request_suspend_state() -> 准备执行late resume -> 开始执行late resume，之后整个系统就这样被唤醒了。
if (Earlysuspend_debug_mask & DEBUG_USER_STATE) {
struct timespec ts; // 打印出debug信息
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);
}
// eg: [request_suspend_state]: sleep (0->3) at 97985478409 (2010-01-03 09:52:59.637902305 UTC)， 这里对时间的获取和处理，在其他地方可以参考
// ready to enter earlysuspend
if (!old_sleep && new_state != PM_SUSPEND_ON) { // susepnd会进入这里
state |= SUSPEND_REQUESTED; // state = 1
pr_info("[request_suspend_state]:
sys_sync_work_queue early_sys_sync_work/n");
queue_work(sys_sync_work_queue, &early_sys_sync_work);
pr_info("[request_suspend_state]: suspend_work_queue early_suspend_work/n");
queue_work(suspend_work_queue, &early_suspend_work);
// 在wakelocks_init()函数(wakelock.c)中会创建这两个工作队列和工作者线程来专门负责处理sys_sync和early suspend的工作。关于工作队列的详情参考我工作队列的文章
}
// ready to enter lateresume
else if (old_sleep && new_state == PM_SUSPEND_ON) {
state &= ~SUSPEND_REQUESTED; // state = 2
wake_lock(&main_wake_lock); // 对main wakelock上锁
pr_info("[request_suspend_state]: suspend_work_queue late_resume_work/n" );
if (queue_work(suspend_work_queue, &late_resume_work)) {
// 提交late resume的工作项
//
// In order to synchronize the backlight turn on timing,
// block the thread and wait for fb driver late_resume()
// callback function is completed
//
wait_for_completion(&fb_drv_ready);
// 等待完成量fb_drv_ready，他会在late resume结束之后完成
}
}
requested_suspend_state = new_state;
// 存储本次休眠或者是唤醒的状态，供下次休眠或者唤醒使用
spin_unlock_irqrestore(&state_lock, irqflags);
}
在系统suspend的时候提交的两个工作项会陆续被执行到，那么下面就来看一下执行early suspend的关键函数。
static void early_sys_sync(struct work_struct *work)
{
wake_lock(&sys_sync_wake_lock);
printk("[sys_sync work] start/n");
sys_sync(); // 同步文件系统
printk("[sys_sync wrok] done/n");
wake_unlock(&sys_sync_wake_lock);
}
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)
state |= SUSPENDED; // state = 3
else
abort = 1;
spin_unlock_irqrestore(&state_lock, irqflags);
if (abort) { // suspend 中止退出
if (Earlysuspend_debug_mask & DEBUG_SUSPEND)
pr_info("[early_suspend]: abort, state %d/n", state);
mutex_unlock(&early_suspend_lock);
goto abort;
}
if (Earlysuspend_debug_mask & DEBUG_SUSPEND)
pr_info("[early_suspend]: call handlers/n");
list_for_each_entry(pos, &early_suspend_handlers, link) {
if (pos->suspend != NULL)
pos->suspend(pos);
}
// 函数register_early_suspend()会将每一个early suspend项以优先级大小注册到链表early_suspend_handlers中，这里就是一次取出，然后执行对应的early suspend回调函数
mutex_unlock(&early_suspend_lock);
// Remove sys_sync from early_suspend,
// and use work queue to complete sys_sync
abort:
spin_lock_irqsave(&state_lock, irqflags);
if (state == SUSPEND_REQUESTED_AND_SUSPENDED)
{
pr_info("[early_suspend]: wake_unlock(main)/n");
wake_unlock(&main_wake_lock);
// main wakelock 解锁。看到这里，好像系统执行了early suspend之后就没有往下执行标准linux的suspend流程了，其实不是，android的做法是，不是你执行完了early suspend 的回调就可以马上走标准linux的suspend流程，而是会检查还有没有wakelock被持有，如果所有wakelock全是解锁状态，那么就会执行标准linux的suspend步骤。
}
