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Suspend and Resume


This flowchart depicts the process that powerd's Suspender class follows to suspend and resume a Chromium OS system:

Yellow boxes indicates states where powerd's event loop is running, white ovals indicate discrete tasks that are performed, and arrows indicate events or other occurrences that result in changes to the state.

The process begins when powerd (or some other process) decides that the system should be suspended. Suspender receives a request to suspend the system and generates an ID corresponding to the request. It performs pre-suspend preparation, such as muting audio and turning off the panel backlight. Then, it emits a SuspendImminent D-Bus signal to announce the suspend request to other processes (particularly ones that have registered suspend delays). The SuspendDelayController class is used to track the state of suspend delays.

After all suspend delays have become ready or have timed out, powerd starts a suspend attempt. This involves using powerd_setuid_helper to run the powerd_suspend script as root; this script is responsible for writing "mem" to /sys/power/state (the actual act that tells the kernel to suspend). If powerd_suspend reports that the system suspended and resumed successfully, then the pre-suspend preparations are undone, a SuspendDone D-Bus signal is emitted to inform other process that the suspend request is complete, and Suspender is ready for the next suspend request.

If the suspend attempt fails, Suspender waits for ten seconds before running powerd_suspend again to retry. After ten failed retries, the system is shut down.


The panel backlight brightness and display power are configured in a specific manner to minimize resume time:
  • Before suspending, powerd first sets the backlight brightness to 0 and sets the kernel's "resume brightness" to the panel's original, non-dimmed level.
  • Next, powerd emits the SuspendImminent signal; when Chrome sees it, it uses XRandR to turn the display on (that is not a typo).
  • powerd suspends the system.
  • After the system resumes, and before userspace is awake, the restores the display's previous (on) power state and the resume brightness.
This sequence of events allows the kernel to restore the display's original state quickly when resuming. Note that when the system suspends, the display is actually on but the backlight is turned off.

Wakeup Counts

The kernel provides a /sys/power/wakeup_count file that can be used to avoid races while suspending. Drivers within the kernel increment this count in response to user activity like keyboard or touchpad events.
  • When powerd receives a suspend request, it reads and saves the current count.
  • When the powerd_suspend script is run to suspend the system, it writes the previously-read count back to the wakeup_count file. If the number that it writes doesn't match the kernel's current count, the kernel reports failure, which results in powerd_suspend reporting failure, which results in the suspend attempt failing.
  • When powerd_suspend writes to /sys/power/state, the kernel again reports failure if the wakeup count has changed since powerd_suspend wrote to it.
This has the effect of letting suspend attempts get aborted if user activity is observed by the kernel after the point at which the userspace powerd process starts suspending the system.

powerd's RequestSuspend D-Bus method takes an optional externally-supplied wakeup count. If a count is passed, powerd uses it instead of reading the wakeup_count file. autotests exercising the suspend path typically pass a count after installing a RTC wake alarm to trigger a resume several seconds after suspending. If powerd sees a count mismatch, it aborts the suspend request immediately. If it instead retried it ten seconds later, the retry would probably occur after the wake alarm had fired, resulting in the system sleeping forever.

Dark Resume

On some systems, powerd passes a duration to the kernel in order to periodically wake the system from the S3 state. This results in an awake-with-the-display-off-and-audio-muted mode referred to as "dark resume". After the system wakes into dark resume, powerd checks the battery level. If it is low enough to suggest that the battery will be drained entirely while in S3, resulting in a system that can't be used until it's recharged, powerd shuts down the system. Otherwise, it re-suspends immediately.