Linux内核提供了4种Suspend模式:
freeze （suspend to idle） 进程冻结，CPU进入Idle，外设进入Suspend（s2idle）
standby （suspend to standby） 从核拔除下电，只保留主核，系统正产运行（不使用）
mem （suspend to mem） CPU、外设均下电，进程冻结，DDR进入自刷新（主要流程）
disk （suspend to disk） CPU、DDR、外设均下电，所有数据进入Disk（hibernate，不使用）
通过用户空间向”/sys/power/state”写入”freeze”、”standby”、”mem”来触发对应Suspend流程。
Android Suspend也在用户态通过写节点来触发。
Suspend主要包括5件事：
1、冻结进程
需要将所有进程置为Freeze状态，保存现场上下文，暂停运行。在Resume后重新使其继续运行。
2、设备休眠（DPM）
Device休眠通过dpm框架来管理，DPM是一个PM设备链表，Device电源管理框架在DPM中详细描述。
3、系统休眠（Platform）
suspend_ops在psci初始化时，通过suspend_set_ops注册。在Platform Suspend阶段，通过PSCI下发给ATF完成CPU及整个系统的Suspend动作。
4、Syscore Suspend
syscore允许subsys向suspend过程挂回调，比实现一个pm device，这个量级更轻。
https://lore.kernel.org/lkml/201103100133.06734.rjw@sisk.pl/
5、PM Notifiers
还有一些回调点，需要在suspend前，或resume后完成一些事。这时可以使用PM Notifier，通过register_pm_notifier注册回调。
通过ftrace event可以清楚追踪到这些流程。
- /kernel/power/suspend.c # main suspend flow
- /kernel/power/main.c # /sys/power interface
- /drivers/base/power/main.c # dpm (device power management)
- /drivers/base/syscore.c # pm callback framework
pm_suspend/enter_state
suspend_prepare
suspend_freeze_processes
pm_notifier_call_chain_robust # Notifiers: PM_SUSPEND_PREPARE
suspend_freeze_processes # 冻结进程
pm_notifier_call_chain # Notifiers: PM_POST_SUSPEND
suspend_devices_and_enter
platform_suspend_begin # Platfom:suspend_ops->begin
suspend_console # Disable Printk and Console
dpm_suspend_start # DPM: Suspend all Device
suspend_enter
platform_suspend_prepare # Platfom: suspend_ops->prepare
dpm_suspend_late # DPM: Suspend all Device (late)
platform_suspend_prepare_late
dpm_suspend_noirq # DPM: Suspend all Device (noirq)
platform_suspend_prepare_noirq # Platfom: suspend_ops->prepare_late
pm_sleep_disable_secondary_cpus
arch_suspend_disable_irqs # 禁中断
syscore_suspend # Syscore: Suspend all syscore callbacks
suspend_ops->enter(state) # Platfom: enter state
/sys/power
suspend功能节点
Tracing suspend/resume
* 操作需要在USB断开的情况下进行
cd /sys/kernel/debug/tracing
echo 1 > events/power/suspend_resume/enable
echo 1 > tracing_on
<...>-4745 [001] .... 280208.503406: suspend_resume: suspend_enter[3] begin
<...>-4745 [001] .... 280208.503416: suspend_resume: sync_filesystems[0] begin
<...>-4745 [000] .... 280208.566895: suspend_resume: sync_filesystems[0] end
<...>-4745 [000] .... 280208.577584: suspend_resume: freeze_processes[0] begin
<...>-4745 [000] .... 280208.597040: suspend_resume: freeze_processes[0] end
<...>-4745 [000] .... 280208.597048: suspend_resume: suspend_enter[3] end
<...>-4745 [000] .... 280208.597053: suspend_resume: dpm_prepare[2] begin
<...>-4745 [000] .... 280208.600119: suspend_resume: dpm_prepare[2] end
<...>-4745 [000] .... 280208.600124: suspend_resume: dpm_suspend[2] begin
<...>-4745 [000] .... 280208.600354: lpi_pinctrl_suspend: lpi_pinctrl_suspend: system suspend
<...>-4745 [000] .... 280208.641639: suspend_resume: dpm_suspend[2] end
<...>-4745 [000] .... 280208.641654: suspend_resume: dpm_suspend_late[2] begin
<...>-4745 [000] .... 280208.647185: suspend_resume: dpm_suspend_late[2] end
<...>-4745 [000] .... 280208.648907: suspend_resume: dpm_suspend_noirq[2] begin
<...>-4745 [000] .... 280208.653170: suspend_resume: dpm_suspend_noirq[2] end
<...>-4745 [000] .... 280208.653211: suspend_resume: CPU_OFF[1] begin
<...>-4745 [000] .... 280208.657886: suspend_resume: CPU_OFF[1] end
<...>-4745 [000] .... 280208.657890: suspend_resume: CPU_OFF[2] begin
<...>-4745 [000] .... 280208.661211: suspend_resume: CPU_OFF[2] end
<...>-4745 [000] .... 280208.661214: suspend_resume: CPU_OFF[3] begin
<...>-4745 [000] .... 280208.663980: suspend_resume: CPU_OFF[3] end
