-
Device Power Management
-
-Device power management encompasses two areas - the ability to save
-state and transition a device to a low-power state when the system is
-entering a low-power state; and the ability to transition a device to
-a low-power state while the system is running (and independently of
-any other power management activity).
-
-
-Methods
-
-The methods to suspend and resume devices reside in struct bus_type:
-
-struct bus_type {
- ...
- int (*suspend)(struct device * dev, pm_message_t state);
- int (*resume)(struct device * dev);
+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 (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
+shared by everything that hooks up to the driver model core. Read it as
+background for the domain-specific work you'd do with any specific driver.
+
+
+Two Models for Device Power Management
+======================================
+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" (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
+ cleanly power down hardware and software subsystems, then reactivate
+ them without loss of data.
+
+ Some drivers can manage hardware wakeup events, which make the system
+ 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:
+ 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
+or removal (for PCMCIA, MMC/SD, USB, and so on).
+
+
+Interfaces for Entering System Sleep States
+===========================================
+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);
};
-Each bus driver is responsible implementing these methods, translating
-the call into a bus-specific request and forwarding the call to the
-bus-specific drivers. For example, PCI drivers implement suspend() and
-resume() methods in struct pci_driver. The PCI core is simply
-responsible for translating the pointers to PCI-specific ones and
-calling the low-level driver.
-
-This is done to a) ease transition to the new power management methods
-and leverage the existing PM code in various bus drivers; b) allow
-buses to implement generic and default PM routines for devices, and c)
-make the flow of execution obvious to the reader.
-
-
-System Power Management
-
-When the system enters a low-power state, the device tree is walked in
-a depth-first fashion to transition each device into a low-power
-state. The ordering of the device tree is guaranteed by the order in
-which devices get registered - children are never registered before
-their ancestors, and devices are placed at the back of the list when
-registered. By walking the list in reverse order, we are guaranteed to
-suspend devices in the proper order.
-
-Devices are suspended once with interrupts enabled. Drivers are
-expected to stop I/O transactions, save device state, and place the
-device into a low-power state. Drivers may sleep, allocate memory,
-etc. at will.
-
-Some devices are broken and will inevitably have problems powering
-down or disabling themselves with interrupts enabled. For these
-special cases, they may return -EAGAIN. This will put the device on a
-list to be taken care of later. When interrupts are disabled, before
-we enter the low-power state, their drivers are called again to put
-their device to sleep.
-
-On resume, the devices that returned -EAGAIN will be called to power
-themselves back on with interrupts disabled. Once interrupts have been
-re-enabled, the rest of the drivers will be called to resume their
-devices. On resume, a driver is responsible for powering back on each
-device, restoring state, and re-enabling I/O transactions for that
-device.
-
-System devices follow a slightly different API, which can be found in
+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.
+
+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.
+
+
+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.
+
+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 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.
+
+
+/sys/devices/.../power/wakeup files
+-----------------------------------
+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.
+
+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 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 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 might prepare the devices for triggering system wakeup
+events.
+
+
+Call Sequence Guarantees
+------------------------
+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.
+
+The ordering of the device tree is defined by the order in which devices
+get registered: a child can never be registered, probed or resumed before
+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 (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.
+
+
+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).
+
+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
+
+ dev->class->pm.suspend(dev);
+ dev->type->pm.suspend(dev);
+ dev->bus->pm.suspend(dev);
+
+before moving on to the next device, whereas during the resume phase the core
+invokes
+
+ dev->bus->pm.resume(dev);
+ dev->type->pm.resume(dev);
+ dev->class->pm.resume(dev);
+
+These callbacks may in turn invoke device- or driver-specific methods stored in
+dev->driver->pm, but they don't have to.
+
+
+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.
+
+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.
+
+
+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 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.
+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 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".
+
+Some busses define rules about what different suspend states mean. PCI
+gives one example: after the suspend sequence completes, a non-legacy
+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
+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).
+
+Some details here may be platform-specific. Systems may have devices that
+can be fully active in certain sleep states, such as an LCD display that's
+refreshed using DMA while most of the system is sleeping lightly ... and
+its frame buffer might even be updated by a DSP or other non-Linux CPU while
+the Linux control processor stays idle.
+
+Moreover, the specific actions taken may depend on the target system state.
+One target system state might allow a given device to be very operational;
+another might require a hard shut down with re-initialization on resume.
+And two different target systems might use the same device in different
+ways; the aforementioned LCD might be active in one product's "standby",
+but a different product using the same SOC might work differently.
+
+
+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.
+
+For details refer to Documentation/power/notifiers.txt.
Runtime Power Management
-
-Many devices are able to dynamically power down while the system is
-still running. This feature is useful for devices that are not being
-used, and can offer significant power savings on a running system.
-
-In each device's directory, there is a 'power' directory, which
-contains at least a 'state' file. Reading from this file displays what
-power state the device is currently in. Writing to this file initiates
-a transition to the specified power state, which must be a decimal in
-the range 1-3, inclusive; or 0 for 'On'.
