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The battery life on modern portable gadgets is much lower than ten or twenty years ago, mainly because electronics technology is developing much more quickly than battery technology. Batteries are struggling to keep up with all the amazing new things that portable gadgets are capable of.
With this in mind, it's important to use a portable device's battery as efficiently as possible. This tutorial will look at some simple ways to do this on the Nokia tablets. The point is to put the most effective measures first but it can't be guaranteed that it compares straight to reality. There are too many scenarios for every aspect to keep the order always 110% right.
Before reading on, have a look at the N900 User Guide on your tablet, and read the section Prolong battery life in the chapter Find help.
This is a rather odd problem, but is actually quite common. It is alone sufficient to spend a fully charged battery in less than a day.
The keyboard slide is detected by a magnetic coupling and nearby magnets may confuse this function. So if you put your device in a pouch or jacket pocket that uses a magnetic clasp, the device might believe that the keyboard is opened and closed all the time.
This applies more to the , but if you use a 3G-enabled cell phone to connect your older tablet to the web, this tip should help your phones' battery life.
Even when you would have a good 3G coverage, turning the connection to GSM only will help battery life immensely. Your connection speed will slow down if you do it, but not as bad as you would imagine. The phone will use EDGE speed (2.5G) which is 384 kbps at theoretical maximum. This is in many situations sufficient.
Turning off 3G will help your battery life even more under spotty or non-existent 3G coverage when the phone doesn't have to search for the 3G connection all the time.
There is a
that lets you switch easily between full 3G and GSM only. Using such an app lets you mostly have the phone in GSM mode for background tasks, and switch to 3G only when you actively use the internet.
Many users find that they have about 50% battery left after a full day's worth of usage if they only use 3G when really needed (and don't have the magnetic clasp problem).
One of the major sources of battery drain on a portable device is the screen. If it's very large and very bright, it will use up a lot of battery life when it's lit.
One of the best things you can do to save battery life is to turn down the brightness on your tablet's screen to the lowest level that you feel comfortable with. The lower it is, the more battery life you will save. Just click on the screen brightness icon (the icon that looks like a sun in the status bar at the top) and adjust the brightness on the sliding scale.
For Maemo 5, you can use the
(available in Extras) to have easy access to the brightness controls.
Like all portable gadgets, Nokia's internet tablets have an automatic dimming and shut down period on their screens. If you don't use a tablet within a certain time period, the screen will dim, and if it continues to be unused the screen will switch off. You can make it switch back on at any time just by touching the screen.
To get the maximum battery life, set the time-out period for screen dimming and shut down to the shortest possible time. To do this click on the brightness icon in the status bar (the one that looks like a sun at the top of the screen), click on "Display Settings", set Brightness Period to 10 seconds and Switch Off Display to 30 seconds. Click "OK".
If you get annoyed by how quickly the screen dims or shuts off, you can always increase the Brightness Period and Switch Off Display time until you feel more comfortable. However, this will use up more battery life than the minimum times.
has a very convenient way to keep display on without timeout: just click and hold the rectangle "screen" icon on left side of slider
The Bluetooth wireless system is intended to let the tablet easily connect to nearby accessories or computers, but if you're not using the connection then it's worth switching it off to avoid it needlessly wasting battery life.
Bluetooth doesn't use much power in standby mode (some 2..3mA) so this may not have much of an impact on battery life, but if you genuinely don't need it then there's no point having it switched on. Tests proved that BT doesn't use any significant power on N900 when set to active+invisible.
To switch off Bluetooth just click on the status bar at the top (or open the system-settings dialog), click on "Bluetooth Settings", and tick or untick the box marked "Bluetooth On". Click "OK" and the Bluetooth will switch on/off accordingly.
The very next checkbox in the same dialog is for selecting visibility status of BT. Setting it to "off" (aka "not visible") will make BT not send own name all the time, thus saving part of the above mentioned power consumption of BT.
Again, like Bluetooth the Wi-Fi wireless standard doesn't actually use that much power when it's in standby mode (but there are still some nasty bugs in the wifi subsystem that can drain your battery in an hour, like ), but if you really don't need the Wi-Fi connection then there's no point having it switched on.
To disconnect Wi-Fi, just click on the connection icon in the status bar at the top (it looks like a small ball with curved lines radiating upwards) and then click on "Disconnect". Click "OK" and the Wi-Fi connection will be disconnected.