spin_unlock_irqrestore(&state_lock, irqflags);
}
static void late_resume(struct work_struct *work)
{
struct early_suspend *pos;
unsigned long irqflags;
int abort = 0;
int completed = 0;
mutex_lock(&early_suspend_lock);
spin_lock_irqsave(&state_lock, irqflags);
// return back from suspend
if (state == SUSPENDED)
state &= ~SUSPENDED; // state = 0
else
abort = 1;
spin_unlock_irqrestore(&state_lock, irqflags);
if (abort) {
if (Earlysuspend_debug_mask & DEBUG_SUSPEND)
pr_info("[late_resume]: abort, state %d/n", state);
goto abort;
}
if (Earlysuspend_debug_mask & DEBUG_SUSPEND)
pr_info("[late_resume]: call handlers/n");
list_for_each_entry_reverse(pos, &early_suspend_handlers, link)
{
if (!completed && pos->level < EARLY_SUSPEND_LEVEL_DISABLE_FB) {
complete(&fb_drv_ready);
completed = 1;
}
if (pos->resume != NULL)
pos->resume(pos);
}
// 以和early suspend的逆序执行链表early_suspend_handlers上的late resume回调函数
if (Earlysuspend_debug_mask & DEBUG_SUSPEND)
pr_info("[late_resume]: done/n");
abort:
if (!completed)
complete(&fb_drv_ready); // 设置完成量ok
mutex_unlock(&early_suspend_lock);
}
四、android层源码解析
在linux之上经过android的软件堆层层封装,最终在上层的java应用程序中使用。休眠唤醒也是从最上层发出的命令，然后一层一层地将参数解析，往最底层传，最后走上标准linux的休眠唤醒之路。
这一部分将会初略分析休眠唤醒机制上linux之上所走的路线。
在linux之上，存在一个hal层，专门做和linux内核设备打交道的事情，这里也不例外。休眠唤醒机制的hal层源码位于：@hardware/libhardware_legacy/power/power.c
该文件源码比较简单，下面列举重点片段：
enum {
ACQUIRE_PARTIAL_WAKE_LOCK = 0,
RELEASE_WAKE_LOCK,
REQUEST_STATE,
OUR_FD_COUNT
};
const char * const NEW_PATHS[] = {
"/sys/power/wake_lock",
"/sys/power/wake_unlock",
"/sys/power/state"
};
static int g_initialized = 0;
static int g_fds[OUR_FD_COUNT];
static const char *off_state = "mem";
static const char *on_state = "on";
static int open_file_descriptors(const char * const paths[])
{
int i;
for (i=0; i<OUR_FD_COUNT; i++) {
int fd = open(paths, O_RDWR);
if (fd < 0) {
fprintf(stderr, "fatal error opening /"%s/"/n", paths);
g_error = errno;
return -1;
}
g_fds = fd;
}
g_error = 0;
return 0;
}
static inline void initialize_fds(void)
{
if (g_initialized == 0) {
if(open_file_descriptors(NEW_PATHS) < 0) {
open_file_descriptors(OLD_PATHS);
on_state = "wake";
off_state = "standby";
}
g_initialized = 1;
}
}
int acquire_wake_lock(int lock, const char* id)
{
initialize_fds();
if (g_error) return g_error;
int fd;
if (lock == PARTIAL_WAKE_LOCK) { // 上层传下来的lock type
fd = g_fds[ACQUIRE_PARTIAL_WAKE_LOCK];
}
else {
return EINVAL;
}
return write(fd, id, strlen(id));
}
int release_wake_lock(const char* id)
{
initialize_fds();
// LOGI("release_wake_lock id='%s'/n", id);
if (g_error) return g_error;
ssize_t len = write(g_fds[RELEASE_WAKE_LOCK], id, strlen(id));
return len >= 0;
}
int set_screen_state(int on)
{
QEMU_FALLBACK(set_screen_state(on));
LOGI("*** set_screen_state %d", on);
initialize_fds();
if (g_error) return g_error;
char buf[32];
int len;
if(on)
len = sprintf(buf, on_state);
else
len = sprintf(buf, off_state);
len = write(g_fds[REQUEST_STATE], buf, len);
if(len < 0) {
