Loading Documentation/ABI/testing/sysfs-power +20 −9 Original line number Original line Diff line number Diff line Loading @@ -7,19 +7,30 @@ Description: subsystem. subsystem. What: /sys/power/state What: /sys/power/state Date: August 2006 Date: May 2014 Contact: Rafael J. Wysocki <rjw@rjwysocki.net> Contact: Rafael J. Wysocki <rjw@rjwysocki.net> Description: Description: The /sys/power/state file controls the system power state. The /sys/power/state file controls system sleep states. Reading from this file returns what states are supported, Reading from this file returns the available sleep state which is hard-coded to 'freeze' (Low-Power Idle), 'standby' labels, which may be "mem", "standby", "freeze" and "disk" (Power-On Suspend), 'mem' (Suspend-to-RAM), and 'disk' (hibernation). The meanings of the first three labels depend on (Suspend-to-Disk). the relative_sleep_states command line argument as follows: 1) relative_sleep_states = 1 "mem", "standby", "freeze" represent non-hibernation sleep states from the deepest ("mem", always present) to the shallowest ("freeze"). "standby" and "freeze" may or may not be present depending on the capabilities of the platform. "freeze" can only be present if "standby" is present. 2) relative_sleep_states = 0 (default) "mem" - "suspend-to-RAM", present if supported. "standby" - "power-on suspend", present if supported. "freeze" - "suspend-to-idle", always present. Writing to this file one of these strings causes the system to Writing to this file one of these strings causes the system to transition into that state. Please see the file transition into the corresponding state, if available. See Documentation/power/states.txt for a description of each of Documentation/power/states.txt for a description of what these states. "suspend-to-RAM", "power-on suspend" and "suspend-to-idle" mean. What: /sys/power/disk What: /sys/power/disk Date: September 2006 Date: September 2006 Loading Documentation/kernel-parameters.txt +7 −0 Original line number Original line Diff line number Diff line Loading @@ -2889,6 +2889,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted. [KNL, SMP] Set scheduler's default relax_domain_level. [KNL, SMP] Set scheduler's default relax_domain_level. See Documentation/cgroups/cpusets.txt. See Documentation/cgroups/cpusets.txt. relative_sleep_states= [SUSPEND] Use sleep state labeling where the deepest state available other than hibernation is always "mem". Format: { "0" | "1" } 0 -- Traditional sleep state labels. 1 -- Relative sleep state labels. reserve= [KNL,BUGS] Force the kernel to ignore some iomem area reserve= [KNL,BUGS] Force the kernel to ignore some iomem area reservetop= [X86-32] reservetop= [X86-32] Loading Documentation/power/devices.txt +30 −4 Original line number Original line Diff line number Diff line Loading @@ -2,6 +2,7 @@ Device Power Management Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu> Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu> Copyright (c) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> Most of the code in Linux is device drivers, so most of the Linux power Most of the code in Linux is device drivers, so most of the Linux power Loading Loading @@ -326,6 +327,20 @@ the phases are: driver in some way for the upcoming system power transition, but it driver in some way for the upcoming system power transition, but it should not put the device into a low-power state. should not put the device into a low-power state. For devices supporting runtime power management, the return value of the prepare callback can be used to indicate to the PM core that it may safely leave the device in runtime suspend (if runtime-suspended already), provided that all of the device's descendants are also left in runtime suspend. Namely, if the prepare callback returns a positive number and that happens for all of the descendants of the device too, and all of them (including the device itself) are runtime-suspended, the PM core will skip the suspend, suspend_late and suspend_noirq suspend phases as well as the resume_noirq, resume_early and resume phases of the following system resume for all of these devices. In that case, the complete callback will be called directly after the prepare callback and is entirely responsible for bringing the device back to the functional state as appropriate. 2. The suspend methods should quiesce the device to stop it from performing 2. The suspend methods should quiesce the device to stop it from performing I/O. They also may save the device registers and put it into the I/O. They also may save the device registers and put it into the appropriate low-power state, depending on the bus type the device is on, appropriate low-power state, depending on the bus type the device is on, Loading Loading @@ -400,12 +415,23 @@ When resuming from freeze, standby or memory sleep, the phases are: the resume callbacks occur; it's not necessary to wait until the the resume callbacks occur; it's not necessary to wait until the complete phase. complete phase. Moreover, if the preceding prepare callback returned a positive number, the device may have been left in runtime suspend throughout the whole system suspend and resume (the suspend, suspend_late, suspend_noirq