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Commit 4fc08400 authored by David Brownell's avatar David Brownell Committed by Greg Kroah-Hartman
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updated Documentation/power/devices.txt 

This turned into a rewrite of Documentation/power/devices.txt:

 - Provide more of the "big picture"

 - Fixup some of the horribly ancient/obsolete description of device suspend()
   semantics; lots of text just got deleted.

 - Add a decent description of PM_EVENT_* codes, including the new PRETHAW code
   needed in some swsusp scenarios.

 - Describe the new PM factorization from Linus:
     * class suspend, current suspend, then suspend_late
     * NOT suspend_prepare, it wasn't really usable
     * resume_early, current resume, class resume.

 - Updates power/state docs to be correct, and deprecate its usage except for
   driver testing.
Signed-off-by: default avatarDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@suse.de>
parent f1cc0a89
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Documentation/power/devices.txt
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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.
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-to-ram", or (mostly for systems with
disks) "hibernate" (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 that low-power state. This feature may be disabled using the
relevant /sys/devices/.../power/wakeup file; enabling it may cost some
power usage, but let the whole system enter low power states more often.
Runtime Power Management model:
Drivers may also enter low power states while the system is running,
independently of other power management activity. Upstream drivers
will normally not know (or care) if the device is in some low power
state when issuing requests; the driver will auto-resume anything
that's needed when it gets a request.
This doesn't have, or need much infrastructure; it's just something you
should do when writing your drivers. For example, clk_disable() unused
clocks as part of minimizing power drain for currently-unused hardware.
Of course, sometimes clusters of drivers will collaborate with each
other, which could involve task-specific power management.
There's not a lot to be said about those low power states except that they
are very system-specific, and often device-specific. Also, that if enough
drivers put themselves into low power states (at "runtime"), the effect may be
the same as entering some system-wide low-power state (system sleep) ... and
that synergies exist, so that several drivers using runtime pm might put the
system into a state where even deeper power saving options are available.
Most suspended devices will have quiesced all I/O: no more DMA or irqs, no
more data read or written, and requests from upstream drivers are no longer
accepted. A given bus or platform may have different requirements though.
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
===========================================
Most of the programming interfaces a device driver needs to know about
relate to that first model: entering a system-wide low power state,
rather than just minimizing power consumption by one device.
Bus Driver Methods
------------------
The core methods to suspend and resume devices reside in struct bus_type.
These are mostly of interest to people writing infrastructure for busses
like PCI or USB, or because they define the primitives that device drivers
may need to apply in domain-specific ways to their devices:
Device Power Management
struct bus_type {
...
int (*suspend)(struct device *dev, pm_message_t state);
int (*suspend_late)(struct device *dev, pm_message_t state);
int (*resume_early)(struct device *dev);
int (*resume)(struct device *dev);
};
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).
Bus drivers implement those methods as appropriate for the hardware and
the drivers using it; PCI works differently from USB, and so on. Not many
people write bus drivers; most driver code is a "device driver" that
builds on top of bus-specific framework code.
For more information on these driver calls, see the description later;
they are called in phases for every device, respecting the parent-child
sequencing in the driver model tree. Note that as this is being written,
only the suspend() and resume() are widely available; not many bus drivers
leverage all of those phases, or pass them down to lower driver levels.
/sys/devices/.../power/wakeup files
-----------------------------------
All devices in the driver model have two flags to control handling of
wakeup events, which are hardware signals that can force the device and/or
system out of a low power state. These are initialized by bus or device
driver code using device_init_wakeup(dev,can_wakeup).
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.
For devices that can issue wakeup events, a separate flag controls whether
that device should try to use its wakeup mechanism. The initial value of
device_may_wakeup() will be true, so that the device's "wakeup" file holds
the value "enabled". Userspace can change that to "disabled" so that
device_may_wakeup() returns false; or change it back to "enabled" (so that
it returns true again).
EXAMPLE: PCI Device Driver Methods
-----------------------------------
PCI framework software calls these methods when the PCI device driver bound
to a device device has provided them:
struct pci_driver {
...
int (*suspend)(struct pci_device *pdev, pm_message_t state);
int (*suspend_late)(struct pci_device *pdev, pm_message_t state);
int (*resume_early)(struct pci_device *pdev);
int (*resume)(struct pci_device *pdev);
};
Drivers will implement those methods, and call PCI-specific procedures
like pci_set_power_state(), pci_enable_wake(), pci_save_state(), and
pci_restore_state() to manage PCI-specific mechanisms. (PCI config space
could be saved during driver probe, if it weren't for the fact that some
systems rely on userspace tweaking using setpci.) Devices are suspended
before their bridges enter low power states, and likewise bridges resume
before their devices.
