PM / Sleep: Update documentation related to system wakeup

/sys/devices/.../power/wakeup files

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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

All device objects in the driver model contain fields that control the handling

of system wakeup events (hardware signals that can force the system out of a

sleep state).  These fields 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

The "power.can_wakeup" flag just records whether the device (and its driver) can

physically support wakeup events.  The device_set_wakeup_capable() routine

affects this flag.  The "should_wakeup" flag controls whether the device should

try to use its wakeup mechanism.  device_set_wakeup_enable() affects this flag;

for the most part drivers should not change its value.  The initial value of

should_wakeup is supposed to be false for the majority of devices; the major

exceptions are power buttons, keyboards, and Ethernet adapters whose WoL

(wake-on-LAN) feature has been set up with ethtool.  It should also default

to true for devices that don't generate wakeup requests on their own but merely

forward wakeup requests from one bus to another (like PCI bridges).

affects this flag.  The "power.wakeup" field is a pointer to an object of type

struct wakeup_source used for controlling whether or not the device should use

its system wakeup mechanism and for notifying the PM core of system wakeup

events signaled by the device.  This object is only present for wakeup-capable

devices (i.e. devices whose "can_wakeup" flags are set) and is created (or

removed) by device_set_wakeup_capable().

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.  This file is only present

for wakeup-capable devices (i.e. devices whose "can_wakeup" flags are set)

and is created (or removed) by device_set_wakeup_capable().  Reads from the

file will return the corresponding string.

The device_may_wakeup() routine returns true only if both flags are set.

"power/wakeup" file.  User space can write the strings "enabled" or "disabled"

to it to indicate whether or not, respectively, the device is supposed to signal

system wakeup.  This file is only present if the "power.wakeup" object exists

for the given device and is created (or removed) along with that object, by

device_set_wakeup_capable().  Reads from the file will return the corresponding

string.

The "power/wakeup" file is supposed to contain the "disabled" string initially

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.  It should also default to "enabled" for devices that don't generate

wakeup requests on their own but merely forward wakeup requests from one bus to

another (like PCI Express ports).

The device_may_wakeup() routine returns true only if the "power.wakeup" object

exists and the corresponding "power/wakeup" file contains the string "enabled".

This information is used by subsystems, like the PCI bus type code, to see

whether or not to enable the devices' wakeup mechanisms.  If device wakeup

mechanisms are enabled or disabled directly by drivers, they also should use

device_may_wakeup() to decide what to do during a system sleep transition.

However for runtime power management, wakeup events should be enabled whenever

the device and driver both support them, regardless of the should_wakeup flag.

Device drivers, however, are not supposed to call device_set_wakeup_enable()

directly in any case.

It ought to be noted that system wakeup is conceptually different from "remote

wakeup" used by runtime power management, although it may be supported by the

same physical mechanism.  Remote wakeup is a feature allowing devices in

low-power states to trigger specific interrupts to signal conditions in which

they should be put into the full-power state.  Those interrupts may or may not

be used to signal system wakeup events, depending on the hardware design.  On

some systems it is impossible to trigger them from system sleep states.  In any

case, remote wakeup should always be enabled for runtime power management for

all devices and drivers that support it.

/sys/devices/.../power/control files

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