phases of system suspend and the resume_noirq, resume_early, resume phases of system resume may have been skipped for it). In that case, the complete callback is entirely responsible for bringing the device back to the functional state after system suspend if necessary. [For example, it may need to queue up a runtime resume request for the device for this purpose.] To check if that is the case, the complete callback can consult the device's power.direct_complete flag. Namely, if that flag is set when the complete callback is being run, it has been called directly after the preceding prepare and special action may be required to make the device work correctly afterward. At the end of these phases, drivers should be as functional as they were before At the end of these phases, drivers should be as functional as they were before suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are gated on. Even if the device was in a low-power state before the system sleep gated on. because of runtime power management, afterwards it should be back in its full-power state. There are multiple reasons why it's best to do this; they are discussed in more detail in Documentation/power/runtime_pm.txt. However, the details here may again be platform-specific. For example, However, the details here may again be platform-specific. For example, some systems support multiple "run" states, and the mode in effect at some systems support multiple "run" states, and the mode in effect at Loading Documentation/power/runtime_pm.txt +17 −0 Original line number Original line Diff line number Diff line Loading @@ -2,6 +2,7 @@ Runtime Power Management Framework for I/O Devices (C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. (C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. (C) 2010 Alan Stern <stern@rowland.harvard.edu> (C) 2010 Alan Stern <stern@rowland.harvard.edu> (C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> 1. Introduction 1. Introduction Loading Loading @@ -444,6 +445,10 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h: bool pm_runtime_status_suspended(struct device *dev); bool pm_runtime_status_suspended(struct device *dev); - return true if the device's runtime PM status is 'suspended' - return true if the device's runtime PM status is 'suspended' bool pm_runtime_suspended_if_enabled(struct device *dev); - return true if the device's runtime PM status is 'suspended' and its 'power.disable_depth' field is equal to 1 void pm_runtime_allow(struct device *dev); void pm_runtime_allow(struct device *dev); - set the power.runtime_auto flag for the device and decrease its usage - set the power.runtime_auto flag for the device and decrease its usage counter (used by the /sys/devices/.../power/control interface to counter (used by the /sys/devices/.../power/control interface to Loading Loading @@ -644,6 +649,18 @@ place (in particular, if the system is not waking up from hibernation), it may be more efficient to leave the devices that had been suspended before the system be more efficient to leave the devices that had been suspended before the system suspend began in the suspended state. suspend began in the suspended state. To this end, the PM core provides a mechanism allowing some coordination between different levels of device hierarchy. Namely, if a system suspend .prepare() callback returns a positive number for a device, that indicates to the PM core that the device appears to be runtime-suspended and its state is fine, so it may be left in runtime suspend provided that all of its descendants are also left in runtime suspend. If that happens, the PM core will not execute any system suspend and resume callbacks for all of those devices, except for the complete callback, which is then entirely responsible for handling the device as appropriate. This only applies to system suspend transitions that are not related to hibernation (see Documentation/power/devices.txt for more information). The PM core does its best to reduce the probability of race conditions between The PM core does its best to reduce the probability of race conditions between the runtime PM and system suspend/resume (and hibernation) callbacks by carrying the runtime PM and system suspend/resume (and hibernation) callbacks by carrying out the following operations: out the following operations: Loading Documentation/power/states.txt +56 −31 Original line number Original line Diff line number Diff line System Power Management Sleep States System Power Management States (C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> The kernel supports up to four system sleep states generically, although three of them depend on the platform support code to implement the low-level details for each state. The kernel supports four power management states generically, though The states are represented by strings that can be read or written to the one is generic and the other three are dependent on platform support /sys/power/state file. Those strings may be "mem", "standby", "freeze" and code to implement the low-level details for each state. "disk", where the last one always represents hibernation (Suspend-To-Disk) and This file describes each state, what they are the meaning of the