LOGE("Failed setting last user activity: g_error=%d/n", g_error);
}
return 0;
}
Hal层的代码在jni层中被使用，源码位于：frameworks/base/core/jni/android_os_Power.cpp，代码片段如下：
static void acquireWakeLock(JNIEnv *env, jobject clazz, jint lock, jstring idObj)
{
if (idObj == NULL) {
throw_NullPointerException(env, "id is null");
return ;
}
const char *id = env->GetStringUTFChars(idObj, NULL);
acquire_wake_lock(lock, id);
env->ReleaseStringUTFChars(idObj, id);
}// 对wakelock加锁函数
static void releaseWakeLock(JNIEnv *env, jobject clazz, jstring idObj)
{
if (idObj == NULL) {
throw_NullPointerException(env, "id is null");
return ;
}
const char *id = env->GetStringUTFChars(idObj, NULL);
release_wake_lock(id);
env->ReleaseStringUTFChars(idObj, id);
}// 对wakelock解锁函数
static int setScreenState(JNIEnv *env, jobject clazz, jboolean on)
{
return set_screen_state(on);
}// 休眠唤醒的函数
Jni的方法需要注册到上层才可以使用，同时也需要在上层的对应java类中声明了native才可以使用。那么这里的方法在java中对应的声明在哪里呢？frameworks/base/core/java/android/os/Power.java，该文件定义一个java类，如下：
public class Power
{
// can't instantiate this class
private Power()
{
}
/**
* Wake lock that ensures that the CPU is running. The screen might
* not be on.
*/
public static final int PARTIAL_WAKE_LOCK = 1;
/**
* Wake lock that ensures that the screen is on.
*/
public static final int FULL_WAKE_LOCK = 2;
public static native void acquireWakeLock(int lock, String id);
public static native void releaseWakeLock(String id);
…
/**
* Turn the screen on or off
*
* @param on Whether you want the screen on or off
*/
public static native int setScreenState(boolean on);
…
}
声明的jni接口应该是被java server在使用，这里就是专门的电源管理服务：PowerManagerService使用，具体源码位置在：frameworks/base/services/java/com/android/server/PowerManagerService.java。android在最上层还提供了现场的android.os.PowerManager类
(frameworks/base/core/java/android/os/PowerManager.java)来供app使用，PowerManager类会调用java服务PowerManagerService的方法来完成与wakelock相关的工作。
@ frameworks/base/core/java/android/os/PowerManager.java
类PowerManager中内嵌了一个WakeLock类，另外还定义了wakelock的类型，下面是代码片段：
public class PowerManager
{
private static final String TAG = "PowerManager";
…
/**
* Wake lock that ensures that the CPU is running. The screen might
* not be on.
*/
public static final int PARTIAL_WAKE_LOCK = WAKE_BIT_CPU_STRONG;
/**
* Wake lock that ensures that the screen and keyboard are on at
* full brightness.
*/
public static final int FULL_WAKE_LOCK = WAKE_BIT_CPU_WEAK | WAKE_BIT_SCREEN_BRIGHT | WAKE_BIT_KEYBOARD_BRIGHT;
/**
* Wake lock that ensures that the screen is on at full brightness;
* the keyboard backlight will be allowed to go off.
*/
public static final int SCREEN_BRIGHT_WAKE_LOCK = WAKE_BIT_CPU_WEAK | WAKE_BIT_SCREEN_BRIGHT;
/**
* Wake lock that ensures that the screen is on (but may be dimmed);
* the keyboard backlight will be allowed to go off.
*/
public static final int SCREEN_DIM_WAKE_LOCK = WAKE_BIT_CPU_WEAK | WAKE_BIT_SCREEN_DIM;
/**
* Wake lock that turns the screen off when the proximity sensor activates.