Upper Layers of Driver Stacks
-----------------------------
Device drivers generally have at least two interfaces, and the methods
sketched above are the ones which apply to the lower level (nearer PCI, USB,
or other bus hardware). The network and block layers are examples of upper
level interfaces, as is a character device talking to userspace.
Power management requests normally need to flow through those upper levels,
which often use domain-oriented requests like "blank that screen". In
some cases those upper levels will have power management intelligence that
relates to end-user activity, or other devices that work in cooperation.
When those interfaces are structured using class interfaces, there is a
standard way to have the upper layer stop issuing requests to a given
class device (and restart later):
struct class {
...
int (*suspend)(struct device *dev, pm_message_t state);
int (*resume)(struct device *dev);
};
Methods
Those calls are issued in specific phases of the process by which the
system enters a low power "suspend" state, or resumes from it.
Calling Drivers to Enter System Sleep States
============================================
When the system enters a low power state, each device's driver is asked
to suspend the device by putting it into state compatible with the target
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.
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
matching resume calls would then completely reinitialize the hardware
before reactivating its class I/O queues.
More power-aware drivers drivers will use more than one device low power
state, either at runtime or during system sleep states, and might trigger
system wakeup events.
Call Sequence Guarantees
------------------------
To ensure that bridges and similar links needed 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.)
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.
The phases are seen by driver notifications issued in this order:
1 class.suspend(dev, message) is called after tasks are frozen, for
devices associated with a class that has such a method. This
method may sleep.
Since I/O activity usually comes from such higher layers, this is
a good place to quiesce all drivers of a given type (and keep such
code out of those drivers).
2 bus.suspend(dev, message) is called next. This method may sleep,
and is often morphed into a device driver call with bus-specific
parameters and/or rules.
This call should handle parts of device suspend logic that require
sleeping. It probably does work to quiesce the device which hasn't
been abstracted into class.suspend() or bus.suspend_late().
3 bus.suspend_late(dev, message) is called with IRQs disabled, and
with only one CPU active. Until the bus.resume_early() phase
completes (see later), IRQs are not enabled again. This method
won't be exposed by all busses; for message based busses like USB,
I2C, or SPI, device interactions normally require IRQs. This bus
call may be morphed into a driver call with bus-specific parameters.
This call might save low level hardware state that might otherwise
be lost in the upcoming low power state, and actually put the
device into a low power state ... so that in some cases the device
may stay partly usable until this late. This "late" call may also
help when coping with hardware that behaves badly.
The pm_message_t parameter is currently used to refine those semantics
(described later).
At the end of those phases, drivers should normally have stopped all I/O
transactions (DMA, IRQs), saved enough state that they can re-initialize
or restore previous state (as needed by the hardware), and placed the
device into a low-power state. On many platforms they will also use
clk_disable() to gate off one or more clock sources; sometimes they will
also switch off power supplies, or reduce voltages. Drivers which have
runtime PM support may already have performed some or all of the steps
needed to prepare for the upcoming system sleep state.
When any driver sees that its device_can_wakeup(dev), it should make sure
to use the relevant hardware signals to trigger a system wakeup event.
For example, enable_irq_wake() might identify GPIO signals hooked up to
a switch or other external hardware, and pci_enable_wake() does something
similar for PCI's PME# signal.
If a driver (or bus, or class) fails it suspend method, the system won't
enter the desired low power state; it will resume all the devices it's
suspended so far.
Note that drivers may need to perform different actions based on the target
system lowpower/sleep state. At this writing, there are only platform
specific APIs through which drivers could determine those target states.
Device Low Power (suspend) States
---------------------------------
Device low-power states aren't very 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.
Meaning of pm_message_t.event
-----------------------------
Parameters to suspend calls include the device affected and a message of
type pm_message_t, which has one field: the event. If driver does not
recognize the event code, suspend calls may abort the request and return
a negative errno. However, most drivers will be fine if they implement
PM_EVENT_SUSPEND semantics for all messages.
The event codes are used to refine the goal of suspending the device, and
mostly matter when creating or resuming system memory image snapshots, as
used with suspend-to-disk:
PM_EVENT_SUSPEND -- quiesce the driver and put hardware into a low-power
state. When used with system sleep states like "suspend-to-RAM" or
"standby", the upcoming resume() call will often be able to rely on
state kept in hardware, or issue system wakeup events. When used
instead with suspend-to-disk, few devices support this capability;
most are completely powered off.
PM_EVENT_FREEZE -- quiesce the driver, but don't necessarily change into
any low power mode. A system snapshot is about to be taken, often
followed by a call to the driver's resume() method. Neither wakeup
events nor DMA are allowed.
PM_EVENT_PRETHAW -- quiesce the driver, knowing that the upcoming resume()
will restore a suspend-to-disk snapshot from a different kernel image.
Drivers that are smart enough to look at their hardware state during
resume() processing need that state to be correct ... a PRETHAW could
be used to invalidate that state (by resetting the device), like a
shutdown() invocation would before a kexec() or system halt. Other
drivers might handle this the same way as PM_EVENT_FREEZE. Neither
wakeup events nor DMA are allowed.
To enter "standby" (ACPI S1) or "Suspend to RAM" (STR, ACPI S3) states, or
the similarly named APM states, only PM_EVENT_SUSPEND is used; for "Suspend
to Disk" (STD, hibernate, ACPI S4), all of those event codes are used.
There's also PM_EVENT_ON, a value which never appears as a suspend event
but is sometimes used to record the "not suspended" device state.
Resuming Devices
----------------
Resuming is done in multiple phases, much like suspending, with all
devices processing each phase's calls before the next phase begins.
The phases are seen by driver notifications issued in this order:
1 bus.resume_early(dev) is called with IRQs disabled, and with
only one CPU active. As with bus.suspend_late(), this method
won't be supported on busses that require IRQs in order to
interact with devices.
This reverses the effects of bus.suspend_late().
2 bus.resume(dev) is called next. This may be morphed into a device
driver call with bus-specific parameters; implementations may sleep.
This reverses the effects of bus.suspend().
3 class.resume(dev) is called for devices associated with a class
that has such a method. Implementations may sleep.
This reverses the effects of class.suspend(), and would usually
reactivate the device's I/O queue.
At the end of those phases, drivers should normally be as functional as
they were before suspending: I/O can be performed using DMA and IRQs, and
the relevant clocks are gated on. The device need not be "fully on"; it
might be in a runtime lowpower/suspend state that acts as if it were.
However, the details here may again be platform-specific. For example,
some systems support multiple "run" states, and the mode in effect at
the end of resume() might not be the one which preceded suspension.
That means availability of certain clocks or power supplies changed,
which could easily affect how a driver works.
Drivers need to be able to handle hardware which has been reset since the
suspend methods were called, for example by complete reinitialization.
This may be the hardest part, and the one most protected by NDA'd documents
and chip errata. It's simplest if the hardware state hasn't changed since
the suspend() was called, but that can't always be guaranteed.
Drivers must also be prepared to notice that the device has been removed
while the system was powered off, whenever that's physically possible.
PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
where common Linux platforms will see such removal. Details of how drivers
will notice and handle such removals are currently bus-specific, and often
involve a separate thread.
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);
};
Note that the bus-specific runtime PM wakeup mechanism can exist, and might
be defined to share some of the same driver code as for system wakeup. For
example, a bus-specific device driver's resume() method might be used there,
so it wouldn't only be called from bus.resume() during system-wide wakeup.
See bus-specific information about how runtime wakeup events are handled.
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
--------------
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.
System devices will only be suspended with interrupts disabled, and after
all other devices have been suspended. On resume, they will be resumed
before any other devices, and also with interrupts disabled.
That is, IRQs are disabled, the suspend_late() phase begins, then the
sysdev_driver.suspend() phase, and the system enters a sleep state. Then
the sysdev_driver.resume() phase begins, followed by the resume_early()
phase, after which IRQs are enabled.
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'.
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.
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 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
#
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. 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
usually include hardware states that are also used in system sleep states.
However, note that if a driver puts a device into a runtime low power state
and the system then goes into a system-wide sleep state, it normally ought
to resume into that runtime low power state rather than "full on". Such
distinctions would be part of the driver-internal state machine for that
hardware; the whole point of runtime power management is to be sure that
drivers are decoupled in that way from the state machine governing phases
of the system-wide power/sleep state transitions.