remaining ones depends on the relative_sleep_states command commonly called, what ACPI state they map to, and what string to write line argument. to /sys/power/state to enter that state state: Freeze / Low-Power Idle For relative_sleep_states=1, the strings "mem", "standby" and "freeze" label the available non-hibernation sleep states from the deepest to the shallowest, respectively. In that case, "mem" is always present in /sys/power/state, because there is at least one non-hibernation sleep state in every system. If the given system supports two non-hibernation sleep states, "standby" is present in /sys/power/state in addition to "mem". If the system supports three non-hibernation sleep states, "freeze" will be present in /sys/power/state in addition to "mem" and "standby". For relative_sleep_states=0, which is the default, the following descriptions apply. state: Suspend-To-Idle ACPI state: S0 ACPI state: S0 String: "freeze" Label: "freeze" This state is a generic, pure software, light-weight, low-power state. This state is a generic, pure software, light-weight, system sleep state. It allows more energy to be saved relative to idle by freezing user It allows more energy to be saved relative to runtime idle by freezing user space and putting all I/O devices into low-power states (possibly space and putting all I/O devices into low-power states (possibly lower-power than available at run time), such that the processors can lower-power than available at run time), such that the processors can spend more time in their idle states. spend more time in their idle states. This state can be used for platforms without Standby/Suspend-to-RAM This state can be used for platforms without Power-On Suspend/Suspend-to-RAM support, or it can be used in addition to Suspend-to-RAM (memory sleep) support, or it can be used in addition to Suspend-to-RAM (memory sleep) to provide reduced resume latency. to provide reduced resume latency. It is always supported. State: Standby / Power-On Suspend State: Standby / Power-On Suspend ACPI State: S1 ACPI State: S1 String: "standby" Label: "standby" This state offers minimal, though real, power savings, while providing This state, if supported, offers moderate, though real, power savings, while a very low-latency transition back to a working system. No operating providing a relatively low-latency transition back to a working system. No state is lost (the CPU retains power), so the system easily starts up operating state is lost (the CPU retains power), so the system easily starts up again where it left off. again where it left off. We try to put devices in a low-power state equivalent to D1, which In addition to freezing user space and putting all I/O devices into low-power also offers low power savings, but low resume latency. Not all devices states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline support D1, and those that don't are left on. and all low-level system functions are suspended during transitions into this state. For this reason, it should allow more energy to be saved relative to Suspend-To-Idle, but the resume latency will generally be greater than for that state. State: Suspend-to-RAM State: Suspend-to-RAM ACPI State: S3 ACPI State: S3 String: "mem" Label: "mem" This state offers significant power savings as everything in the This state, if supported, offers significant power savings as everything in the system is put into a low-power state, except for memory, which is system is put into a low-power state, except for memory, which should be placed placed in self-refresh mode to retain its contents. into the self-refresh mode to retain its contents. All of the steps carried out when entering Power-On Suspend are also carried out during transitions to STR. Additional operations may take place depending on the platform capabilities. In particular, on ACPI systems the kernel passes control to the BIOS (platform firmware) as the last step during STR transitions and that usually results in powering down some more low-level components that aren't directly controlled by the kernel. System and device state is saved and kept in memory. All devices are System and device state is saved and kept in memory. All devices are suspended suspended and put into D3. In many cases, all peripheral buses lose and put into low-power states. In many cases, all peripheral buses lose power power when entering STR, so devices must be able to handle the when entering STR, so devices must be able to handle the transition back to the transition back to the On state. "on" state. For at least ACPI, STR requires some minimal boot-strapping code to For at least ACPI, STR requires some minimal boot-strapping code to resume the resume the system from STR. This may be true on other platforms. system from it. This may be the case on other platforms too. State: Suspend-to-disk State: Suspend-to-disk ACPI State: S4 ACPI State: S4 String: "disk" Label: "disk" This state offers the greatest power savings, and can be used even in This state offers the greatest power savings, and can be used even in the absence of low-level platform support for power management. This the absence of low-level platform support for power management. This Loading Loading
