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diff --git a/Documentation/power/devices.txt b/Documentation/power/devices.txt index c9abbd8..57080cd 100644 --- a/Documentation/power/devices.txt +++ b/Documentation/power/devices.txt @@ -1,7 +1,13 @@ +Device Power Management + +Copyright (c) 2010 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc. +Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu> + + Most of the code in Linux is device drivers, so most of the Linux power -management code is also driver-specific. Most drivers will do very little; -others, especially for platforms with small batteries (like cell phones), -will do a lot. +management (PM) code is also driver-specific. Most drivers will do very +little; others, especially for platforms with small batteries (like cell +phones), will do a lot. This writeup gives an overview of how drivers interact with system-wide power management goals, emphasizing the models and interfaces that are @@ -15,9 +21,10 @@ Drivers will use one or both of these models to put devices into low-power states: System Sleep model: - Drivers can enter low power states as part of entering system-wide - low-power states like "suspend-to-ram", or (mostly for systems with - disks) "hibernate" (suspend-to-disk). + Drivers can enter low-power states as part of entering system-wide + low-power states like "suspend" (also known as "suspend-to-RAM"), or + (mostly for systems with disks) "hibernation" (also known as + "suspend-to-disk"). This is something that device, bus, and class drivers collaborate on by implementing various role-specific suspend and resume methods to @@ -25,33 +32,41 @@ states: them without loss of data. Some drivers can manage hardware wakeup events, which make the system - leave that low-power state. This feature may be disabled using the - relevant /sys/devices/.../power/wakeup file; enabling it may cost some - power usage, but let the whole system enter low power states more often. + leave the low-power state. This feature may be enabled or disabled + using the relevant /sys/devices/.../power/wakeup file (for Ethernet + drivers the ioctl interface used by ethtool may also be used for this + purpose); enabling it may cost some power usage, but let the whole + system enter low-power states more often. Runtime Power Management model: - Drivers may also enter low power states while the system is running, - independently of other power management activity. Upstream drivers - will normally not know (or care) if the device is in some low power - state when issuing requests; the driver will auto-resume anything - that's needed when it gets a request. - - This doesn't have, or need much infrastructure; it's just something you - should do when writing your drivers. For example, clk_disable() unused - clocks as part of minimizing power drain for currently-unused hardware. - Of course, sometimes clusters of drivers will collaborate with each - other, which could involve task-specific power management. - -There's not a lot to be said about those low power states except that they -are very system-specific, and often device-specific. Also, that if enough -drivers put themselves into low power states (at "runtime"), the effect may be -the same as entering some system-wide low-power state (system sleep) ... and -that synergies exist, so that several drivers using runtime pm might put the -system into a state where even deeper power saving options are available. - -Most suspended devices will have quiesced all I/O: no more DMA or irqs, no -more data read or written, and requests from upstream drivers are no longer -accepted. A given bus or platform may have different requirements though. + Devices may also be put into low-power states while the system is + running, independently of other power management activity in principle. + However, devices are not generally independent of each other (for + example, a parent device cannot be suspended unless all of its child + devices have been suspended). Moreover, depending on the bus type the + device is on, it may be necessary to carry out some bus-specific + operations on the device for this purpose. Devices put into low power + states at run time may require special handling during system-wide power + transitions (suspend or hibernation). + + For these reasons not only the device driver itself, but also the + appropriate subsystem (bus type, device type or device class) driver and + the PM core are involved in runtime power management. As in the system + sleep power management case, they need to collaborate by implementing + various role-specific suspend and resume methods, so that the hardware + is cleanly powered down and reactivated without data or service loss. + +There's not a lot to be said about those low-power states except that they are +very system-specific, and often device-specific. Also, that if enough devices +have been put into low-power states (at runtime), the effect may be very similar +to entering some system-wide low-power state (system sleep) ... and that +synergies exist, so that several drivers using runtime PM might put the system +into a state where even deeper power saving options are available. + +Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except +for wakeup events), no more data read or written, and requests from upstream +drivers are no longer accepted. A given bus or platform may have different +requirements though. Examples of hardware wakeup events include an alarm from a real time clock, network wake-on-LAN packets, keyboard or mouse activity, and media insertion @@ -60,129 +75,152 @@ or removal (for PCMCIA, MMC/SD, USB, and so on). Interfaces for Entering System Sleep States =========================================== -Most of the programming interfaces a device driver needs to know about -relate to that first model: entering a system-wide low power state, -rather than just minimizing power consumption by one device. - - -Bus Driver Methods ------------------- -The core methods to suspend and resume devices reside in struct bus_type. -These are mostly of interest to people