-
-The PM core will call the ->suspend() method in the bus_type object
-that the device belongs to if the specified state is not 0, or
-->resume() if it is.
-
-Nothing will happen if the specified state is the same state the
-device is currently in.
-
-If the device is already in a low-power state, and the specified state
-is another, but different, low-power state, the ->resume() method will
-first be called to power the device back on, then ->suspend() will be
-called again with the new state.
-
-The driver is responsible for saving the working state of the device
-and putting it into the low-power state specified. If this was
-successful, it returns 0, and the device's power_state field is
-updated.
-
-The driver must take care to know whether or not it is able to
-properly resume the device, including all step of reinitialization
-necessary. (This is the hardest part, and the one most protected by
-NDA'd documents).
-
-The driver must also take care not to suspend a device that is
-currently in use. It is their responsibility to provide their own
-exclusion mechanisms.
-
-The runtime power transition happens with interrupts enabled. If a
-device cannot support being powered down with interrupts, it may
-return -EAGAIN (as it would during a system power management
-transition), but it will _not_ be called again, and the transaction
-will fail.
-
-There is currently no way to know what states a device or driver
-supports a priori. This will change in the future.
-
-pm_message_t meaning
-
-pm_message_t has two fields. event ("major"), and flags. If driver
-does not know event code, it aborts the request, returning error. Some
-drivers may need to deal with special cases based on the actual type
-of suspend operation being done at the system level. This is why
-there are flags.
-
-Event codes are:
-
-ON -- no need to do anything except special cases like broken
-HW.
-
-# NOTIFICATION -- pretty much same as ON?
-
-FREEZE -- stop DMA and interrupts, and be prepared to reinit HW from
-scratch. That probably means stop accepting upstream requests, the
-actual policy of what to do with them beeing specific to a given
-driver. It's acceptable for a network driver to just drop packets
-while a block driver is expected to block the queue so no request is
-lost. (Use IDE as an example on how to do that). FREEZE requires no
-power state change, and it's expected for drivers to be able to
-quickly transition back to operating state.
-
-SUSPEND -- like FREEZE, but also put hardware into low-power state. If
-there's need to distinguish several levels of sleep, additional flag
-is probably best way to do that.
-
-Transitions are only from a resumed state to a suspended state, never
-between 2 suspended states. (ON -> FREEZE or ON -> SUSPEND can happen,
-FREEZE -> SUSPEND or SUSPEND -> FREEZE can not).
-
-All events are:
-
-[NOTE NOTE NOTE: If you are driver author, you should not care; you
-should only look at event, and ignore flags.]
-
-#Prepare for suspend -- userland is still running but we are going to
-#enter suspend state. This gives drivers chance to load firmware from
-#disk and store it in memory, or do other activities taht require
-#operating userland, ability to kmalloc GFP_KERNEL, etc... All of these
-#are forbiden once the suspend dance is started.. event = ON, flags =
-#PREPARE_TO_SUSPEND
-
-Apm standby -- prepare for APM event. Quiesce devices to make life
-easier for APM BIOS. event = FREEZE, flags = APM_STANDBY
-
-Apm suspend -- same as APM_STANDBY, but it we should probably avoid
-spinning down disks. event = FREEZE, flags = APM_SUSPEND
-
-System halt, reboot -- quiesce devices to make life easier for BIOS. event
-= FREEZE, flags = SYSTEM_HALT or SYSTEM_REBOOT
-
-System shutdown -- at least disks need to be spun down, or data may be
-lost. Quiesce devices, just to make life easier for BIOS. event =
-FREEZE, flags = SYSTEM_SHUTDOWN
-
-Kexec -- turn off DMAs and put hardware into some state where new
-kernel can take over. event = FREEZE, flags = KEXEC
-
-Powerdown at end of swsusp -- very similar to SYSTEM_SHUTDOWN, except wake
-may need to be enabled on some devices. This actually has at least 3
-subtypes, system can reboot, enter S4 and enter S5 at the end of
-swsusp. event = FREEZE, flags = SWSUSP and one of SYSTEM_REBOOT,
-SYSTEM_SHUTDOWN, SYSTEM_S4
-
-Suspend to ram -- put devices into low power state. event = SUSPEND,
-flags = SUSPEND_TO_RAM
-
-Freeze for swsusp snapshot -- stop DMA and interrupts. No need to put
-devices into low power mode, but you must be able to reinitialize
-device from scratch in resume method. This has two flavors, its done
-once on suspending kernel, once on resuming kernel. event = FREEZE,
-flags = DURING_SUSPEND or DURING_RESUME
-
-Device detach requested from /sys -- deinitialize device; proably same as
-SYSTEM_SHUTDOWN, I do not understand this one too much. probably event
-= FREEZE, flags = DEV_DETACH.