Documentation/ABI/testing/sysfs-power +20 −9 Original line number Original line Diff line number Diff line Loading @@ -7,19 +7,30 @@ Description: subsystem. subsystem. What: /sys/power/state What: /sys/power/state Date: August 2006 Date: May 2014 Contact: Rafael J. Wysocki <rjw@rjwysocki.net> Contact: Rafael J. Wysocki <rjw@rjwysocki.net> Description: Description: The /sys/power/state file controls the system power state. The /sys/power/state file controls system sleep states. Reading from this file returns what states are supported, Reading from this file returns the available sleep state which is hard-coded to 'freeze' (Low-Power Idle), 'standby' labels, which may be "mem", "standby", "freeze" and "disk" (Power-On Suspend), 'mem' (Suspend-to-RAM), and 'disk' (hibernation). The meanings of the first three labels depend on (Suspend-to-Disk). the relative_sleep_states command line argument as follows: 1) relative_sleep_states = 1 "mem", "standby", "freeze" represent non-hibernation sleep states from the deepest ("mem", always present) to the shallowest ("freeze"). "standby" and "freeze" may or may not be present depending on the capabilities of the platform. "freeze" can only be present if "standby" is present. 2) relative_sleep_states = 0 (default) "mem" - "suspend-to-RAM", present if supported. "standby" - "power-on suspend", present if supported. "freeze" - "suspend-to-idle", always present. Writing to this file one of these strings causes the system to Writing to this file one of these strings causes the system to transition into that state. Please see the file transition into the corresponding state, if available. See Documentation/power/states.txt for a description of each of Documentation/power/states.txt for a description of what these states. "suspend-to-RAM", "power-on suspend" and "suspend-to-idle" mean. What: /sys/power/disk What: /sys/power/disk Date: September 2006 Date: September 2006 Loading
Documentation/kernel-parameters.txt +7 −0 Original line number Original line Diff line number Diff line Loading @@ -2889,6 +2889,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted. [KNL, SMP] Set scheduler's default relax_domain_level. [KNL, SMP] Set scheduler's default relax_domain_level. See Documentation/cgroups/cpusets.txt. See Documentation/cgroups/cpusets.txt. relative_sleep_states= [SUSPEND] Use sleep state labeling where the deepest state available other than hibernation is always "mem". Format: { "0" | "1" } 0 -- Traditional sleep state labels. 1 -- Relative sleep state labels. reserve= [KNL,BUGS] Force the kernel to ignore some iomem area reserve= [KNL,BUGS] Force the kernel to ignore some iomem area reservetop= [X86-32] reservetop= [X86-32] Loading
Documentation/power/devices.txt +30 −4 Original line number Original line Diff line number Diff line Loading @@ -2,6 +2,7 @@ Device Power Management Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu> Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu> Copyright (c) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> Most of the code in Linux is device drivers, so most of the Linux power Most of the code in Linux is device drivers, so most of the Linux power Loading Loading @@ -326,6 +327,20 @@ the phases are: driver in some way for the upcoming system power transition, but it driver in some way for the upcoming system power transition, but it should not put the device into a low-power state. should not put the device into a low-power state. For devices supporting runtime power management, the return value of the prepare callback can be used to indicate to the PM core that it may safely leave the device in runtime suspend (if runtime-suspended already), provided that all of the device's descendants are also left in runtime suspend. Namely, if the prepare callback returns a positive number and that happens for all of the descendants of the device too, and all of them (including the device itself) are runtime-suspended, the PM core will skip the suspend, suspend_late and suspend_noirq suspend phases as well as the resume_noirq, resume_early and resume phases of the following system resume for all of these devices. In that case, the complete callback will be called directly after the prepare callback and is entirely responsible for bringing the device back to the functional state as appropriate. 2. The suspend methods should quiesce the device to stop it from performing 2. The suspend methods should quiesce the device to stop it from performing I/O. They also may save the device registers and put it into the I/O. They also may save the device registers and put it into the appropriate low-power state, depending on the bus type the device is on, appropriate low-power state, depending on the bus type the device is on, Loading Loading @@ -400,12 +415,23 @@ When resuming from freeze, standby or memory sleep, the phases are: the resume callbacks occur; it's not necessary to wait until the the resume callbacks occur; it's not necessary to wait until the complete phase. complete phase. Moreover, if the preceding prepare callback returned a positive number, the device may have been left in runtime suspend throughout the whole system suspend and resume (the suspend, suspend_late, suspend_noirq