To re-connect, click on the same icon, then "Select Connection", then click on the connection you were using before and press "Connect".
It's worth noticing that the WLAN chip is using considerable amounts of battery energy during scanning for APs. This is where the setting for scan period becomes relevant: set it to a period as long as acceptable for your usage patterns. A scan may easily take longer than 60 seconds, during which the WLAN chip is consuming full power, so for shortest "search interval" setting in "settings -& internet connections"
of 5 minutes you have a so called duty cycle of 60/300 = 20%. Do your math for search interval = 30 minutes.
Main article:
The Nokia tablets support power saving mode (PSM) for wifi. You can find out more about how to turn it on, and the positive and possible negative consequences in .
Yes, that's right, if you want to save battery life then do NOT switch off the tablet when you've finished using it.
You can charge it while it's switched on, and if you want to carry it you can lock the touchscreen and keys so it doesn't accidentally activate anything while in your pocket or bag.
Leaving the tablet on to save power might sound absolutely insane, but it does work apparently, and there are good reasons why it works.
When you switch on any modern computing device such as a PC, smartphone or pocket computer, it has to go through a complex and intensive process of getting the device ready for use. This takes a lot of effort by the computer, and anyone who has booted up a PC knows how long they have to wait between powering up and actually getting to use it. This effort uses a lot more power than simply staying switched on doing nothing.
If you switch off your tablet and then switch it on a few hours later, this apparently uses up more battery life than just leaving the tablet switched on for several days. For this reason, it saves energy if you leave your tablet switched on all the time, just like you'd leave a mobile phone switched on all the time.
N.B. Calls and texts cannot be made/sent or received in these modes, so ensure call-forwarding is active before enabling either flight- or tablet- mode.
A quick and easy way to shut down the tablet's radio transmissions (ie. WWAN aka GSM, Bluetooth and Wi-Fi) is to put the tablet into offline mode, which is the same thing as flight mode.
To do this, press the power button briefly so that a menu appears, click on "Offline Mode", click on "OK" as necessary, and you will be in offline mode.
To get back to normal mode, just press the power button briefly again, click on "Normal Mode" from the menu, click on "OK" as necessary, and you will be back in normal mode.
The reason this is also known as flight mode is because many airlines have a ban on passengers using devices which transmit radio signals (including phone signals, bluetooth and wi-fi). If a device is in flight mode it does not transmit any radio signals, so many airlines will allow its use in this mode.
This is useful also at night if you don't want to be disturbed by messages etc with N900. Just put it to flight mode when going to sleep and online when you wake up. No need to be afraid that silent mode has the alarm too silent etc.
Traditionally, tablet computers have no GSM/GPRS/EDGE/UTMS/etc. connection but can connect to wifi, bluetooth and hardware connections. Tablet-mode replicates this by preventing phone functionality and preserving access to other connections. Therefore, tablet-mode reduces battery consumption compared with "normal" mode, yet allows some connectivity. As such, it is a useful alternative to the complete blackout of flight-mode, even though it is not quite as power-conserving.
For the N900 (Maemo5), Cellular Modem Control Buttons
is available in - its package name is "cell-modem-ui". This software allows tablet-mode to be en/dis -abled in the same way as flight-mode: from the power button menu.
As it turns out, an old SIM card will have a lot of scratches on its pins. Some of them may even be bent inwards or otherwise damaged.The SIM cards are not designed to last more than couple of years and they really should not be taken out of the phone frequently as this damages the pins.
Long story short, old SIM cards "leak" power due to bad contacts and increased internal resistance [citation needed]. In addition, other users have mentioned that depending on their age, different SIM cards operate in different voltages (5V, 3V, 1.8V). The N900 has support for legacy SIM cards which means older SIMs will require more power.
The operator probably will give you a new sim for free if you ask kindly.
If you have an application running in the background, this uses up battery life. If you're not using an app, it's a good idea to shut it down (or "kill" it) using the task icon in the bottom left hand corner of the screen: just click on it and click on the X next to whichever app you want to shut down.
Some applications, especially unofficial ones, may use up far more battery life than they're supposed to because they're badly programmed. If you find that your tablet's battery life has suddenly turned to mush since you installed an application even though you don't use it very much, bad programming in the application may well be to blame. The easiest way to check if this is the case is to remove the application and see if the battery life improves.