-
-#These are not really events sent:
-#
-#System fully on -- device is working normally; this is probably never
-#passed to suspend() method... event = ON, flags = 0
-#
-#Ready after resume -- userland is now running, again. Time to free any
-#memory you ate during prepare to suspend... event = ON, flags =
-#READY_AFTER_RESUME
-#
-
-Driver Detach Power Management
-
-The kernel now supports the ability to place a device in a low-power
-state when it is detached from its driver, which happens when its
-module is removed.
-
-Each device contains a 'detach_state' file in its sysfs directory
-which can be used to control this state. Reading from this file
-displays what the current detach state is set to. This is 0 (On) by
-default. A user may write a positive integer value to this file in the
-range of 1-4 inclusive.
-
-A value of 1-3 will indicate the device should be placed in that
-low-power state, which will cause ->suspend() to be called for that
-device. A value of 4 indicates that the device should be shutdown, so
-->shutdown() will be called for that device.
-
-The driver is responsible for reinitializing the device when the
-module is re-inserted during it's ->probe() (or equivalent) method.
-The driver core will not call any extra functions when binding the
-device to the driver.
-
-pm_message_t meaning
-
-pm_message_t has two fields. event ("major"), and flags. If driver
-does not know event code, it aborts the request, returning error. Some
-drivers may need to deal with special cases based on the actual type
-of suspend operation being done at the system level. This is why
-there are flags.
-
-Event codes are:
-
-ON -- no need to do anything except special cases like broken
-HW.
-
-# NOTIFICATION -- pretty much same as ON?
-
-FREEZE -- stop DMA and interrupts, and be prepared to reinit HW from
-scratch. That probably means stop accepting upstream requests, the
-actual policy of what to do with them being specific to a given
-driver. It's acceptable for a network driver to just drop packets
-while a block driver is expected to block the queue so no request is
-lost. (Use IDE as an example on how to do that). FREEZE requires no
-power state change, and it's expected for drivers to be able to
-quickly transition back to operating state.
-
-SUSPEND -- like FREEZE, but also put hardware into low-power state. If
-there's need to distinguish several levels of sleep, additional flag
-is probably best way to do that.
-
-Transitions are only from a resumed state to a suspended state, never
-between 2 suspended states. (ON -> FREEZE or ON -> SUSPEND can happen,
-FREEZE -> SUSPEND or SUSPEND -> FREEZE can not).
-
-All events are:
-
-[NOTE NOTE NOTE: If you are driver author, you should not care; you
-should only look at event, and ignore flags.]
-
-#Prepare for suspend -- userland is still running but we are going to
-#enter suspend state. This gives drivers chance to load firmware from
-#disk and store it in memory, or do other activities taht require
-#operating userland, ability to kmalloc GFP_KERNEL, etc... All of these
-#are forbiden once the suspend dance is started.. event = ON, flags =
-#PREPARE_TO_SUSPEND
-
-Apm standby -- prepare for APM event. Quiesce devices to make life
-easier for APM BIOS. event = FREEZE, flags = APM_STANDBY
-
-Apm suspend -- same as APM_STANDBY, but it we should probably avoid
-spinning down disks. event = FREEZE, flags = APM_SUSPEND
-
-System halt, reboot -- quiesce devices to make life easier for BIOS. event
-= FREEZE, flags = SYSTEM_HALT or SYSTEM_REBOOT
-
-System shutdown -- at least disks need to be spun down, or data may be
-lost. Quiesce devices, just to make life easier for BIOS. event =
-FREEZE, flags = SYSTEM_SHUTDOWN
-
-Kexec -- turn off DMAs and put hardware into some state where new
-kernel can take over. event = FREEZE, flags = KEXEC
-
-Powerdown at end of swsusp -- very similar to SYSTEM_SHUTDOWN, except wake
-may need to be enabled on some devices. This actually has at least 3
-subtypes, system can reboot, enter S4 and enter S5 at the end of
-swsusp. event = FREEZE, flags = SWSUSP and one of SYSTEM_REBOOT,
-SYSTEM_SHUTDOWN, SYSTEM_S4
-
-Suspend to ram -- put devices into low power state. event = SUSPEND,
-flags = SUSPEND_TO_RAM
-
-Freeze for swsusp snapshot -- stop DMA and interrupts. No need to put
-devices into low power mode, but you must be able to reinitialize
-device from scratch in resume method. This has two flavors, its done
-once on suspending kernel, once on resuming kernel. event = FREEZE,
-flags = DURING_SUSPEND or DURING_RESUME
-
-Device detach requested from /sys -- deinitialize device; proably same as
-SYSTEM_SHUTDOWN, I do not understand this one too much. probably event
-= FREEZE, flags = DEV_DETACH.
-
-#These are not really events sent:
-#
-#System fully on -- device is working normally; this is probably never
-#passed to suspend() method... event = ON, flags = 0
-#
-#Ready after resume -- userland is now running, again. Time to free any
-#memory you ate during prepare to suspend... event = ON, flags =
-#READY_AFTER_RESUME
-#
+========================
+Many devices are able to dynamically power down while the system is still
+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 the bus the device uses, and will
+usually include hardware states that are also used in system sleep states.
+
+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.
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff