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PM: Improve device power management document

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Improve the device power management document after it's been
updated by the previous patch.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
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Alan Stern 2010-03-26 23:53:55 +01:00 committed by Rafael J. Wysocki
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Documentation/power/devices.txt
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@ -1,11 +1,13 @@
Device Power Management
(C) 2010 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
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
@ -19,9 +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
@ -29,41 +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 enabled or disabled
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.
system enter low-power states more often.
Runtime Power Management model:
Devices may also be put into low power states while the system is
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, parent device cannot be suspended unless all of its child
devices have been suspended). Moreover, depending on the bus type the
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. Also, devices put into low
power states at run time may require special handling during system-wide
power transitions, like suspend to RAM.
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 the runtime power management of devices.
Like 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.
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 "run time"), 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.
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, 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.
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
@ -72,10 +75,10 @@ or removal (for PCMCIA, MMC/SD, USB, and so on).
Interfaces for Entering System Sleep States
===========================================
There are programming interfaces provided for subsystem (bus type, device type,
device class) and device drivers in order to allow them to participate in the
power management of devices they are concerned with. They cover the system
sleep power management as well as the runtime power management of devices.
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
@ -106,16 +109,15 @@ struct dev_pm_ops {
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 of them are
specific to runtime power management, while the remaining ones are used during
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 an "old" or "legacy", deprecated way of implementing power
management operations available at least for some subsystems. This approach
does not use struct dev_pm_ops objects and it only is suitable 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.
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
@ -125,10 +127,10 @@ 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.
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
@ -137,66 +139,78 @@ 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, 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().
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. When that flag is clear, the sysfs
"wakeup" file is empty, and device_may_wakeup() returns false.
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.
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 false for the majority of devices, except for
power buttons, keyboards, and Ethernet adapters whose WoL (wake-on-LAN) feature
has been set up with ethtool. Thus in the majority of cases the device's
"wakeup" file will initially hold the value "disabled". Userspace can change
that to "enabled", so that device_may_wakeup() returns true, or change it back
to "disabled", so that it returns false again.
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
------------------------------------
All devices in the driver model have a flag to control the desired behavior of
its driver with respect 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(), depending on whether or not the
driver is supposed to power manage the device at run time by default,
respectively.
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.
This setting may be adjusted by user space by writing either "on" or "auto" to
the device's "control" file. If "auto" is written, the device's runtime_auto
flag will be set and the driver will be allowed to power manage the device if
capable of doing that. If "on" is written, the driver is not allowed to power
manage the device which in turn is supposed to remain in the full power state at
run time. User space can check the current value of the runtime_auto flag by
reading from the device's "control" file.
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 by its driver. 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 (like "suspend-to-RAM") even though its
runtime_auto flag is unset (in which case its "control" file contains "on").
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 for devices
refer to Documentation/power/runtime_pm.txt.
For more information about the runtime power management framework, refer to
Documentation/power/runtime_pm.txt.
Calling Drivers to Enter 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 state compatible with the target
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.
@ -224,168 +238,299 @@ 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).
Generally, different callbacks are used depending on whether the system is
going to the standby or memory sleep state ("suspend-to-RAM") or it is going to
be hibernated ("suspend-to-disk").
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
If the system goes to the standby or memory sleep state the phases are seen by
driver notifications issued in this order:
dev->class->pm.suspend(dev);
dev->type->pm.suspend(dev);
dev->bus->pm.suspend(dev);
1 bus->pm.prepare(dev) is called after tasks are frozen and it is supposed
to call the device driver's ->pm.prepare() method.
before moving on to the next device, whereas during the resume phase the core
invokes
The purpose of this method is mainly to prevent new children of the
device from being registered after it has returned. It also may be used
to generally prepare the device for the upcoming system transition, but
it should not put the device into a low power state.
dev->bus->pm.resume(dev);
dev->type->pm.resume(dev);
dev->class->pm.resume(dev);
2 class->pm.suspend(dev) is called if dev is associated with a class that
has such a method. It may invoke the device driver's ->pm.suspend()
method, unless type->pm.suspend(dev) or bus->pm.suspend() does that.
These callbacks may in turn invoke device- or driver-specific methods stored in
dev->driver->pm, but they don't have to.
3 type->pm.suspend(dev) is called if dev is associated with a device type
that has such a method. It may invoke the device driver's
->pm.suspend() method, unless class->pm.suspend(dev) or
bus->pm.suspend() does that.
4 bus->pm.suspend(dev) is called, if implemented. It usually calls the
device driver's ->pm.suspend() method.
Entering System Suspend
-----------------------
When the system goes into the standby or memory sleep state, the phases are:
This call should generally quiesce the device so that it doesn't do any
I/O after the call has returned. It also may save the device registers
and put it into the appropriate low power state, depending on the bus
type the device is on.
prepare, suspend, suspend_noirq.
5 bus->pm.suspend_noirq(dev) is called, if implemented. It may call the
device driver's ->pm.suspend_noirq() method, depending on the bus type
in question.
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.
This method is invoked after device interrupts have been suspended,
which means that the driver's interrupt handler will not be called
while it is running. It 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 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
method. However, bus types allowing devices to share interrupt vectors,
like PCI, generally need to use it to prevent interrupt handling issues
from happening during suspend.
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 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
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 the steps needed to prepare for the upcoming system
state transition.]
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 when the system is in the sleep state to
trigger a system wakeup event. For example, enable_irq_wake() might identify
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 a driver (or subsystem) 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.
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.
Hibernation Phases
------------------
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 a system image and saving it.
Therefore there are more phases of hibernation and special device PM methods are
used in this case.
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.
First, it is necessary to prepare the system for creating a hibernation image.
This is similar to putting the system into the standby or memory sleep state,
although it generally doesn't require that devices be put into low power states
(that is even not desirable at this point). Driver notifications are then
issued in the following order:
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:
1 bus->pm.prepare(dev) is called after tasks have been frozen and enough
memory has been freed.
prepare, freeze, freeze_noirq, thaw_noirq, thaw, complete,
prepare, poweroff, poweroff_noirq
2 class->pm.freeze(dev) is called if implemented. It may invoke the
device driver's ->pm.freeze() method, unless type->pm.freeze(dev) or
bus->pm.freeze() does that.
1. The prepare phase is discussed in the "Entering System Suspend" section
above.
3 type->pm.freeze(dev) is called if implemented. It may invoke the device
driver's ->pm.suspend() method, unless class->pm.freeze(dev) or
bus->pm.freeze() does that.
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.
4 bus->pm.freeze(dev) is called, if implemented. It usually calls the
device driver's ->pm.freeze() method.
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.
5 bus->pm.freeze_noirq(dev) is called, if implemented. It may call the
device driver's ->pm.freeze_noirq() method, depending on the bus type
in question.
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.
The difference between ->pm.freeze() and the corresponding ->pm.suspend() (and
similarly for the "noirq" variants) is that the former should avoid preparing
devices to trigger system wakeup events and putting devices into low power
states, although they generally have to save the values of device registers
so that it's possible to restore them during system resume.
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.
Second, after the system image has been created, the functionality of devices
has to be restored so that the image can be saved. That is similar to resuming
devices after the system has been woken up from the standby or memory sleep
state, which is described below, and causes the following device notifications
to be issued:
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.
1 bus->pm.thaw_noirq(dev), if implemented; may call the device driver's
->pm.thaw_noirq() method, depending on the bus type in question.
6. The complete phase is discussed in the "Leaving System Suspend" section
above.
2 bus->pm.thaw(dev), if implemented; usually calls the device driver's
->pm.thaw() method.
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.
3 type->pm.thaw(dev), if implemented; may call the device driver's
->pm.thaw() method if not called by the bus type or class.
7. The prepare phase is discussed above.
4 class->pm.thaw(dev), if implemented; may call the device driver's
->pm.thaw() method if not called by the bus type or device type.
8. The poweroff phase is analogous to the suspend phase.
5 bus->pm.complete(dev), if implemented; may call the device driver's
->pm.complete() method.
