wakeup source（唤醒源），就是在系统下电后（suspend to ram），可以将系统重新唤醒上电的中断。
唤醒机制是ARM提供的，Kernel看，就是一个被配置了wake的irq。
- /drivers/base/power/wakeup.c
wakeup_source_create
创建wakeup source对象
wakeup_source_register
创建wakeup source对象，并在/sys/devices/virtual/wakeup创建对应device的节点。
wakeup_sources_read_lock
wakeup sources list read lock
for_each_wakeup_source
Loopper for all wakeup sources
device_wakeup_enable
为device创建对应的wakeup source对象，并绑定wakeup irq。
device_wakeup_attach_irq
为device的wakeup source设置wakeup irq
device_wakeup_arm_wake_irqs
将所有wakeup source的irq设为wake irq（使能这些irq的唤醒功能）
这个函数在suspend流程中被执行，suspend时将所有wakeup source对应的中断置为wake状态
device_wakeup_disarm_wake_irqs
将所有wakeup source的irq的wake disable（禁用这些irq的唤醒功能）
这个函数在resume流程中被执行，resume时将所有wakeup source对应的中断置为diswake状态
device_init_wakeup
为device创建对应的wakeup source对象，并绑定wakeup irq。
pm_stay_awake
有中断来时，在中断处理函数中，上报wakeup event，并阻止suspend
pm_relax
中断处理结束后，释放wakeup event，重新允许suspend
dev_pm_set_wake_irq
为device设置wakeup irq
wakeup_source_activate
将一个wakeup source标记为active状态，表示有event正在被处理中
wakeup_source_deactivate
将一个wakeup source标记为deactive状态，表示event已经处理完毕
对于一个device，如果其可以作为唤醒源，可以在dts中配置wakeup-source
在driver中，解析wakeup-source，如果有，则通过dev_pm_set_wake_irq配置device的wakeup irq
然后调用wakup框架相关的device_init_wakeup，为device创建出wakeup source，并绑定wakeup irq
系统suspend时，会通过device_wakeup_arm_wake_irqs为device的wakeup irq配置wake
当中断发生是，系统被唤醒，resume流程被执行
在中断回调函数中，driver使用pm_stay_awake上报event，在处理结束后，通过pm_relax结束处理。
同时，pm_stay_awake被用于wakelock，当wakelock被设置时，也会作为一个wakeup event上报，将suspend流程禁用。
所有唤醒源列表（/drivers/base/power/wakeup.c）
root@test:/ # cat /sys/kernel/debug/wakeup_sources
name active_count event_count wakeup_count expire_count active_since total_time max_time last_change prevent_suspend_time
event1 40644 40644 0 0 0 31294 30 537054822 0
event4 4496 4496 0 0 0 13369 22 20913677 0
event5 4496 4496 0 0 0 13048 22 20913677 0
event0 4540 4540 0 0 0 27995 277 258270184 0
eventpoll 40688 54176 0 0 0 217 5 537054822 0
NETLINK 2175 2175 0 0 0 16960 59 537058523 0
root@AGX-Orin:/sys/class/wakeup# ls -l
total 0
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup1 -> ../../devices/platform/c2a0000.rtc/wakeup/wakeup1
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup10 -> ../../devices/platform/3400000.sdhci/wakeup/wakeup10
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup11 -> ../../devices/platform/3610000.xhci/usb1/1-4/1-4.4/wakeup/wakeup11
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup12 -> ../../devices/platform/gpio-keys/wakeup/wakeup12
lrwxrwxrwx 1 root root 0 May 25 14:04 wakeup13 -> ../../devices/platform/c240000.i2c/i2c-1/1-0008/wakeup/wakeup13
lrwxrwxrwx 1 root root 0 May 25 14:04 wakeup14 -> ../../devices/platform/c240000.i2c/i2c-1/1-0008/power_supply/ucsi-source-psy-1-00081/wakeup14
lrwxrwxrwx 1 root root 0 May 25 15:17 wakeup15 -> ../../devices/platform/c240000.i2c/i2c-1/1-0008/power_supply/ucsi-source-psy-1-00082/wakeup15
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup2 -> ../../devices/platform/c2a0000.rtc/rtc/rtc1/alarmtimer.0.auto/wakeup/wakeup2
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup3 -> ../../devices/platform/bpmp/bpmp:i2c/i2c-4/4-003c/nvvrs-pseq-rtc/wakeup/wakeup3
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup4 -> ../../devices/platform/efuse-burn/wakeup/wakeup4
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup5 -> ../../devices/platform/3610000.xhci/usb1/wakeup/wakeup5
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup6 -> ../../devices/platform/3610000.xhci/usb2/wakeup/wakeup6
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup7 -> ../../devices/platform/3610000.xhci/wakeup/wakeup7
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup8 -> ../../devices/virtual/wakeup/wakeup8
lrwxrwxrwx 1 root root 0 Mar 15 2023 wakeup9 -> ../../devices/virtual/wakeup/wakeup9
所有唤醒源绑定的device列表（/drivers/base/power/wakeup_stats.c）
/sys/devices/virtual/wakeup/wakeup<dev_id>
uevent
event_count
max_time_ms
wakeup_count
total_time_ms
expire_count
active_count
subsystem
last_change_ms
prevent_suspend_time_ms
name
active_time_ms
控制Device的唤醒源：
在/sys/devices/<device>/power、/sys/devices/<device>/<wakeup>下，会创建wakeup控制节点，可以enable/disable一个device的唤醒源。
当系统被唤醒时，可以通过/sys/power/pm_wakeup_irq查看唤醒源。
http://www.wowotech.net/pm_subsystem/wakeup_events_framework.html