Most applications work fine and don't mess around with battery life, but some do, and it's this minority that you have to watch out for. One pretty notorious 'candidate' is skype!
This appliese to every app that has a setting for refresh rate. Do you really need to have every time max 29 minute old weather forecasts or could you cope with 2 hours old data? What about fetching email every 15 minutes, what if you would get mail once in a hour? And one can always manually poll the mailbox when needed.
It probably isn't very great idea but if you need to really optimize battery life, you should remove every widget.
But every time a widget refreshes it's display, it uses the processor and consumes battery.
If your mailbox has 50 folders, it will slow the fetching and therefore consume much more battery.
The batteries wear out eventually. To increase using time and see some tips when buying new batteries, look through
article first. It has a lot of useful information. Also, charge up when you have the chance.
If you really need to stay out long with your device without a chance to charge, invest in a spare battery. These are even available with standalone chargers on eBay, so you can charge the spare battery while using your device with the other battery.
Do you really need your phone to vibrate and play a sound as loud as possible when an email is received? Or could the notification light do the trick fully?
This is also very minimal amount of battery drainage but still worth considering.
Some of these techniques are more efficient than others, and some affect the user-experience more than others. Try out different combinations to improve your battery life, but don't compromise too much. Find the best way for your own usage and keep your device usable!
There are good apps to help you keep tabs on your battery usage profile like
that shows a history-graph of your battery levels and has some nice usage statistics,
that shows everything Battery-Eye shows and can show the state of use of your processor in each time (thus showing any leakage of processing by an undesired program), so that you can see if you really have a bad process running in background and
that (among other useful things) shows your current CPU load and the most CPU-intensive apps running at the moment. All are available from the . Take care since all those monitor apps are usually quite CPU heavy and thus power hungry by themselves - all the info shown has to get acquired first, and then depending on app it needs frequent complex rendering and display updating.
One of the simplest uses of
is to detect when the phone is locked or unlocked and run scripts to reduce the cpu frequency, system load, etc.
See Xagoln's post:
Over on the
page, Fecn describes a set of scripts to modify the
settings on phone lock/unlock, basically to underclock the CPU (or at least, undo overclocking it) whilst the device is locked.
Note that this is contrary to the general "rush to idle" policy used maemo
Not just for , the
package helps to save battery power and makes it easy to configure CPU frequency scaling. You should be using this for power saving purposes even if you never choose to go over the stock 600 MHz.[citation needed]
is stable up to 900Mhz with =&KP50 (recommended)
Having both VDD1 & VDD2 enabled will increase your battery life. [citation needed]
The powersave bias setting modifies the behaviour of the ondemand governor to save more power by reducing the target frequency by a specified percentage. This can be used to limit the extent to which the phone will overclock, according to user-specified criteria.[citation needed]
This page was last modified on 7 April 2014, at 16:18.This page has been accessed 99,799 times.1713人阅读
