| Power Management Interface for System Sleep | |
| Copyright (c) 2016 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> | |
| The power management subsystem provides userspace with a unified sysfs interface | |
| for system sleep regardless of the underlying system architecture or platform. | |
| The interface is located in the /sys/power/ directory (assuming that sysfs is | |
| mounted at /sys). | |
| /sys/power/state is the system sleep state control file. | |
| Reading from it returns a list of supported sleep states, encoded as: | |
| 'freeze' (Suspend-to-Idle) | |
| 'standby' (Power-On Suspend) | |
| 'mem' (Suspend-to-RAM) | |
| 'disk' (Suspend-to-Disk) | |
| Suspend-to-Idle is always supported. Suspend-to-Disk is always supported | |
| too as long the kernel has been configured to support hibernation at all | |
| (ie. CONFIG_HIBERNATION is set in the kernel configuration file). Support | |
| for Suspend-to-RAM and Power-On Suspend depends on the capabilities of the | |
| platform. | |
| If one of the strings listed in /sys/power/state is written to it, the system | |
| will attempt to transition into the corresponding sleep state. Refer to | |
| Documentation/power/states.txt for a description of each of those states. | |
| /sys/power/disk controls the operating mode of hibernation (Suspend-to-Disk). | |
| Specifically, it tells the kernel what to do after creating a hibernation image. | |
| Reading from it returns a list of supported options encoded as: | |
| 'platform' (put the system into sleep using a platform-provided method) | |
| 'shutdown' (shut the system down) | |
| 'reboot' (reboot the system) | |
| 'suspend' (trigger a Suspend-to-RAM transition) | |
| 'test_resume' (resume-after-hibernation test mode) | |
| The currently selected option is printed in square brackets. | |
| The 'platform' option is only available if the platform provides a special | |
| mechanism to put the system to sleep after creating a hibernation image (ACPI | |
| does that, for example). The 'suspend' option is available if Suspend-to-RAM | |
| is supported. Refer to Documentation/power/basic_pm_debugging.txt for the | |
| description of the 'test_resume' option. | |
| To select an option, write the string representing it to /sys/power/disk. | |
| /sys/power/image_size controls the size of hibernation images. | |
| It can be written a string representing a non-negative integer that will be | |
| used as a best-effort upper limit of the image size, in bytes. The hibernation | |
| core will do its best to ensure that the image size will not exceed that number. | |
| However, if that turns out to be impossible to achieve, a hibernation image will | |
| still be created and its size will be as small as possible. In particular, | |
| writing '0' to this file will enforce hibernation images to be as small as | |
| possible. | |
| Reading from this file returns the current image size limit, which is set to | |
| around 2/5 of available RAM by default. | |
| /sys/power/pm_trace controls the PM trace mechanism saving the last suspend | |
| or resume event point in the RTC across reboots. | |
| It helps to debug hard lockups or reboots due to device driver failures that | |
| occur during system suspend or resume (which is more common) more effectively. | |
| If /sys/power/pm_trace contains '1', the fingerprint of each suspend/resume | |
| event point in turn will be stored in the RTC memory (overwriting the actual | |
| RTC information), so it will survive a system crash if one occurs right after | |
| storing it and it can be used later to identify the driver that caused the crash | |
| to happen (see Documentation/power/s2ram.txt for more information). | |
| Initially it contains '0' which may be changed to '1' by writing a string | |
| representing a nonzero integer into it. |