<...>-4745 [000] .... 280208.663983: suspend_resume: CPU_OFF[4] begin
<...>-4745 [000] .... 280208.666587: suspend_resume: CPU_OFF[4] end
<...>-4745 [000] .... 280208.666589: suspend_resume: CPU_OFF[5] begin
<...>-4745 [000] .... 280208.670508: suspend_resume: CPU_OFF[5] end
<...>-4745 [000] .... 280208.670511: suspend_resume: CPU_OFF[6] begin
<...>-4745 [000] .... 280208.674076: suspend_resume: CPU_OFF[6] end
<...>-4745 [000] .... 280208.674079: suspend_resume: CPU_OFF[7] begin
<...>-4745 [000] .... 280208.677074: suspend_resume: CPU_OFF[7] end
<...>-4745 [000] d... 280208.677145: suspend_resume: syscore_suspend[0] begin
<...>-4745 [000] d... 280208.677163: suspend_resume: syscore_suspend[0] end
<...>-4745 [000] d... 280208.677163: suspend_resume: machine_suspend[3] begin
<...>-4745 [000] d... 280208.677163: suspend_resume: machine_suspend[3] end
<...>-4745 [000] d... 280208.677163: suspend_resume: syscore_resume[0] begin
<...>-4745 [000] dn.. 280208.677212: suspend_resume: syscore_resume[0] end
<...>-4745 [000] .... 280208.677703: suspend_resume: CPU_ON[1] begin
<...>-4745 [000] .... 280208.680579: suspend_resume: CPU_ON[1] end
<...>-4745 [000] .... 280208.680957: suspend_resume: CPU_ON[2] begin
<...>-4745 [000] .... 280208.682403: suspend_resume: CPU_ON[2] end
<...>-4745 [000] .... 280208.683170: suspend_resume: CPU_ON[3] begin
<...>-4745 [000] .... 280208.684575: suspend_resume: CPU_ON[3] end
<...>-4745 [000] .... 280208.684727: suspend_resume: CPU_ON[4] begin
<...>-4745 [000] .... 280208.686221: suspend_resume: CPU_ON[4] end
<...>-4745 [000] .... 280208.686371: suspend_resume: CPU_ON[5] begin
<...>-4745 [000] .... 280208.687832: suspend_resume: CPU_ON[5] end
<...>-4745 [000] .... 280208.687978: suspend_resume: CPU_ON[6] begin
<...>-4745 [000] .... 280208.689916: suspend_resume: CPU_ON[6] end
<...>-4745 [000] .... 280208.690110: suspend_resume: CPU_ON[7] begin
<...>-4745 [000] .... 280208.691842: suspend_resume: CPU_ON[7] end
<...>-4745 [000] .... 280208.692035: suspend_resume: dpm_resume_noirq[16] begin
<...>-4745 [006] .... 280208.696154: suspend_resume: dpm_resume_noirq[16] end
<...>-4745 [007] .... 280208.697518: suspend_resume: dpm_resume_early[16] begin
<...>-4745 [005] .... 280208.702554: suspend_resume: dpm_resume_early[16] end
<...>-4745 [005] .... 280208.702563: suspend_resume: dpm_resume[16] begin
<...>-4745 [002] .... 280208.721405: suspend_resume: dpm_resume[16] end
<...>-4745 [002] .... 280208.721412: suspend_resume: dpm_complete[16] begin
<...>-4745 [002] .n.. 280208.724622: suspend_resume: dpm_complete[16] end
<...>-4745 [002] .... 280208.724784: suspend_resume: resume_console[3] begin
<...>-4745 [002] .... 280208.724786: suspend_resume: resume_console[3] end
<...>-4745 [002] .... 280208.724790: suspend_resume: thaw_processes[0] begin
<...>-4745 [002] .... 280208.734586: suspend_resume: thaw_processes[0] end
可以看到suspend/resume的整个框架流程。
查看唤醒源：
cat /sys/power/pm_wakeup_irq # 唤醒源中断号
cat /proc/interrupt # 查看中断对应设备名
/sys/kernel/debug/suspend_stats
suspend成功失败次数
Suspend States and Power sysfs interface:
https://www.kernel.org/doc/html/v5.0/admin-guide/pm/sleep-states.html
Suspend Flow:
https://docs.kernel.org/admin-guide/pm/suspend-flows.html
Suspend Debug:
https://www.kernel.org/doc/html/v5.7/power/basic-pm-debugging.html#testing-suspend-to-ram-str
https://01.org/blogs/rzhang/2015/best-practice-debug-linux-suspend/hibernate-issues
https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-power
https://wiki.ubuntu.com/DebuggingKernelSuspend
Others:
https://zhuanlan.zhihu.com/p/542445635