Linux Kernel（18）
Linux内核提供了三种Suspend: Freeze、Standby和STR(Suspend to RAM)，在用户空间向”/sys/power/state”文件分别写入”freeze”、”standby”和”mem”，即可触发它们。
内核中，Suspend及Resume过程涉及到PM Core、Device PM、各个设备的驱动、Platform dependent PM、CPU control等多个模块，涉及了console switch、process freeze、CPU hotplug、wakeup处理等过个知识点。就让我们跟着内核代码，一一见识它们吧。
2. Suspend功能有关的代码分布
内核中Suspend功能有关的代码包括PM core、Device PM、Platform PM等几大块，具体如下：
1）PM Core
kernel/power/main.c----提供用户空间接口(/sys/power/state)
kernel/power/suspend.c----Suspend功能的主逻辑
kernel/power/suspend_test.c----Suspend功能的测试逻辑
kernel/power/console.c----Suspend过程中对控制台的处理逻辑
kernel/power/process.c----Suspend过程中对进程的处理逻辑
2）Device PM
drivers/base/power/*----具体可参考“”的描述。
设备驱动----具体设备驱动的位置，不再涉及。
3）Platform dependent PM
include/linux/suspend.h----定义platform dependent PM有关的操作函数集
arch/xxx/mach-xxx/xxx.c或者
arch/xxx/plat-xxx/xxx.c----平台相关的电源管理操作
3. suspend&resume过程概述
下面图片对Linux suspend&resume过程做了一个概述，读者可以顺着这个流程阅读内核源代码。具体的说明，可以参考后面的代码分析。
4. 代码分析
4.1 suspend入口
在用户空间执行如下操作：
echo &freeze& & /sys/power/state
echo &standby& & /sys/power/state
echo &mem& & /sys/power/state
会通过sysfs触发suspend的执行，相应的处理代码如下：
static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr,
const char *buf, size_t n){
suspend_state_&
error = pm_autosleep_lock();
if (error)
if (pm_autosleep_state() & PM_SUSPEND_ON) {
error = -EBUSY;
state = decode_state(buf, n);
if (state & PM_SUSPEND_MAX)
error = pm_suspend(state);
else if (state == PM_SUSPEND_MAX)
error = hibernate();
error = -EINVAL;& out:
pm_autosleep_unlock();
return error ? error :}&power_attr(state);
power_attr定义了一个名称为state的attribute文件，该文件的store接口为state_store，该接口在lock住autosleep功能后，解析用户传入的buffer（freeze、standby or mem），转换成state参数。
state参数的类型为suspend_state_t，在include\linux\suspend.h中定义，为电源管理状态在内核中的表示。具体如下：
typedef int __bitwise suspend_state_t;&#define PM_SUSPEND_ON
((__force suspend_state_t) 0)#define PM_SUSPEND_FREEZE
((__force suspend_state_t) 1)#define PM_SUSPEND_STANDBY
((__force suspend_state_t) 2)#define PM_SUSPEND_MEM
((__force suspend_state_t) 3)#define PM_SUSPEND_MIN
PM_SUSPEND_FREEZE#define PM_SUSPEND_MAX
((__force suspend_state_t) 4)
根据state的值，如果不是（PM_SUSPEND_MAX，对应hibernate功能），则调用pm_suspend接口，进行后续的处理。&
pm_suspend在kernel/power/suspend.c定义，处理所有的suspend过程。&&
4.2 pm_suspend & enter_state
pm_suspend的实现非常简单，简单的做一下参数合法性判断，直接调用enter_state接口，如下：
int pm_suspend(suspend_state_t state){&
if (state &= PM_SUSPEND_ON || state &= PM_SUSPEND_MAX)
return -EINVAL;&
error = enter_state(state);
if (error) {
suspend_stats.fail++;
dpm_save_failed_errno(error);
suspend_stats.success++;
enter_state代码为：
static int enter_state(suspend_state_t state){&
if (!valid_state(state))
return -ENODEV;&
if (!mutex_trylock(&pm_mutex))
return -EBUSY;&
if (state == PM_SUSPEND_FREEZE)
freeze_begin();&
printk(KERN_INFO &PM: Syncing filesystems ... &);
sys_sync();
printk(&done.\n&);&
pr_debug(&PM: Preparing system for %s sleep\n&, pm_states[state]);
error = suspend_prepare(state);
if (error)
if (suspend_test(TEST_FREEZER))
pr_debug(&PM: Entering %s sleep\n&, pm_states[state]);
pm_restrict_gfp_mask();
error = suspend_devices_and_enter(state);
pm_restore_gfp_mask();& Finish:
pr_debug(&PM: Finishing wakeup.\n&);
suspend_finish(); Unlock:
mutex_unlock(&pm_mutex);}
主要工作包括：
a）调用valid_state，判断该平台是否支持该电源状态。
suspend的最终目的，是让系统进入可恢复的挂起状态，而该功能必须有平台相关代码的参与才能完成，因此内核PM Core就提供了一系列的回调函数（封装在platform_suspend_ops中），让平台代码（如arch/arm/mach-xxx/pm.c）实现，然后由PM Core在合适的时机调用。这些回调函数包含一个valid函数，就是用来告知PM Core，支持哪些state。
最后看一下valid_state的实现（删除了无关代码）：
bool valid_state(suspend_state_t state){
if (state == PM_SUSPEND_FREEZE) {
* PM_SUSPEND_STANDBY and PM_SUSPEND_MEMORY states need lowlevel
* support and need to be valid to the lowlevel
* implementation, no valid callback implies that none are valid.
