How does Suspend work on Linux (CentOS)

How does suspend work on Linux:

Step1: suspend on Userspace

pm-suspend is shell script, which attempts to prepare the system for suspend. For example, it will tell NetworkManager to shut down networking, and use vbetool (run code at video option ROM) to save VGA state. Finally, it wil echo string “mem” to /sys/power/state. This jumps to kernel, and the userspace is stopped.

Detailed info:

pm-suspend: /usr/lib/pm-utils/bin/pm-action

functions: /usr/lib/pm-utils/functions

10NetworkManager: /usr/lib/pm-utils/sleep.d/10NetwrokManager

20video: /usr/lib/pm-utils/sleep.d/20video

Step2: suspend in kernel

After run “echo -n mem > /sys/power/state”, the kernel goes through the device tree and calls suspend method on each bound driver. In the mean while, kernel execute couple of ACPI methods acpi_enter_sleep_state_prep() and acpi_enter_sleep_state(). In this method, kernel will set the SLP_TYP register at south-bridge, which tell BIOS to wakeup from sleep state next time.

Detailed info:

acpi_enter_sleep_state_prep(): /linuxsrc/drivers/acpi/hardware/hwsleep.c

acpi_enter_sleep_state(): /linuxsrc/drivers/acpi/hardware/hwsleep.c

main.c /linux/src/drivers/acpi/sleep/main.c

Suspend Time measurements:

Userspace: 1.5 seconds

Kenerl: 1 second