writing infrastructure for busses -like PCI or USB, or because they define the primitives that device drivers -may need to apply in domain-specific ways to their devices: - -struct bus_type { - ... - int (*suspend)(struct device *dev, pm_message_t state); - int (*resume)(struct device *dev); +There are programming interfaces provided for subsystems (bus type, device type, +device class) and device drivers to allow them to participate in the power +management of devices they are concerned with. These interfaces cover both +system sleep and runtime power management. + + +Device Power Management Operations +---------------------------------- +Device power management operations, at the subsystem level as well as at the +device driver level, are implemented by defining and populating objects of type +struct dev_pm_ops: + +struct dev_pm_ops { + int (*prepare)(struct device *dev); + void (*complete)(struct device *dev); + int (*suspend)(struct device *dev); + int (*resume)(struct device *dev); + int (*freeze)(struct device *dev); + int (*thaw)(struct device *dev); + int (*poweroff)(struct device *dev); + int (*restore)(struct device *dev); + int (*suspend_noirq)(struct device *dev); + int (*resume_noirq)(struct device *dev); + int (*freeze_noirq)(struct device *dev); + int (*thaw_noirq)(struct device *dev); + int (*poweroff_noirq)(struct device *dev); + int (*restore_noirq)(struct device *dev); + int (*runtime_suspend)(struct device *dev); + int (*runtime_resume)(struct device *dev); + int (*runtime_idle)(struct device *dev); }; -Bus drivers implement those methods as appropriate for the hardware and -the drivers using it; PCI works differently from USB, and so on. Not many -people write bus drivers; most driver code is a "device driver" that -builds on top of bus-specific framework code. +This structure is defined in include/linux/pm.h and the methods included in it +are also described in that file. Their roles will be explained in what follows. +For now, it should be sufficient to remember that the last three methods are +specific to runtime power management while the remaining ones are used during +system-wide power transitions. -For more information on these driver calls, see the description later; -they are called in phases for every device, respecting the parent-child -sequencing in the driver model tree. Note that as this is being written, -only the suspend() and resume() are widely available; not many bus drivers -leverage all of those phases, or pass them down to lower driver levels. +There also is a deprecated "old" or "legacy" interface for power management +operations available at least for some subsystems. This approach does not use +struct dev_pm_ops objects and it is suitable only for implementing system sleep +power management methods. Therefore it is not described in this document, so +please refer directly to the source code for more information about it. -/sys/devices/.../power/wakeup files ------------------------------------ -All devices in the driver model have two flags to control handling of -wakeup events, which are hardware signals that can force the device and/or -system out of a low power state. These are initialized by bus or device -driver code using device_init_wakeup(dev,can_wakeup). +Subsystem-Level Methods +----------------------- +The core methods to suspend and resume devices reside in struct dev_pm_ops +pointed to by the pm member of struct bus_type, struct device_type and +struct class. They are mostly of interest to the people writing infrastructure +for buses, like PCI or USB, or device type and device class drivers. -The "can_wakeup" flag just records whether the device (and its driver) can -physically support wakeup events. When that flag is clear, the sysfs -"wakeup" file is empty, and device_may_wakeup() returns false. +Bus drivers implement these methods as appropriate for the hardware and the +drivers using it; PCI works differently from USB, and so on. Not many people +write subsystem-level drivers; most driver code is a "device driver" that builds +on top of bus-specific framework code. -For devices that can issue wakeup events, a separate flag controls whether -that device should try to use its wakeup mechanism. The initial value of -device_may_wakeup() will be true, so that the device's "wakeup" file holds -the value "enabled". Userspace can change that to "disabled" so that -device_may_wakeup() returns false; or change it back to "enabled" (so that -it returns true again). +For more information on these driver calls, see the description later; +they are called in phases for every device, respecting the parent-child +sequencing in the driver model tree. -EXAMPLE: PCI Device Driver Methods +/sys/devices/.../power/wakeup files ----------------------------------- -PCI framework software calls these methods when the PCI device driver bound -to a device device has provided them: - -struct pci_driver { - ... - int (*suspend)(struct pci_device *pdev, pm_message_t state); - int (*suspend_late)(struct pci_device *pdev, pm_message_t state); +All devices in the driver model have two flags to control handling of wakeup +events (hardware signals that can force the device and/or system out of a low +power state). These flags are initialized by bus or device driver code using +device_set_wakeup_capable() and device_set_wakeup_enable(), defined in +include/linux/pm_wakeup.h. - int (*resume_early)(struct pci_device *pdev); - int (*resume)(struct pci_device *pdev); -}; - -Drivers will implement those methods, and call PCI-specific procedures -like pci_set_power_state(), pci_enable_wake(), pci_save_state(), and -pci_restore_state() to manage PCI-specific mechanisms. (PCI config space -could be saved during driver probe, if it weren't for the fact that some -systems rely on userspace tweaking using setpci.) Devices are suspended -before their bridges enter low power states, and likewise bridges resume -before their devices. - - -Upper Layers of Driver Stacks ------------------------------ -Device drivers generally have at least two interfaces, and the methods -sketched above are the