* Since not all devices have proximity sensors, use
* {@link #getSupportedWakeLockFlags() getSupportedWakeLockFlags()} to determine if
* this wake lock mode is supported.
*
* {@hide}
*/
public static final int PROXIMITY_SCREEN_OFF_WAKE_LOCK
= WAKE_BIT_PROXIMITY_SCREEN_OFF;
…
public class WakeLock
{
…
WakeLock(int flags, String tag)
{
switch (flags & LOCK_MASK) {
case PARTIAL_WAKE_LOCK:
case SCREEN_DIM_WAKE_LOCK:
case SCREEN_BRIGHT_WAKE_LOCK:
case FULL_WAKE_LOCK:
case PROXIMITY_SCREEN_OFF_WAKE_LOCK:
break;
default:
throw new IllegalArgumentException();
}
mFlags = flags;
mTag = tag;
mToken = new Binder();
}
public void acquire()
{
synchronized (mToken) {
if (!mRefCounted || mCount++ == 0) {
try {
mService.acquireWakeLock(mFlags, mToken, mTag);
} catch (RemoteException e) {
}
mHeld = true;
}
}
}
public void release(int flags)
{
synchronized (mToken) {
if (!mRefCounted || --mCount == 0) {
try {
mService.releaseWakeLock(mToken, flags);
} catch (RemoteException e) {
}
mHeld = false;
}
if (mCount < 0) {
throw new RuntimeException("WakeLock under-locked " + mTag);
}
}
}
…
}
…
public WakeLock newWakeLock(int flags, String tag)
{
if (tag == null) {
throw new NullPointerException("tag is
null in PowerManager.newWakeLock");
}
return new WakeLock(flags, tag);
}
public void goToSleep(long time)
{
try {
mService.goToSleep(time);
} catch (RemoteException e) {
}
}
…
public PowerManager(IPowerManager service, Handler handler)
{
mService = service;
mHandler = handler;
}
IPowerManager mService;
Handler mHandler;
}
应用实例：
PowerManager pm = (PowerManager)getSystemService(Context.POWER_SERVICE);
PowerManager.WakeLock wl =
pm.newWakeLock(PowerManager.SCREEN_DIM_WAKE_LOCK, “Tag”);
wl.acquire(); //申请锁这个里面会调用PowerManagerService里面acquireWakeLock()
…
wl.release(); //释放锁，显示的释放，如果申请的锁不在此释放系统就不会进入休眠。
接下来就会调用到java服务PowerManagerService中：
public void acquireWakeLock(int flags, IBinder lock, String tag) {
int uid = Binder.getCallingUid();
if (uid != Process.myUid()) {
mContext.enforceCallingOrSelfPermission(android.Manifest.permission.WAKE_LOCK, null);
}
long ident = Binder.clearCallingIdentity();
try {
synchronized (mLocks) {
acquireWakeLockLocked(flags, lock, uid, tag); // 内部方法
}
} finally {
Binder.restoreCallingIdentity(ident);
}
}
acquireWakeLockLocked(flags, lock, uid, tag)会调用函数power类的方法：
Power.acquireWakeLock(Power.PARTIAL_WAKE_LOCK,PARTIAL_NAME)。
public void releaseWakeLock(IBinder lock, int flags) {
int uid = Binder.getCallingUid();
if (uid != Process.myUid()) {
mContext.enforceCallingOrSelfPermission(android.Manifest.permission.WAKE_LOCK, null);
}
synchronized (mLocks) {
releaseWakeLockLocked(lock, flags, false);
}
}
releaseWakeLockLocked(lock, flags, false)函数会调用power类的方法：
Power.releaseWakeLock(PARTIAL_NAME);
上层休眠唤醒都是调用PowerManagerService类的方法：
goToSleep()
à goToSleepWithReason()
à goToSleepLocked()
à setPowerState()
à setScreenStateLocked()
à Power.setScreenState()
à jni方法
Android层的代码分析得不是很详细，这里只关注框架和流程。下图是网上的一个框架,可以参考一下:
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