Based on kernel version 4.0. Page generated on 2015-04-14 21:25 EST.
1 Power Management Interface 2 3 4 The power management subsystem provides a unified sysfs interface to 5 userspace, regardless of what architecture or platform one is 6 running. The interface exists in /sys/power/ directory (assuming sysfs 7 is mounted at /sys). 8 9 /sys/power/state controls system power state. Reading from this file 10 returns what states are supported, which is hard-coded to 'freeze', 11 'standby' (Power-On Suspend), 'mem' (Suspend-to-RAM), and 'disk' 12 (Suspend-to-Disk). 13 14 Writing to this file one of those strings causes the system to 15 transition into that state. Please see the file 16 Documentation/power/states.txt for a description of each of those 17 states. 18 19 20 /sys/power/disk controls the operating mode of the suspend-to-disk 21 mechanism. Suspend-to-disk can be handled in several ways. We have a 22 few options for putting the system to sleep - using the platform driver 23 (e.g. ACPI or other suspend_ops), powering off the system or rebooting the 24 system (for testing). 25 26 Additionally, /sys/power/disk can be used to turn on one of the two testing 27 modes of the suspend-to-disk mechanism: 'testproc' or 'test'. If the 28 suspend-to-disk mechanism is in the 'testproc' mode, writing 'disk' to 29 /sys/power/state will cause the kernel to disable nonboot CPUs and freeze 30 tasks, wait for 5 seconds, unfreeze tasks and enable nonboot CPUs. If it is 31 in the 'test' mode, writing 'disk' to /sys/power/state will cause the kernel 32 to disable nonboot CPUs and freeze tasks, shrink memory, suspend devices, wait 33 for 5 seconds, resume devices, unfreeze tasks and enable nonboot CPUs. Then, 34 we are able to look in the log messages and work out, for example, which code 35 is being slow and which device drivers are misbehaving. 36 37 Reading from this file will display all supported modes and the currently 38 selected one in brackets, for example 39 40 [shutdown] reboot test testproc 41 42 Writing to this file will accept one of 43 44 'platform' (only if the platform supports it) 45 'shutdown' 46 'reboot' 47 'testproc' 48 'test' 49 50 /sys/power/image_size controls the size of the image created by 51 the suspend-to-disk mechanism. It can be written a string 52 representing a non-negative integer that will be used as an upper 53 limit of the image size, in bytes. The suspend-to-disk mechanism will 54 do its best to ensure the image size will not exceed that number. However, 55 if this turns out to be impossible, it will try to suspend anyway using the 56 smallest image possible. In particular, if "0" is written to this file, the 57 suspend image will be as small as possible. 58 59 Reading from this file will display the current image size limit, which 60 is set to 2/5 of available RAM by default. 61 62 /sys/power/pm_trace controls the code which saves the last PM event point in 63 the RTC across reboots, so that you can debug a machine that just hangs 64 during suspend (or more commonly, during resume). Namely, the RTC is only 65 used to save the last PM event point if this file contains '1'. Initially it 66 contains '0' which may be changed to '1' by writing a string representing a 67 nonzero integer into it. 68 69 To use this debugging feature you should attempt to suspend the machine, then 70 reboot it and run 71 72 dmesg -s 1000000 | grep 'hash matches' 73 74 CAUTION: Using it will cause your machine's real-time (CMOS) clock to be 75 set to a random invalid time after a resume.