ones which apply to the lower level (nearer PCI, USB, -or other bus hardware). The network and block layers are examples of upper -level interfaces, as is a character device talking to userspace. - -Power management requests normally need to flow through those upper levels, -which often use domain-oriented requests like "blank that screen". In -some cases those upper levels will have power management intelligence that -relates to end-user activity, or other devices that work in cooperation. - -When those interfaces are structured using class interfaces, there is a -standard way to have the upper layer stop issuing requests to a given -class device (and restart later): - -struct class { - ... - int (*suspend)(struct device *dev, pm_message_t state); - int (*resume)(struct device *dev); -}; - -Those calls are issued in specific phases of the process by which the -system enters a low power "suspend" state, or resumes from it. - - -Calling Drivers to Enter System Sleep States -============================================ -When the system enters a low power state, each device's driver is asked -to suspend the device by putting it into state compatible with the target +The "can_wakeup" flag just records whether the device (and its driver) can +physically support wakeup events. The device_set_wakeup_capable() routine +affects this flag. The "should_wakeup" flag controls whether the device should +try to use its wakeup mechanism. device_set_wakeup_enable() affects this flag; +for the most part drivers should not change its value. The initial value of +should_wakeup is supposed to be false for the majority of devices; the major +exceptions are power buttons, keyboards, and Ethernet adapters whose WoL +(wake-on-LAN) feature has been set up with ethtool. + +Whether or not a device is capable of issuing wakeup events is a hardware +matter, and the kernel is responsible for keeping track of it. By contrast, +whether or not a wakeup-capable device should issue wakeup events is a policy +decision, and it is managed by user space through a sysfs attribute: the +power/wakeup file. User space can write the strings "enabled" or "disabled" to +set or clear the should_wakeup flag, respectively. Reads from the file will +return the corresponding string if can_wakeup is true, but if can_wakeup is +false then reads will return an empty string, to indicate that the device +doesn't support wakeup events. (But even though the file appears empty, writes +will still affect the should_wakeup flag.) + +The device_may_wakeup() routine returns true only if both flags are set. +Drivers should check this routine when putting devices in a low-power state +during a system sleep transition, to see whether or not to enable the devices' +wakeup mechanisms. However for runtime power management, wakeup events should +be enabled whenever the device and driver both support them, regardless of the +should_wakeup flag. + + +/sys/devices/.../power/control files +------------------------------------ +Each device in the driver model has a flag to control whether it is subject to +runtime power management. This flag, called runtime_auto, is initialized by the +bus type (or generally subsystem) code using pm_runtime_allow() or +pm_runtime_forbid(); the default is to allow runtime power management. + +The setting can be adjusted by user space by writing either "on" or "auto" to +the device's power/control sysfs file. Writing "auto" calls pm_runtime_allow(), +setting the flag and allowing the device to be runtime power-managed by its +driver. Writing "on" calls pm_runtime_forbid(), clearing the flag, returning +the device to full power if it was in a low-power state, and preventing the +device from being runtime power-managed. User space can check the current value +of the runtime_auto flag by reading the file. + +The device's runtime_auto flag has no effect on the handling of system-wide +power transitions. In particular, the device can (and in the majority of cases +should and will) be put into a low-power state during a system-wide transition +to a sleep state even though its runtime_auto flag is clear. + +For more information about the runtime power management framework, refer to +Documentation/power/runtime_pm.txt. + + +Calling Drivers to Enter and Leave System Sleep States +====================================================== +When the system goes into a sleep state, each device's driver is asked to +suspend the device by putting it into a state compatible with the target system state. That's usually some version of "off", but the details are system-specific. Also, wakeup-enabled devices will usually stay partly functional in order to wake the system. -When the system leaves that low power state, the device's driver is asked -to resume it. The suspend and resume operations always go together, and -both are multi-phase operations. +When the system leaves that low-power state, the device's driver is asked to +resume it by returning it to full power. The suspend and resume operations +always go together, and both are multi-phase operations. -For simple drivers, suspend might quiesce the device using the class code -and then turn its hardware as "off" as possible with late_suspend. The +For simple drivers, suspend might quiesce the device using class code +and then turn its hardware as "off" as possible during suspend_noirq. The matching resume calls would then completely reinitialize the hardware before reactivating its class I/O queues. -More power-aware drivers drivers will use more than one device low power -state, either at runtime or during system sleep states, and might trigger -system wakeup events. +More power-aware drivers might prepare the devices for triggering system wakeup +events. Call Sequence Guarantees ------------------------ -To ensure that bridges and similar links needed to talk to a device are +To ensure that bridges and similar links needing to talk to a device are available when the device is suspended or resumed, the device tree is walked in a bottom-up order to suspend devices. A top-down order is used to resume those devices. @@ -194,67 +232,310 @@ its parent; and can't be removed or suspended after that parent. The policy is that the device tree should match hardware bus topology. (Or at least the control bus, for devices which use multiple busses.) In particular, this means that a device registration may fail if the parent of -the device is suspending (ie. has been chosen by the PM core as the next +the device is suspending (i.e. has been chosen by the PM core as the next device to suspend) or has already suspended, as well as after all of the other devices have been suspended. Device drivers must be prepared to cope with such situations. -Suspending Devices ------------------- -Suspending a given device is done in several phases. Suspending the -system always includes every phase, executing calls for every device -before the next phase begins. Not all busses or classes support all -these callbacks; and not all drivers use all the callbacks. +System Power Management Phases +------------------------------ +Suspending or resuming the system is done in several phases. Different phases +are used for standby or memory sleep states ("suspend-to-RAM") and the +hibernation state ("suspend-to-disk"). Each phase involves executing callbacks +for every device before the next phase begins. Not all busses or classes +support all these callbacks and not all drivers use all the callbacks. The +various phases always run after tasks have been frozen and before they are +unfrozen. Furthermore, the *_noirq phases run at a time when IRQ handlers have +been disabled (except for those marked with the IRQ_WAKEUP flag). -The phases are seen by driver notifications issued in this order: +Most phases use bus, type, and class callbacks (that is, methods defined in +dev->bus->pm, dev->type->pm, and dev->class->pm). The prepare and complete +phases are exceptions; they use only bus callbacks. When multiple callbacks +are used in a phase, they are invoked in the order: <class, type, bus> during +power-down transitions and in the opposite order during power-up transitions. +For example, during the suspend phase the PM core invokes - 1 class.suspend(dev, message) is called after tasks are frozen, for - devices associated with a class that has such a method. This - method may sleep. + dev->class->pm.suspend(dev); + dev->type->pm.suspend(dev); + dev->bus->pm.suspend(dev); - Since I/O activity usually comes from such higher layers, this is - a good place to quiesce all drivers of a given type (and keep such - code out of those drivers). +before moving on to the next device, whereas during the resume phase the core +invokes - 2 bus.suspend(dev, message) is called next. This method may sleep, - and is often morphed into a device driver call with bus-specific - parameters and/or rules. + dev->bus->pm.resume(dev); + dev->type->pm.resume(dev); + dev->class->pm.resume(dev); - This call should handle parts of device suspend logic that require - sleeping. It probably does work to quiesce the device which hasn't - been abstracted into class.suspend(). +These callbacks may in turn invoke device- or driver-specific methods stored in +dev->driver->pm, but they don't have to. -The pm_message_t parameter is currently used to refine those semantics -(described later). -At the end of those phases, drivers should normally have stopped all I/O -transactions (DMA, IRQs), saved enough state that they can re-initialize -or restore previous state (as needed by the hardware), and placed the -device into a low-power state. On many platforms they will also use -clk_disable() to gate off one or more clock sources; sometimes they will -also switch off power supplies, or reduce voltages. Drivers which have -runtime PM support may already have performed some or all of the steps -needed to prepare for the upcoming system sleep state. +Entering System Suspend +----------------------- +When the system goes into the standby or memory sleep state, the phases are: + + prepare, suspend, suspend_noirq. + + 1. The prepare phase is meant to prevent races by preventing new devices + from being registered; the PM core would never know that all the + children of a device had been suspended if new children could be + registered at will. (By contrast, devices may be unregistered at any + time.) Unlike the other suspend-related phases, during the prepare + phase the device tree is traversed top-down. + + The prepare phase uses only a bus callback. After the callback method + returns, no new children may be registered below the device. The method + may also prepare the device or driver in some way for the upcoming + system power transition, but it should not put the device into a + low-power state. + + 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 + appropriate low-power state, depending on the bus type the device is on, + and they may enable wakeup events. + + 3. The suspend_noirq phase occurs after IRQ handlers have been disabled, + which means that the driver's interrupt handler will not be called while + the callback method is running. The methods should save the values of + the device's registers that weren't saved previously and finally put the + device into the appropriate low-power state. + + The majority of subsystems and device drivers need not implement this + callback. However, bus types allowing devices to share interrupt + vectors, like PCI, generally need it; otherwise a driver might encounter + an error during the suspend phase by fielding a shared interrupt + generated by some other device after its own device had been set to low + power. + +At the end of these phases, drivers should have stopped all I/O transactions +(DMA, IRQs), saved enough state that they can re-initialize or restore previous +state (as needed by the hardware), and placed the device into a low-power state. +On many platforms they will gate off one or more clock sources; sometimes they +will also switch off power supplies or reduce voltages. (Drivers supporting +runtime PM may already have performed some or all of these steps.) + +If device_may_wakeup(dev) returns true, the device should be prepared for +generating