return suspend_ops && suspend_ops-&valid && suspend_ops-&valid(state);}
如果是freeze，无需平台代码参与即可支持，直接返回true。对于standby和mem，则需要调用suspend_ops的valid回掉，由底层平台代码判断是否支持。&
b）加互斥锁，只允许一个实例处理suspend。
c）如果state是freeze，调用freeze_begin，进行suspend to freeze相关的特殊动作。我会在后面统一分析freeze的特殊动作，这里暂不描述。
d）打印提示信息，同步文件系统。
e）调用suspend_prepare，进行suspend前的准备，主要包括switch console和process&thread freezing。如果失败，则终止suspend过程。
f）调用suspend_test，判断是否为suspend to freeze，如果是，则已经完成，直接跳至h步骤。
g）然后，调用suspend_devices_and_enter接口，该接口负责suspend和resume的所有实际动作。前半部分，suspend console、suspend device、关中断、调用平台相关的suspend_ops使系统进入低功耗状态。后半部分，在系统被事件唤醒后，处理相关动作，调用平台相关的suspend_ops恢复系统、开中断、resume device、resume console。
h）最后，调用suspend_finish，恢复（或等待恢复）process&thread，还原console。 &
4.3 suspend_prepare
suspend_prepare的代码如下：
static int suspend_prepare(suspend_state_t state){&
if (need_suspend_ops(state) && (!suspend_ops || !suspend_ops-&enter))
return -EPERM;&
pm_prepare_console();&
error = pm_notifier_call_chain(PM_SUSPEND_PREPARE);
if (error)
error = suspend_freeze_processes();
if (!error)
return 0;&
suspend_stats.failed_freeze++;
dpm_save_failed_step(SUSPEND_FREEZE); Finish:
pm_notifier_call_chain(PM_POST_SUSPEND);
pm_restore_console();}
主要工作为：
a）调用suspend_ops的enter回调（有的话），通知平台代码，以便让其作相应的准备（需要的话）
b）调用pm_prepare_console，将当前console切换到一个虚拟console并重定向内核的kmsg（需要的话）。该功能称作VT switch，后面我会在稍微详细的介绍一下，但Linux控制台子系统是相当复杂的，更具体的分析，要在控制台子系统的分析文章中说明。
c）调用pm_notifier_call_chain，发送suspend开始的消息（PM_SUSPEND_PREPARE），后面会详细描述。
d）调用suspend_freeze_processes，freeze用户空间进程和一些内核线程。该功能称作freezing-of-tasks，我会专门用一篇文章去分析它。本文就不再详细说明了。
e）如果freezing-of-tasks失败，调用pm_restore_console，将console切换回原来的console，并返回错误，以便能终止suspend。&&
4.4 suspend_devices_and_enter
suspend_devices_and_enter的过程较为复杂，代码实现如下：
int suspend_devices_and_enter(suspend_state_t state){
bool wakeup =&
if (need_suspend_ops(state) && !suspend_ops)
return -ENOSYS;&
trace_machine_suspend(state);
if (need_suspend_ops(state) && suspend_ops-&begin) {
error = suspend_ops-&begin(state);
if (error)
suspend_console();
ftrace_stop();
suspend_test_start();
error = dpm_suspend_start(PMSG_SUSPEND);
if (error) {
printk(KERN_ERR &PM: Some devices failed to suspend\n&);
goto Recover_
suspend_test_finish(&suspend devices&);
if (suspend_test(TEST_DEVICES))
goto Recover_&
error = suspend_enter(state, &wakeup);
} while (!error && !wakeup && need_suspend_ops(state)
&& suspend_ops-&suspend_again && suspend_ops-&suspend_again());& Resume_devices:
suspend_test_start();
dpm_resume_end(PMSG_RESUME);
suspend_test_finish(&resume devices&);
ftrace_start();
resume_console(); Close:
if (need_suspend_ops(state) && suspend_ops-&end)
suspend_ops-&end();
trace_machine_suspend(PWR_EVENT_EXIT);& Recover_platform:
if (need_suspend_ops(state) && suspend_ops-&recover)
suspend_ops-&recover();
goto Resume_}
a）再次检查平台代码是否需要提供以及是否提供了suspend_ops。
b）调用suspend_ops的begin回调（有的话），通知平台代码，以便让其作相应的处理（需要的话）。可能失败，需要跳至Close处执行恢复操作（suspend_ops-&end）。
c）调用suspend_console，挂起console。该接口由&kernel\printk.c&实现，主要是hold住一个lock，该lock会阻止其它代码访问console。
d）调用ftrace_stop，停止ftrace功能。ftrace是一个很有意思的功能，后面再介绍。
e）调用dpm_suspend_start，调用所有设备的-&prepare和-&suspend回调函数（具体可参考“”的描述），suspend需要正常suspend的设备。suspend device可能失败，需要跳至
Recover_platform，执行recover操作（suspend_ops-&recover）。
f）以上都是suspend前的准备工作，此时，调用suspend_enter接口，使系统进入指定的电源状态。该接口的内容如下：
static int suspend_enter(suspend_state_t state, bool *wakeup){&
if (need_suspend_ops(state) && suspend_ops-&prepare) {
error = suspend_ops-&prepare();
if (error)
goto Platform_
error = dpm_suspend_end(PMSG_SUSPEND);
if (error) {
printk(KERN_ERR &PM: Some devices failed to power down\n&);
goto Platform_
if (need_suspend_ops(state) && suspend_ops-&prepare_late) {
error = suspend_ops-&prepare_late();
if (error)
goto Platform_
if (suspend_test(TEST_PLATFORM))
goto Platform_&
* PM_SUSPEND_FREEZE equals
* frozen processes + suspended devices + idle processors.