phases of system suspend and the resume_noirq, resume_early, resume phases of system resume may have been skipped for it). In that case, the complete callback is entirely responsible for bringing the device back to the functional state after system suspend if necessary. [For example, it may need to queue up a runtime resume request for the device for this purpose.] To check if that is the case, the complete callback can consult the device's power.direct_complete flag. Namely, if that flag is set when the complete callback is being run, it has been called directly after the preceding prepare and special action may be required to make the device work correctly afterward. At the end of these phases, drivers should be as functional as they were before At the end of these phases, drivers should be as functional as they were before suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are gated on. Even if the device was in a low-power state before the system sleep gated on. because of runtime power management, afterwards it should be back in its full-power state. There are multiple reasons why it's best to do this; they are discussed in more detail in Documentation/power/runtime_pm.txt. However, the details here may again be platform-specific. For example, However, the details here may again be platform-specific. For example, some systems support multiple "run" states, and the mode in effect at some systems support multiple "run" states, and the mode in effect at Loading
Documentation/power/runtime_pm.txt +17 −0 Original line number Original line Diff line number Diff line Loading @@ -2,6 +2,7 @@ Runtime Power Management Framework for I/O Devices (C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. (C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. (C) 2010 Alan Stern <stern@rowland.harvard.edu> (C) 2010 Alan Stern <stern@rowland.harvard.edu> (C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> 1. Introduction 1. Introduction Loading Loading @@ -444,6 +445,10 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h: bool pm_runtime_status_suspended(struct device *dev); bool pm_runtime_status_suspended(struct device *dev); - return true if the device's runtime PM status is 'suspended' - return true if the device's runtime PM status is 'suspended' bool pm_runtime_suspended_if_enabled(struct device *dev); - return true if the device's runtime PM status is 'suspended' and its 'power.disable_depth' field is equal to 1 void pm_runtime_allow(struct device *dev); void pm_runtime_allow(struct device *dev); - set the power.runtime_auto flag for the device and decrease its usage - set the power.runtime_auto flag for the device and decrease its usage counter (used by the /sys/devices/.../power/control interface to counter (used by the /sys/devices/.../power/control interface to Loading Loading @@ -644,6 +649,18 @@ place (in particular, if the system is not waking up from hibernation), it may be more efficient to leave the devices that had been suspended before the system be more efficient to leave the devices that had been suspended before the system suspend began in the suspended state. suspend began in the suspended state. To this end, the PM core provides a mechanism allowing some coordination between different levels of device hierarchy. Namely, if a system suspend .prepare() callback returns a positive number for a device, that indicates to the PM core that the device appears to be runtime-suspended and its state is fine, so it may be left in runtime suspend provided that all of its descendants are also left in runtime suspend. If that happens, the PM core will not execute any system suspend and resume callbacks for all of those devices, except for the complete callback, which is then entirely responsible for handling the device as appropriate. This only applies to system suspend transitions that are not related to hibernation (see Documentation/power/devices.txt for more information). The PM core does its best to reduce the probability of race conditions between The PM core does its best to reduce the probability of race conditions between the runtime PM and system suspend/resume (and hibernation) callbacks by carrying the runtime PM and system suspend/resume (and hibernation) callbacks by carrying out the following operations: out the following operations: Loading
Documentation/power/states.txt +56 −31 Original line number Original line Diff line number Diff line System Power Management Sleep States System Power Management States (C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com> The kernel supports up to four system sleep states generically, although three of them depend on the platform support code to implement the low-level details for each state. The kernel supports four power management states generically, though The states are represented by strings that can be read or written to the one is generic and the other three are dependent on platform support /sys/power/state file. Those strings may be "mem", "standby", "freeze" and code to implement the low-level details for each state. "disk", where the last one always represents hibernation (Suspend-To-Disk) and This file describes each state, what they are the meaning of the