hardware wakeup signals to trigger a system wakeup event when the +system is in the sleep state. For example, enable_irq_wake() might identify +GPIO signals hooked up to a switch or other external hardware, and +pci_enable_wake() does something similar for the PCI PME signal. + +If any of these callbacks returns an error, the system won't enter the desired +low-power state. Instead the PM core will unwind its actions by resuming all +the devices that were suspended. + + +Leaving System Suspend +---------------------- +When resuming from standby or memory sleep, the phases are: + + resume_noirq, resume, complete. + + 1. The resume_noirq callback methods should perform any actions needed + before the driver's interrupt handlers are invoked. This generally + means undoing the actions of the suspend_noirq phase. If the bus type + permits devices to share interrupt vectors, like PCI, the method should + bring the device and its driver into a state in which the driver can + recognize if the device is the source of incoming interrupts, if any, + and handle them correctly. + + For example, the PCI bus type's ->pm.resume_noirq() puts the device into + the full-power state (D0 in the PCI terminology) and restores the + standard configuration registers of the device. Then it calls the + device driver's ->pm.resume_noirq() method to perform device-specific + actions. + + 2. The resume methods should bring the the device back to its operating + state, so that it can perform normal I/O. This generally involves + undoing the actions of the suspend phase. + + 3. The complete phase uses only a bus callback. The method should undo the + actions of the prepare phase. Note, however, that new children may be + registered below the device as soon as the resume callbacks occur; it's + not necessary to wait until the complete phase. + +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 +gated on. Even if the device was in a low-power state before the system sleep +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. -When any driver sees that its device_can_wakeup(dev), it should make sure -to use the relevant hardware signals to trigger a system wakeup event. -For example, enable_irq_wake() might identify GPIO signals hooked up to -a switch or other external hardware, and pci_enable_wake() does something -similar for PCI's PME# signal. +However, the details here may again be platform-specific. For example, +some systems support multiple "run" states, and the mode in effect at +the end of resume might not be the one which preceded suspension. +That means availability of certain clocks or power supplies changed, +which could easily affect how a driver works. + +Drivers need to be able to handle hardware which has been reset since the +suspend methods were called, for example by complete reinitialization. +This may be the hardest part, and the one most protected by NDA'd documents +and chip errata. It's simplest if the hardware state hasn't changed since +the suspend was carried out, but that can't be guaranteed (in fact, it ususally +is not the case). + +Drivers must also be prepared to notice that the device has been removed +while the system was powered down, whenever that's physically possible. +PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses +where common Linux platforms will see such removal. Details of how drivers +will notice and handle such removals are currently bus-specific, and often +involve a separate thread. + +These callbacks may return an error value, but the PM core will ignore such +errors since there's nothing it can do about them other than printing them in +the system log. + + +Entering Hibernation +-------------------- +Hibernating the system is more complicated than putting it into the standby or +memory sleep state, because it involves creating and saving a system image. +Therefore there are more phases for hibernation, with a different set of +callbacks. These phases always run after tasks have been frozen and memory has +been freed. + +The general procedure for hibernation is to quiesce all devices (freeze), create +an image of the system memory while everything is stable, reactivate all +devices (thaw), write the image to permanent storage, and finally shut down the +system (poweroff). The phases used to accomplish this are: + + prepare, freeze, freeze_noirq, thaw_noirq, thaw, complete, + prepare, poweroff, poweroff_noirq + + 1. The prepare phase is discussed in the "Entering System Suspend" section + above. + + 2. The freeze methods should quiesce the device so that it doesn't generate + IRQs or DMA, and they may need to save the values of device registers. + However the device does not have to be put in a low-power state, and to + save time it's best not to do so. Also, the device should not be + prepared to generate wakeup events. + + 3. The freeze_noirq phase is analogous to the suspend_noirq phase discussed + above, except again that the device should not be put in a low-power + state and should not be allowed to generate wakeup events. + +At this point the system image is created. All devices should be inactive and +the contents of memory should remain undisturbed while this happens, so that the +image forms an atomic snapshot of the system state. + + 4. The thaw_noirq phase is analogous to the resume_noirq phase discussed + above. The main difference is that its methods can assume the device is + in the same state as at the end of the freeze_noirq phase. + + 5. The thaw phase is analogous to the resume phase discussed above. Its + methods should bring the device back to an operating state, so that it + can be used for saving the image if necessary. + + 6. The complete phase is discussed in the "Leaving System Suspend" section + above. + +At this point the system image is saved, and the devices then need to be +prepared for the upcoming system shutdown. This is much like suspending them +before putting the system into the standby or memory sleep state, and the phases +are similar. + + 7. The prepare phase is discussed above. + + 8. The poweroff phase is analogous to the suspend