* Thus we should invoke freeze_enter() soon after
* all the devices are suspended.
if (state == PM_SUSPEND_FREEZE) {
freeze_enter();
goto Platform_
error = disable_nonboot_cpus();
if (error || suspend_test(TEST_CPUS))
goto Enable_&
arch_suspend_disable_irqs();
BUG_ON(!irqs_disabled());&
error = syscore_suspend();
if (!error) {
*wakeup = pm_wakeup_pending();
if (!(suspend_test(TEST_CORE) || *wakeup)) {
error = suspend_ops-&enter(state);
events_check_enabled =
syscore_resume();
arch_suspend_enable_irqs();
BUG_ON(irqs_disabled());& Enable_cpus:
enable_nonboot_cpus();& Platform_wake:
if (need_suspend_ops(state) && suspend_ops-&wake)
suspend_ops-&wake();&
dpm_resume_start(PMSG_RESUME);& Platform_finish:
if (need_suspend_ops(state) && suspend_ops-&finish)
suspend_ops-&finish();&}
&&&&&&& f1）该接口处理完后，会通过返回值告知是否enter成功，同时通过wakeup指针，告知调用者，是否有wakeup事件发生，导致电源状态切换失败。
&&&&&&& f2）调用suspend_ops的prepare回调（有的话），通知平台代码，以便让其在即将进行状态切换之时，再做一些处理（需要的话）。该回调可能失败（平台代码出现意外），失败的话，需要跳至Platform_finish处，调用suspend_ops的finish回调，执行恢复操作。
&&&&&&& f3）调用dpm_suspend_end，调用所有设备的-&suspend_late和-&suspend_noirq回调函数（具体可参考“”的描述），suspend late suspend设备和需要在关中断下suspend的设备。需要说明的是，这里的noirq，是通过禁止所有的中断线的形式，而不是通过关全局中断的方式。同样，该操作可能会失败，失败的话，跳至Platform_finish处，执行恢复动作。
&&&&&&& f4）调用suspend_ops的prepare_late回调（有的话），通知平台代码，以便让其在最后关头，再做一些处理（需要的话）。该回调可能失败（平台代码出现意外），失败的话，需要跳至Platform_wake处，调用suspend_ops的wake回调，执行device的resume、调用suspend_ops的finish回调，执行恢复操作。
&&&&&&& f5）如果是suspend to freeze，执行相应的操作后，跳至Platform_wake处（后面再重点描述）。
&&&&&&& f6）调用disable_nonboot_cpus，禁止所有的非boot cpu。也会失败，执行恢复操作即可。
&&&&&&& f7）调用arch_suspend_disable_irqs，关全局中断。如果无法关闭，则为bug。
&&&&&&& f8）调用syscore_suspend，suspend system core。同样会失败，执行恢复操作即可。有关syscore，我会在另一篇文章中详细描述。
&&&&&&& f9）如果很幸运，以上操作都成功了，那么，切换吧。不过，别高兴太早，还得调用pm_wakeup_pending检查一下，这段时间内，是否有唤醒事件发生，如果有就要终止suspend。
&&&&&&& f10）如果一切顺利，调用suspend_ops的enter回调，进行状态切换。这时，系统应该已经suspend了……
&&&&&&& f11）suspend过程中，唤醒事件发生，系统唤醒，该函数接着执行resume动作，并最终返回。resume动作基本上是suspend的反动作，就不再继续分析了。
&&&&&&& f12）或者，由于意外，suspend终止，该函数也会返回。
g）suspend_enter返回，如果返回原因不是发生错误，且不是wakeup事件。则调用suspend_ops的suspend_again回调，检查是否需要再次suspend。再什么情况下要再次suspend呢？需要看具体的平台了，谁知道呢。
h）继续resume操作，resume device、start ftrace、resume console、suspend_ops-&end等等。
i）该函数返回后，表示系统已经resume。&
4.5 suspend_finish
比较简单：
static void suspend_finish(void){
suspend_thaw_processes();
pm_notifier_call_chain(PM_POST_SUSPEND);
pm_restore_console();}
