mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-12-06 02:46:42 +07:00
4fc0840006
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: David Brownell <dbrownell@users.sourceforge.net> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
554 lines
26 KiB
Plaintext
554 lines
26 KiB
Plaintext
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:
|
|
|
|
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);
|
|
};
|
|
|
|
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);
|
|
};
|
|
|
|
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.
|
|
|
|
|
|
Note that the bus-specific runtime PM wakeup mechanism can exist, and might
|
|
be defined to share some of the same driver code as for system wakeup. For
|
|
example, a bus-specific device driver's resume() method might be used there,
|
|
so it wouldn't only be called from bus.resume() during system-wide wakeup.
|
|
See bus-specific information about how runtime wakeup events are handled.
|
|
|
|
|
|
System Devices
|
|
--------------
|
|
System devices follow a slightly different API, which can be found in
|
|
|
|
include/linux/sysdev.h
|
|
drivers/base/sys.c
|
|
|
|
System devices will only be suspended with interrupts disabled, and after
|
|
all other devices have been suspended. On resume, they will be resumed
|
|
before any other devices, and also with interrupts disabled.
|
|
|
|
That is, IRQs are disabled, the suspend_late() phase begins, then the
|
|
sysdev_driver.suspend() phase, and the system enters a sleep state. Then
|
|
the sysdev_driver.resume() phase begins, followed by the resume_early()
|
|
phase, after which IRQs are enabled.
|
|
|
|
Code to actually enter and exit the system-wide low power state sometimes
|
|
involves hardware details that are only known to the boot firmware, and
|
|
may leave a CPU running software (from SRAM or flash memory) that monitors
|
|
the system and manages its wakeup sequence.
|
|
|
|
|
|
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.
|