phase. + + 9. The poweroff_noirq phase is analogous to the suspend_noirq phase. + +The poweroff and poweroff_noirq callbacks should do essentially the same things +as the suspend and suspend_noirq callbacks. The only notable difference is that +they need not store the device register values, because the registers should +already have been stored during the freeze or freeze_noirq phases. + + +Leaving Hibernation +------------------- +Resuming from hibernation is, again, more complicated than resuming from a sleep +state in which the contents of main memory are preserved, because it requires +a system image to be loaded into memory and the pre-hibernation memory contents +to be restored before control can be passed back to the image kernel. + +Although in principle, the image might be loaded into memory and the +pre-hibernation memory contents restored by the boot loader, in practice this +can't be done because boot loaders aren't smart enough and there is no +established protocol for passing the necessary information. So instead, the +boot loader loads a fresh instance of the kernel, called the boot kernel, into +memory and passes control to it in the usual way. Then the boot kernel reads +the system image, restores the pre-hibernation memory contents, and passes +control to the image kernel. Thus two different kernels are involved in +resuming from hibernation. In fact, the boot kernel may be completely different +from the image kernel: a different configuration and even a different version. +This has important consequences for device drivers and their subsystems. + +To be able to load the system image into memory, the boot kernel needs to +include at least a subset of device drivers allowing it to access the storage +medium containing the image, although it doesn't need to include all of the +drivers present in the image kernel. After the image has been loaded, the +devices managed by the boot kernel need to be prepared for passing control back +to the image kernel. This is very similar to the initial steps involved in +creating a system image, and it is accomplished in the same way, using prepare, +freeze, and freeze_noirq phases. However the devices affected by these phases +are only those having drivers in the boot kernel; other devices will still be in +whatever state the boot loader left them. + +Should the restoration of the pre-hibernation memory contents fail, the boot +kernel would go through the "thawing" procedure described above, using the +thaw_noirq, thaw, and complete phases, and then continue running normally. This +happens only rarely. Most often the pre-hibernation memory contents are +restored successfully and control is passed to the image kernel, which then +becomes responsible for bringing the system back to the working state. + +To achieve this, the image kernel must restore the devices' pre-hibernation +functionality. The operation is much like waking up from the memory sleep +state, although it involves different phases: + + restore_noirq, restore, complete + + 1. The restore_noirq phase is analogous to the resume_noirq phase. + + 2. The restore phase is analogous to the resume phase. + + 3. The complete phase is discussed above. + +The main difference from resume[_noirq] is that restore[_noirq] must assume the +device has been accessed and reconfigured by the boot loader or the boot kernel. +Consequently the state of the device may be different from the state remembered +from the freeze and freeze_noirq phases. The device may even need to be reset +and completely re-initialized. In many cases this difference doesn't matter, so +the resume[_noirq] and restore[_norq] method pointers can be set to the same +routines. Nevertheless, different callback pointers are used in case there is a +situation where it actually matters. -If a driver (or bus, or class) fails it suspend method, the system won't -enter the desired low power state; it will resume all the devices it's -suspended so far. -Note that drivers may need to perform different actions based on the target -system lowpower/sleep state. At this writing, there are only platform -specific APIs through which drivers could determine those target states. +System Devices +-------------- +System devices (sysdevs) follow a slightly different API, which can be found in + + include/linux/sysdev.h + drivers/base/sys.c + +System devices will be suspended with interrupts disabled, and after all other +devices have been suspended. On resume, they will be resumed before any other +devices, and also with interrupts disabled. These things occur in special +"sysdev_driver" phases, which affect only system devices. + +Thus, after the suspend_noirq (or freeze_noirq or poweroff_noirq) phase, when +the non-boot CPUs are all offline and IRQs are disabled on the remaining online +CPU, then a sysdev_driver.suspend phase is carried out, and the system enters a +sleep state (or a system image is created). During resume (or after the image +has been created or loaded) a sysdev_driver.resume phase is carried out, IRQs +are enabled on the only online CPU, the non-boot CPUs are enabled, and the +resume_noirq (or thaw_noirq or restore_noirq) phase begins. + +Code to actually enter and exit the system-wide low power state sometimes +involves hardware details that are only known to the boot firmware, and +may leave a CPU running software (from SRAM or flash memory) that monitors +the system and manages its wakeup sequence. Device Low Power (suspend) States --------------------------------- -Device low-power states aren't very standard. One device might only handle +Device low-power states aren't standard. One device might only handle "on" and "off, while another might support a dozen different versions of "on" (how many engines are active?), plus a state that gets back to "on" faster than from a full "off". @@ -265,7 +546,7 @@ PCI device may not perform DMA or issue IRQs, and any wakeup events it issues would be issued through the PME# bus signal. Plus, there are several PCI-standard device states, some of which are optional. -In contrast, integrated system-on-chip