9. The poweroff_noirq phase is analogous to the suspend_noirq phase.
Generally, the role of the ->pm.thaw() methods (including the "noirq" variants)
is to bring the device back to the fully functional state, so that it may be
used for saving the image, if necessary. The role of bus->pm.complete() is to
reverse whatever bus->pm.prepare() did (likewise for the analogous device driver
callbacks).
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.
After the image has been saved, the devices need to be prepared for putting the
system into the low power state. That is analogous to suspending them before
putting the system into the standby or memory sleep state and involves the
following device notifications:
1 bus->pm.prepare(dev).
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.
2 class->pm.poweroff(dev), if implemented; may invoke the device driver's
->pm.poweroff() method if not called by the bus type or device type.
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.
3 type->pm.poweroff(dev), if implemented; may invoke the device driver's
->pm.poweroff() method if not called by the bus type or device class.
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.
4 bus->pm.poweroff(dev), if implemented; usually calls the device driver's
->pm.poweroff() method (if not called by the device class or type).
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.
5 bus->pm.poweroff_noirq(dev), if implemented; may call the device
driver's ->pm.poweroff_noirq() method, depending on the bus type
in question.
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:
The difference between ->pm.poweroff() and the corresponding ->pm.suspend() (and
analogously for the "noirq" variants) is that the former need not save the
device's registers. Still, they should prepare the device for triggering
system wakeup events if necessary and finally put it into the appropriate low
power state.
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 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
@ -420,204 +565,15 @@ ways; the aforementioned LCD might be active in one product's "standby",
but a different product using the same SOC might work differently.
Resuming Devices
----------------
Resuming is done in multiple phases, much like suspending, with all
devices processing each phase's calls before the next phase begins.
Again, however, different callbacks are used depending on whether the system is
waking up from the standby or memory sleep state ("suspend-to-RAM") or from
hibernation ("suspend-to-disk").
If the system is waking up from the standby or memory sleep state, the phases
are seen by driver notifications issued in this order:
1 bus->pm.resume_noirq(dev) is called, if implemented. It may call the
device driver's ->pm.resume_noirq() method, depending on the bus type in
question.
The role of this method is to perform actions that need to be performed
before device drivers' interrupt handlers are allowed to be invoked. If
the given bus type permits devices to share interrupt vectors, like PCI,
this 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 needed at this stage of resume.
2 bus->pm.resume(dev) is called, if implemented. It usually calls the
device driver's ->pm.resume() method.
This call should generally bring the the device back to the working
state, so that it can do I/O as requested after the call has returned.
However, it may be more convenient to use the device class or device
type ->pm.resume() for this purpose, in which case the bus type's
->pm.resume() method need not be implemented at all.
3 type->pm.resume(dev) is called, if implemented. It may invoke the
device driver's ->pm.resume() method, unless class->pm.resume(dev) or
bus->pm.resume() does that.
For devices that are not associated with any bus type or device class
this method plays the role of bus->pm.resume().
4 class->pm.resume(dev) is called, if implemented. It may invoke the
device driver's ->pm.resume() method, unless bus->pm.resume(dev) or
type->pm.resume() does that.
For devices that are not associated with any bus type or device type
this method plays the role of bus->pm.resume().
5 bus->pm.complete(dev) is called, if implemented. It is supposed to
invoke the device driver's ->pm.complete() method.
The role of this method is to reverse whatever bus->pm.prepare(dev)
(or the driver's ->pm.prepare()) did during suspend, if necessary.
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. In principle the device need not be
"fully on"; it might be in a runtime lowpower/suspend state during suspend and
the resume callbacks may try to restore that state, but that need not be
desirable from the user's point of view. In fact, there are multiple reasons
why it's better to always put devices into the "fully working" state in the
system sleep resume callbacks and 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 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.
Resume From 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.
In principle, the image might be loaded into memory and the pre-hibernation
memory contents might be restored by the boot loader. For this purpose,
however, the boot loader would need to know the image kernel's entry point and
there's no protocol defined for passing that information to boot loaders. As
a workaround, the boot loader loads a fresh instance of the kernel, called the
boot kernel, into memory and passes control to it in a usual way. Then, the
boot kernel reads the hibernation image, restores the pre-hibernation memory
contents and passes control to the image kernel. Thus, in fact, two different
kernels are involved in resuming from hibernation and in general they are not
only different because they play different roles in this operation. Actually,
the boot kernel may be completely different from the image kernel. Not only
the configuration of it, but also the version of it may be different.
The consequences of this are important to device drivers and their subsystems
(bus types, device classes and device types) too.
Namely, to be able to load the hibernation image into memory, the boot kernel
needs to include at least the subset of device drivers allowing it to access the
storage medium containing the image, although it generally doesn't need to
include all of the drivers included into the image kernel. After the image has
been loaded the devices handled by those drivers need to be prepared for passing
control back to the image kernel. This is very similar to the preparation of
devices for creating a hibernation image described above. In fact, it is done
in the same way, with the help of the ->pm.prepare(), ->pm.freeze() and
->pm.freeze_noirq() callbacks, but only for device drivers included in the boot
kernel (whose versions may generally be different from the versions of the
analogous drivers from the image kernel).
Should the restoration of the pre-hibernation memory contents fail, the boot
kernel would carry out the procedure of "thawing" devices described above, using
the ->pm.thaw_noirq(), ->pm.thaw(), and ->pm.complete() callbacks provided by
subsystems and device drivers. This, however, is a very rare condition. Most
often the pre-hibernation memory contents are restored successfully and control
is passed to the image kernel that is now responsible for bringing the system
back to the working state.
To achieve this goal, among other things, the image kernel restores the
pre-hibernation functionality of devices. This operation is analogous to the
resuming of devices after waking up from the memory sleep state, although it
involves different device notifications which are the following:
1 bus->pm.restore_noirq(dev), if implemented; may call the device driver's
->pm.restore_noirq() method, depending on the bus type in question.
2 bus->pm.restore(dev), if implemented; usually calls the device driver's
->pm.restore() method.
3 type->pm.restore(dev), if implemented; may call the device driver's
->pm.restore() method if not called by the bus type or class.
4 class->pm.restore(dev), if implemented; may call the device driver's
->pm.restore() method if not called by the bus type or device type.
5 bus->pm.complete(dev), if implemented; may call the device driver's
->pm.complete() method.
The roles of the ->pm.restore_noirq() and ->pm.restore() callbacks are analogous
to the roles of the corresponding resume callbacks, but they must assume that
the device may have been accessed before by the boot kernel. Consequently, the
state of the device before they are called may be different from the state of it
right prior to calling the resume callbacks. That difference usually doesn't
matter, so the majority of device drivers can set their resume and restore
callback pointers to the same routine. Nevertheless, different callback
pointers are used in case there is a situation where it actually matters.
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, when the non-boot CPUs are all offline and IRQs are disabled on the
remaining online CPU, then the sysdev_driver.suspend() phase is carried out, and
the system enters a sleep state (or hibernation image is created). During
resume (or after the image has been created) the sysdev_driver.resume() phase
is carried out, IRQs are enabled on the only online CPU, the non-boot CPUs are
enabled and that is followed by the "early resume" phase (in which the "noirq"
callbacks provided by subsystems and device drivers are invoked).
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.
Power Management Notifiers
--------------------------
As stated in Documentation/power/notifiers.txt, there are some operations that
cannot be carried out by the power management callbacks discussed above, because
carrying them out at these points would be 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 in a wrong way.
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.
@ -629,24 +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.
Note, however, that a system-wide power transition can be started while some
devices are in low power states due to the runtime power management. The system
sleep PM callbacks should generally recognize such situations and react to them
appropriately, but the recommended actions to be taken in that cases are
subsystem-specific.
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 some
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 system-wide power
transition, but in some other cases the device ought to be put back into the
full power state, for example to be configured for system wakeup or so that its
system wakeup capability can be disabled. That all depends on the hardware
and the design of the subsystem and device driver in question.
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 better 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
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.