Power Saving Techniques
-----------------------
Normally runtime power management is handled by the drivers without specific
userspace or kernel intervention, by device-aware use of techniques like:
Using information provided by other system layers
- stay deeply "off" except between open() and close()
- if transceiver/PHY indicates "nobody connected", stay "off"
- application protocols may include power commands or hints
Using fewer CPU cycles
- using DMA instead of PIO
- removing timers, or making them lower frequency
- shortening "hot" code paths
- eliminating cache misses
- (sometimes) offloading work to device firmware
Reducing other resource costs
- gating off unused clocks in software (or hardware)
- switching off unused power supplies
- eliminating (or delaying/merging) IRQs
- tuning DMA to use word and/or burst modes
Using device-specific low power states
- using lower voltages
- avoiding needless DMA transfers
Read your hardware documentation carefully to see the opportunities that
may be available. If you can, measure the actual power usage and check
it against the budget established for your project.
Examples: USB hosts, system timer, system CPU
----------------------------------------------
USB host controllers make interesting, if complex, examples. In many cases
these have no work to do: no USB devices are connected, or all of them are
in the USB "suspend" state. Linux host controller drivers can then disable
periodic DMA transfers that would otherwise be a constant power drain on the
memory subsystem, and enter a suspend state. In power-aware controllers,
entering that suspend state may disable the clock used with USB signaling,
saving a certain amount of power.
The controller will be woken from that state (with an IRQ) by changes to the
signal state on the data lines of a given port, for example by an existing
peripheral requesting "remote wakeup" or by plugging a new peripheral. The
same wakeup mechanism usually works from "standby" sleep states, and on some
systems also from "suspend to RAM" (or even "suspend to disk") states.
(Except that ACPI may be involved instead of normal IRQs, on some hardware.)
System devices like timers and CPUs may have special roles in the platform
power management scheme. For example, system timers using a "dynamic tick"
approach don't just save CPU cycles (by eliminating needless timer IRQs),
but they may also open the door to using lower power CPU "idle" states that
cost more than a jiffie to enter and exit. On x86 systems these are states
like "C3"; note that periodic DMA transfers from a USB host controller will
also prevent entry to a C3 state, much like a periodic timer IRQ.
That kind of runtime mechanism interaction is common. "System On Chip" (SOC)
processors often have low power idle modes that can't be entered unless
certain medium-speed clocks (often 12 or 48 MHz) are gated off. When the
drivers gate those clocks effectively, then the system idle task may be able
to use the lower power idle modes and thereby increase battery life.
If the CPU can have a "cpufreq" driver, there also may be opportunities
to shift to lower voltage settings and reduce the power cost of executing
a given number of instructions. (Without voltage adjustment, it's rare
for cpufreq to save much power; the cost-per-instruction must go down.)
/sys/devices/.../power/state files
==================================
For now you can also test some of this functionality using sysfs.
DEPRECATED: USE "power/state" ONLY FOR DRIVER TESTING, AND
AVOID USING dev->power.power_state IN DRIVERS.
THESE WILL BE REMOVED. IF THE "power/state" FILE GETS REPLACED,
IT WILL BECOME SOMETHING COUPLED TO THE BUS OR DRIVER.
In each device's directory, there is a 'power' directory, which contains
at least a 'state' file. The value of this field is effectively boolean,
PM_EVENT_ON or PM_EVENT_SUSPEND.
* Reading from this file displays a value corresponding to
the power.power_state.event field. All nonzero values are
displayed as "2", corresponding to a low power state; zero
is displayed as "0", corresponding to normal operation.
* Writing to this file initiates a transition using the
specified event code number; only '0', '2', and '3' are
accepted (without a newline); '2' and '3' are both
mapped to PM_EVENT_SUSPEND.
On writes, the PM core relies on that recorded event code and the device/bus
capabilities to determine whether it uses a partial suspend() or resume()
sequence to change things so that the recorded event corresponds to the
numeric parameter.
- If the bus requires the irqs-disabled suspend_late()/resume_early()
phases, writes fail because those operations are not supported here.
- If the recorded value is the expected value, nothing is done.
- If the recorded value is nonzero, the device is partially resumed,
using the bus.resume() and/or class.resume() methods.
- If the target value is nonzero, the device is partially suspended,
using the class.suspend() and/or bus.suspend() methods and the
PM_EVENT_SUSPEND message.
Drivers have no way to tell whether their suspend() and resume() calls
have come through the sysfs power/state file or as part of entering a
system sleep state, except that when accessed through sysfs the normal
parent/child sequencing rules are ignored. Drivers (such as bus, bridge,
or hub drivers) which expose child devices may need to enforce those rules
on their own.
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