processors often use irqs as the +In contrast, integrated system-on-chip processors often use IRQs as the wakeup event sources (so drivers would call enable_irq_wake) and might be able to treat DMA completion as a wakeup event (sometimes DMA can stay active too, it'd only be the CPU and some peripherals that sleep). @@ -284,120 +565,17 @@ ways; the aforementioned LCD might be active in one product's "standby", but a different product using the same SOC might work differently. -Meaning of pm_message_t.event ------------------------------ -Parameters to suspend calls include the device affected and a message of -type pm_message_t, which has one field: the event. If driver does not -recognize the event code, suspend calls may abort the request and return -a negative errno. However, most drivers will be fine if they implement -PM_EVENT_SUSPEND semantics for all messages. +Power Management Notifiers +-------------------------- +There are some operations that cannot be carried out by the power management +callbacks discussed above, because the callbacks occur too late or too early. +To handle these cases, subsystems and device drivers may register power +management notifiers that are called before tasks are frozen and after they have +been thawed. Generally speaking, the PM notifiers are suitable for performing +actions that either require user space to be available, or at least won't +interfere with user space. -The event codes are used to refine the goal of suspending the device, and -mostly matter when creating or resuming system memory image snapshots, as -used with suspend-to-disk: - - PM_EVENT_SUSPEND -- quiesce the driver and put hardware into a low-power - state. When used with system sleep states like "suspend-to-RAM" or - "standby", the upcoming resume() call will often be able to rely on - state kept in hardware, or issue system wakeup events. - - PM_EVENT_HIBERNATE -- Put hardware into a low-power state and enable wakeup - events as appropriate. It is only used with hibernation - (suspend-to-disk) and few devices are able to wake up the system from - this state; most are completely powered off. - - PM_EVENT_FREEZE -- quiesce the driver, but don't necessarily change into - any low power mode. A system snapshot is about to be taken, often - followed by a call to the driver's resume() method. Neither wakeup - events nor DMA are allowed. - - PM_EVENT_PRETHAW -- quiesce the driver, knowing that the upcoming resume() - will restore a suspend-to-disk snapshot from a different kernel image. - Drivers that are smart enough to look at their hardware state during - resume() processing need that state to be correct ... a PRETHAW could - be used to invalidate that state (by resetting the device), like a - shutdown() invocation would before a kexec() or system halt. Other - drivers might handle this the same way as PM_EVENT_FREEZE. Neither - wakeup events nor DMA are allowed. - -To enter "standby" (ACPI S1) or "Suspend to RAM" (STR, ACPI S3) states, or -the similarly named APM states, only PM_EVENT_SUSPEND is used; the other event -codes are used for hibernation ("Suspend to Disk", STD, ACPI S4). - -There's also PM_EVENT_ON, a value which never appears as a suspend event -but is sometimes used to record the "not suspended" device state. - - -Resuming Devices ----------------- -Resuming is done in multiple phases, much like suspending, with all -devices processing each phase's calls before the next phase begins. - -The phases are seen by driver notifications issued in this order: - - 1 bus.resume(dev) reverses the effects of bus.suspend(). This may - be morphed into a device driver call with bus-specific parameters; - implementations may sleep. - - 2 class.resume(dev) is called for devices associated with a class - that has such a method. Implementations may sleep. - - This reverses the effects of class.suspend(), and would usually - reactivate the device's I/O queue. - -At the end of those phases, drivers should normally be as functional as -they were before suspending: I/O can be performed using DMA and IRQs, and -the relevant clocks are gated on. The device need not be "fully on"; it -might be in a runtime lowpower/suspend state that acts as if it were. - -However, the details here may again be platform-specific. For example, -some systems support multiple "run" states, and the mode in effect at -the end of resume() might not be the one which preceded suspension. -That means availability of certain clocks or power supplies changed, -which could easily affect how a driver works. - - -Drivers need to be able to handle hardware which has been reset since the -suspend methods were called, for example by complete reinitialization. -This may be the hardest part, and the one most protected by NDA'd documents -and chip errata. It's simplest if the hardware state hasn't changed since -the suspend() was called, but that can't always be guaranteed. - -Drivers must also be prepared to notice that the device has been removed -while the system was powered off, whenever that's physically possible. -PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses -where common Linux platforms will see such removal. Details of how drivers -will notice and handle such removals are currently bus-specific, and often -involve a separate thread. - - -Note that the bus-specific runtime PM wakeup mechanism can exist, and might -be defined to share some of the same driver code as for system wakeup. For -example, a bus-specific device driver's resume() method might be used there, -so it wouldn't only be called from bus.resume() during system-wide wakeup. -See bus-specific information about how runtime wakeup events are handled. - - -System Devices --------------- -System devices follow a slightly different API, which can be found in - - include/linux/sysdev.h - drivers/base/sys.c - -System devices will only be suspended with interrupts disabled, and after -all other devices have been suspended. On resume, they will be resumed -before any other devices, and also with interrupts disabled. - -That is, IRQs are disabled, the suspend_late() phase begins, then the -sysdev_driver.suspend() phase, and the system enters a sleep state. Then -the sysdev_driver.resume() phase begins, followed by the resume_early() -phase, after which IRQs are enabled. - -Code to actually enter and exit the system-wide low power state sometimes -involves hardware details that are only known to the boot firmware, and -may leave a CPU running software (from SRAM or flash memory) that monitors -the system and manages its wakeup sequence. +For details refer to Documentation/power/notifiers.txt. Runtime Power Management @@ -407,82 +585,23 @@ running. This feature is useful for devices that are not being used, and can offer significant power savings on a running system. These devices often support a range of runtime power states, which might use names such as "off", "sleep", "idle", "active", and so on. Those states will in some -cases (like PCI) be partially constrained by a bus the device uses, and will +cases (like PCI) be partially constrained by the bus the device uses, and will usually include hardware states that are also used in system sleep states. -However, note that if a driver puts a device into a runtime low power state -and the system then goes into a system-wide sleep state, it normally ought -to resume into that runtime low power state rather than "full on". Such -distinctions would be part of the driver-internal state machine for that -hardware; the whole point of runtime power management is to be sure that -drivers are decoupled in that way from the state machine governing phases -of the system-wide power/sleep state transitions. - - -Power Saving Techniques ------------------------ -Normally runtime power management is handled by the drivers without specific -userspace or kernel intervention, by device-aware use of techniques like: - - Using information provided by other system layers - - stay deeply "off" except between open() and close() - - if transceiver/PHY indicates "nobody connected", stay "off" - - application protocols may include power commands or hints - - Using fewer CPU cycles - - using DMA instead of PIO - - removing timers, or making them lower frequency - - shortening "hot" code paths - - eliminating cache misses - - (sometimes) offloading work to device firmware - - Reducing other resource costs - - gating off unused clocks in software (or hardware) - - switching off unused power supplies - - eliminating (or delaying/merging) IRQs - - tuning DMA to use word and/or burst modes - - Using device-specific low power states - - using lower voltages - - avoiding needless DMA transfers - -Read your hardware documentation carefully to see the opportunities that -may be available. If you can, measure the actual power usage and check -it against the budget established for your project. - - -Examples: USB hosts, system timer, system CPU ----------------------------------------------- -USB host controllers make interesting, if complex, examples. In many cases -these have no work to do: no USB devices are connected, or all of them are -in the USB "suspend" state. Linux host controller drivers can then disable -periodic DMA transfers that would otherwise be a constant power drain on the -memory subsystem, and enter a suspend state. In power-aware controllers, -entering that suspend state may disable the clock used with USB signaling, -saving a certain amount of power. - -The controller will be woken from that state (with an IRQ) by changes to the -signal state on the data lines of a given port, for example by an existing -peripheral requesting "remote wakeup" or by plugging a new peripheral. The -same wakeup mechanism usually works from "standby" sleep states, and on some -systems also from "suspend to RAM" (or even "suspend to disk") states. -(Except that ACPI may be involved instead of normal IRQs, on some hardware.) - -System devices like timers and CPUs may have special roles in the platform -power management scheme. For example, system timers using a "dynamic tick" -approach don't just save CPU cycles (by eliminating needless timer IRQs), -but they may also open the door to using lower power CPU "idle" states that -cost more than a jiffie to enter and exit. On x86 systems these are states -like "C3"; note that periodic DMA transfers from a USB host controller will -also prevent entry to a C3 state, much like a periodic timer IRQ. - -That kind of runtime mechanism interaction is common. "System On Chip" (SOC) -processors often have low power idle modes that can't be entered unless -certain medium-speed clocks (often 12 or 48 MHz) are gated off. When the -drivers gate those clocks effectively, then the system idle task may be able -to use the lower power idle modes and thereby increase battery life. - -If the CPU can have a "cpufreq" driver, there also may be opportunities -to shift to lower voltage settings and reduce the power cost of executing -a given number of instructions. (Without voltage adjustment, it's rare -for cpufreq to save much power; the cost-per-instruction must go down.) +A system-wide power transition can be started while some devices are in low +power states due to runtime power management. The system sleep PM callbacks +should recognize such situations and react to them appropriately, but the +necessary actions are subsystem-specific. + +In some cases the decision may be made at the subsystem level while in other +cases the device driver may be left to decide. In some cases it may be +desirable to leave a suspended device in that state during a system-wide power +transition, but in other cases the device must be put back into the full-power +state temporarily, for example so that its system wakeup capability can be +disabled. This all depends on the hardware and the design of the subsystem and +device driver in question. + +During system-wide resume from a sleep state it's best to put devices into the +full-power state, as explained in Documentation/power/runtime_pm.txt. Refer to +that document for more information regarding this particular issue as well as +for information on the device runtime power management framework in general. |