remaining ones depends on the relative_sleep_states command commonly called, what ACPI state they map to, and what string to write line argument. to /sys/power/state to enter that state state: Freeze / Low-Power Idle For relative_sleep_states=1, the strings "mem", "standby" and "freeze" label the available non-hibernation sleep states from the deepest to the shallowest, respectively. In that case, "mem" is always present in /sys/power/state, because there is at least one non-hibernation sleep state in every system. If the given system supports two non-hibernation sleep states, "standby" is present in /sys/power/state in addition to "mem". If the system supports three non-hibernation sleep states, "freeze" will be present in /sys/power/state in addition to "mem" and "standby". For relative_sleep_states=0, which is the default, the following descriptions apply. state: Suspend-To-Idle ACPI state: S0 ACPI state: S0 String: "freeze" Label: "freeze" This state is a generic, pure software, light-weight, low-power state. This state is a generic, pure software, light-weight, system sleep state. It allows more energy to be saved relative to idle by freezing user It allows more energy to be saved relative to runtime idle by freezing user space and putting all I/O devices into low-power states (possibly space and putting all I/O devices into low-power states (possibly lower-power than available at run time), such that the processors can lower-power than available at run time), such that the processors can spend more time in their idle states. spend more time in their idle states. This state can be used for platforms without Standby/Suspend-to-RAM This state can be used for platforms without Power-On Suspend/Suspend-to-RAM support, or it can be used in addition to Suspend-to-RAM (memory sleep) support, or it can be used in addition to Suspend-to-RAM (memory sleep) to provide reduced resume latency. to provide reduced resume latency. It is always supported. State: Standby / Power-On Suspend State: Standby / Power-On Suspend ACPI State: S1 ACPI State: S1 String: "standby" Label: "standby" This state offers minimal, though real, power savings, while providing This state, if supported, offers moderate, though real, power savings, while a very low-latency transition back to a working system. No operating providing a relatively low-latency transition back to a working system. No state is lost (the CPU retains power), so the system easily starts up operating state is lost (the CPU retains power), so the system easily starts up again where it left off. again where it left off. We try to put devices in a low-power state equivalent to D1, which In addition to freezing user space and putting all I/O devices into low-power also offers low power savings, but low resume latency. Not all devices states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline support D1, and those that don't are left on. and all low-level system functions are suspended during transitions into this state. For this reason, it should allow more energy to be saved relative to Suspend-To-Idle, but the resume latency will generally be greater than for that state. State: Suspend-to-RAM State: Suspend-to-RAM ACPI State: S3 ACPI State: S3 String: "mem" Label: "mem" This state offers significant power savings as everything in the This state, if supported, offers significant power savings as everything in the system is put into a low-power state, except for memory, which is system is put into a low-power state, except for memory, which should be placed placed in self-refresh mode to retain its contents. into the self-refresh mode to retain its contents. All of the steps carried out when entering Power-On Suspend are also carried out during transitions to STR. Additional operations may take place depending on the platform capabilities. In particular, on ACPI systems the kernel passes control to the BIOS (platform firmware) as the last step during STR transitions and that usually results in powering down some more low-level components that aren't directly controlled by the kernel. System and device state is saved and kept in memory. All devices are System and device state is saved and kept in memory. All devices are suspended suspended and put into D3. In many cases, all peripheral buses lose and put into low-power states. In many cases, all peripheral buses lose power power when entering STR, so devices must be able to handle the when entering STR, so devices must be able to handle the transition back to the transition back to the On state. "on" state. For at least ACPI, STR requires some minimal boot-strapping code to For at least ACPI, STR requires some minimal boot-strapping code to resume the resume the system from STR. This may be true on other platforms. system from it. This may be the case on other platforms too. State: Suspend-to-disk State: Suspend-to-disk ACPI State: S4 ACPI State: S4 String: "disk" Label: "disk" This state offers the greatest power savings, and can be used even in This state offers the greatest power savings, and can be used even in the absence of low-level platform support for power management. This the absence of low-level platform support for power management. This Loading
