Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.
1 System Power Management Sleep States 2 3 (C) 2014 Intel Corp., Rafael J. Wysocki <firstname.lastname@example.org> 4 5 The kernel supports up to four system sleep states generically, although three 6 of them depend on the platform support code to implement the low-level details 7 for each state. 8 9 The states are represented by strings that can be read or written to the 10 /sys/power/state file. Those strings may be "mem", "standby", "freeze" and 11 "disk", where the last three always represent Power-On Suspend (if supported), 12 Suspend-To-Idle and hibernation (Suspend-To-Disk), respectively. 13 14 The meaning of the "mem" string is controlled by the /sys/power/mem_sleep file. 15 It contains strings representing the available modes of system suspend that may 16 be triggered by writing "mem" to /sys/power/state. These modes are "s2idle" 17 (Suspend-To-Idle), "shallow" (Power-On Suspend) and "deep" (Suspend-To-RAM). 18 The "s2idle" mode is always available, while the other ones are only available 19 if supported by the platform (if not supported, the strings representing them 20 are not present in /sys/power/mem_sleep). The string representing the suspend 21 mode to be used subsequently is enclosed in square brackets. Writing one of 22 the other strings present in /sys/power/mem_sleep to it causes the suspend mode 23 to be used subsequently to change to the one represented by that string. 24 25 Consequently, there are two ways to cause the system to go into the 26 Suspend-To-Idle sleep state. The first one is to write "freeze" directly to 27 /sys/power/state. The second one is to write "s2idle" to /sys/power/mem_sleep 28 and then to write "mem" to /sys/power/state. Similarly, there are two ways 29 to cause the system to go into the Power-On Suspend sleep state (the strings to 30 write to the control files in that case are "standby" or "shallow" and "mem", 31 respectively) if that state is supported by the platform. In turn, there is 32 only one way to cause the system to go into the Suspend-To-RAM state (write 33 "deep" into /sys/power/mem_sleep and "mem" into /sys/power/state). 34 35 The default suspend mode (ie. the one to be used without writing anything into 36 /sys/power/mem_sleep) is either "deep" (if Suspend-To-RAM is supported) or 37 "s2idle", but it can be overridden by the value of the "mem_sleep_default" 38 parameter in the kernel command line. 39 40 The properties of all of the sleep states are described below. 41 42 43 State: Suspend-To-Idle 44 ACPI state: S0 45 Label: "s2idle" ("freeze") 46 47 This state is a generic, pure software, light-weight, system sleep state. 48 It allows more energy to be saved relative to runtime idle by freezing user 49 space and putting all I/O devices into low-power states (possibly 50 lower-power than available at run time), such that the processors can 51 spend more time in their idle states. 52 53 This state can be used for platforms without Power-On Suspend/Suspend-to-RAM 54 support, or it can be used in addition to Suspend-to-RAM to provide reduced 55 resume latency. It is always supported. 56 57 58 State: Standby / Power-On Suspend 59 ACPI State: S1 60 Label: "shallow" ("standby") 61 62 This state, if supported, offers moderate, though real, power savings, while 63 providing a relatively low-latency transition back to a working system. No 64 operating state is lost (the CPU retains power), so the system easily starts up 65 again where it left off. 66 67 In addition to freezing user space and putting all I/O devices into low-power 68 states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline 69 and all low-level system functions are suspended during transitions into this 70 state. For this reason, it should allow more energy to be saved relative to 71 Suspend-To-Idle, but the resume latency will generally be greater than for that 72 state. 73 74 75 State: Suspend-to-RAM 76 ACPI State: S3 77 Label: "deep" 78 79 This state, if supported, offers significant power savings as everything in the 80 system is put into a low-power state, except for memory, which should be placed 81 into the self-refresh mode to retain its contents. All of the steps carried out 82 when entering Power-On Suspend are also carried out during transitions to STR. 83 Additional operations may take place depending on the platform capabilities. In 84 particular, on ACPI systems the kernel passes control to the BIOS (platform 85 firmware) as the last step during STR transitions and that usually results in 86 powering down some more low-level components that aren't directly controlled by 87 the kernel. 88 89 System and device state is saved and kept in memory. All devices are suspended 90 and put into low-power states. In many cases, all peripheral buses lose power 91 when entering STR, so devices must be able to handle the transition back to the 92 "on" state. 93 94 For at least ACPI, STR requires some minimal boot-strapping code to resume the 95 system from it. This may be the case on other platforms too. 96 97 98 State: Suspend-to-disk 99 ACPI State: S4 100 Label: "disk" 101 102 This state offers the greatest power savings, and can be used even in 103 the absence of low-level platform support for power management. This 104 state operates similarly to Suspend-to-RAM, but includes a final step 105 of writing memory contents to disk. On resume, this is read and memory 106 is restored to its pre-suspend state. 107 108 STD can be handled by the firmware or the kernel. If it is handled by 109 the firmware, it usually requires a dedicated partition that must be 110 setup via another operating system for it to use. Despite the 111 inconvenience, this method requires minimal work by the kernel, since 112 the firmware will also handle restoring memory contents on resume. 113 114 For suspend-to-disk, a mechanism called 'swsusp' (Swap Suspend) is used 115 to write memory contents to free swap space. swsusp has some restrictive 116 requirements, but should work in most cases. Some, albeit outdated, 117 documentation can be found in Documentation/power/swsusp.txt. 118 Alternatively, userspace can do most of the actual suspend to disk work, 119 see userland-swsusp.txt. 120 121 Once memory state is written to disk, the system may either enter a 122 low-power state (like ACPI S4), or it may simply power down. Powering 123 down offers greater savings, and allows this mechanism to work on any 124 system. However, entering a real low-power state allows the user to 125 trigger wake up events (e.g. pressing a key or opening a laptop lid).