a）恢复所有的用户空间进程和内核线程。
b）发送suspend结束的通知。
c）将console切换回原来的。&&&
5. 重要知识点回顾
5.1 VT switch
通常情况下，系统控制台模块（drivers\tty\vt\）会在suspend的过程中，重新分配一个console，并将控制台切换到该console上。然后在resume时，切换回旧的console。这就是VT switch功能。VT switch是很耗时的，因此内核提供了一些机制，控制是否使用这个功能：
1）提供一个接口函数pm_set_vt_switch（drivers\tty\vt\vt_ioctl.c），方便其它内核模块从整体上关闭或者开启VT switch功能。
2）VT switch全局开关处于开启状态时，满足如下的一种条件（可参考kernel\power\console.c相关的描述），即会使能VT switch
&&&&&&& a）有console driver调用pm_vt_switch_required接口，显式的要求使能VT switch。PM core的console模块会把这些信息记录在一个名称为pm_vt_switch_list的链表中。
&&&&&& b）系统禁止在suspend的过程中suspend console（由kernel/printk.c中的console_suspend_enabled变量控制）。很有可能需要使用console查看suspend过程，此时为了使console不混乱，有必要进行VT switch。
&&&&&& c）没有任何console driver关心是否需要VT switch，换句话说没有任何driver调用pm_vt_switch_required接口要求使能或禁止VT switch功能。此时会按照旧的习惯，进行VT switch。&&
因此，suspend过程对console的处理分为4步：
prepare console：负责在需要VT swich时，将当前console切换到SUSPEND console。
int pm_prepare_console(void){
if (!pm_vt_switch())
return 0;&
orig_fgconsole = vt_move_to_console(SUSPEND_CONSOLE, 1);
if (orig_fgconsole & 0)
return 1;&
orig_kmsg = vt_kmsg_redirect(SUSPEND_CONSOLE);
return 0;}
suspend console：挂起console，由kernel/printk.c实现，主要是hold住console用的互斥锁，使他人无法使用console。&&
resume console：对console解锁。
restore console：将console恢复为初始的console。
void pm_restore_console(void){
if (!pm_vt_switch())
if (orig_fgconsole &= 0) {
vt_move_to_console(orig_fgconsole, 0);
vt_kmsg_redirect(orig_kmsg);
也许，您会问，why VT switch？先留着这个疑问吧，等到分析控制台时再回答。
5.2 freezing of task
进程的freezing功能，是suspend、hibernate等电源管理功能的组成部分，在新版本内核中，它被独立出来，作为一个独立的电源管理状态（freeze）。该功能的目的，是在电源管理的状态切换过程中，确保所有用户空间进程和部分内核线程处于一个稳定的状态。有关该功能的具体描述，请参考wowotech后续的文章。&
5.3 PM notifier
PM notifier是基于内核blocking notifier功能实现的。blocking notifier提供了一种kernel内部的消息通知机制，消息接受者通过notifier注册的方式，注册一个回调函数，关注消息发送者发出的notifier。当消息产生时，消息产生者通过调用回调函数的形式，通知消息接受者。这种调用，是可以被阻塞的，因此称作blocking notifier。
那suspend功能为什么使用notifier呢？原因可能有多种，这里我举一个例子，这是我们日常开发中可能会遇到的。
由之前的描述可知，suspend过程中，suspend device发生在进程被freeze之后，resume device发生在进程被恢复之前。那么：
1）如果有些设备就需要在freeze进程之前suspend怎么办？
2）如果有些设备的resume动作需要较多延时，或者要等待什么事情发生，那么如果它的resume动作发生在进程恢复之前，岂不是要阻止所有进程的恢复？更甚者，如果该设备要等待某个进程的数据才能resume，怎么办？
再来看suspend_prepare和suspend_finish中的处理：
static int suspend_prepare(suspend_state_t state) {…
error = pm_notifier_call_chain(PM_SUSPEND_PREPARE);
if (error)
error = suspend_freeze_processes();
…}&static void suspend_finish(void){
suspend_thaw_processes();
pm_notifier_call_chain(PM_POST_SUSPEND);
pm_restore_console();}
原来PM notifier是在设备模型的框架外，开了一个后门，那些比较特殊的driver，可以绕过设备模型，直接接收PM发送的suspend信息，以便执行自身的suspend动作。特别是resume时，可以在其它进程都正好工作的时候，只让suspend进程等待driver的resume。
感兴趣的读者，可以围观一下下面这个活生生的例子（顺便提一下，好的设计是不应该有例外的）：
drivers\video\omap2\dss\core.c&&
5.4 device PM ops 和platform PM ops的调用时机
对Linux驱动工程师来说，device PM ops和platform PM ops就是电源管理（suspend）的全部，只要在合适的地方，实现合适的回调函数，即可实现系统的电源管理。但现实太复杂了，以至于kernel提供的这两个数据结构也很复杂，再回忆一下，如下：
struct dev_pm_ops {
int (*prepare)(struct device *dev);
void (*complete)(struct device *dev);
int (*suspend)(struct device *dev);
int (*resume)(struct device *dev);
int (*freeze)(struct device *dev);
int (*thaw)(struct device *dev);
int (*poweroff)(struct device *dev);
int (*restore)(struct device *dev);
int (*suspend_late)(struct device *dev);
int (*resume_early)(struct device *dev);
int (*freeze_late)(struct device *dev);
int (*thaw_early)(struct device *dev);
int (*poweroff_late)(struct device *dev);
int (*restore_early)(struct device *dev);
int (*suspend_noirq)(struct device *dev);
int (*resume_noirq)(struct device *dev);
int (*freeze_noirq)(struct device *dev);
int (*thaw_noirq)(struct device *dev);
int (*poweroff_noirq)(struct device *dev);
int (*restore_noirq)(struct device *dev);
int (*runtime_suspend)(struct device *dev);
int (*runtime_resume)(struct device *dev);
int (*runtime_idle)(struct device *dev);};&struct platform_suspend_ops {
int (*valid)(suspend_state_t state);
int (*begin)(suspend_state_t state);
int (*prepare)(void);
int (*prepare_late)(void);
int (*enter)(suspend_state_t state);
void (*wake)(void);
void (*finish)(void);
bool (*suspend_again)(void);
void (*end)(void);
void (*recover)(void);};
虽然内核的注释已经相当详细了，但我们一定会犯晕，到底该实现哪些回调？这些回调的应用场景又是什么？蜗蜗以为，要熟练使用这些回调，唯一的方法就是多coding、多理解。除此之外，我们可以总结一下在电源状态切换时，这些回调的调用时机，从侧面帮助理解。如下（只介绍和suspend功能有关的，struct dev_pm_ops简称D，struct platform_suspend_ops简称P）：
5.5 suspend过程的同步和PM wakeup
最重要的事情，如果suspend的过程中，有唤醒事件产生怎么办？正常的流程，应该终止suspend，返回并处理事件。但由于suspend过程的特殊性，进程被freeze、关中断等等，导致事情并没有那么简单，以至于在很久的一段时间内，kernel都不能很好的处理。这也称作suspend过程的同步问题。
在美好的旧时光里，suspend大多用于热关机，因此同步问题的影响并不突出(因为操作并不频繁)。但来到新时代之后，事情变了，Android竟然用suspend作日常的待机（操作就相当频繁了），这时问题就大了。那怎么解决呢？得靠system wakeup framework，也就是suspend过程中所调用的pm_wakeup_pending接口所在的模块。我会在下一篇文章中继续该模块的分析，这里就不再继续了。
本文章转载自蜗窝科技，。
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