Device Power Management
-Copyright (c) 2010 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
+Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
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.)
+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.
-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.
+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.
/sys/devices/.../power/control files
situation where it actually matters.
+Device Power Domains
+--------------------
+Sometimes devices share reference clocks or other power resources. In those
+cases it generally is not possible to put devices into low-power states
+individually. Instead, a set of devices sharing a power resource can be put
+into a low-power state together at the same time by turning off the shared
+power resource. Of course, they also need to be put into the full-power state
+together, by turning the shared power resource on. A set of devices with this
+property is often referred to as a power domain.
+
+Support for power domains is provided through the pwr_domain field of struct
+device. This field is a pointer to an object of type struct dev_power_domain,
+defined in include/linux/pm.h, providing a set of power management callbacks
+analogous to the subsystem-level and device driver callbacks that are executed
+for the given device during all power transitions, in addition to the respective
+subsystem-level callbacks. Specifically, the power domain "suspend" callbacks
+(i.e. ->runtime_suspend(), ->suspend(), ->freeze(), ->poweroff(), etc.) are
+executed after the analogous subsystem-level callbacks, while the power domain
+"resume" callbacks (i.e. ->runtime_resume(), ->resume(), ->thaw(), ->restore,
+etc.) are executed before the analogous subsystem-level callbacks. Error codes
+returned by the "suspend" and "resume" power domain callbacks are ignored.
+
+Power domain ->runtime_idle() callback is executed before the subsystem-level
+->runtime_idle() callback and the result returned by it is not ignored. Namely,
+if it returns error code, the subsystem-level ->runtime_idle() callback will not
+be called and the helper function rpm_idle() executing it will return error
+code. This mechanism is intended to help platforms where saving device state
+is a time consuming operation and should only be carried out if all devices
+in the power domain are idle, before turning off the shared power resource(s).
+Namely, the power domain ->runtime_idle() callback may return error code until
+the pm_runtime_idle() helper (or its asychronous version) has been called for
+all devices in the power domain (it is recommended that the returned error code
+be -EBUSY in those cases), preventing the subsystem-level ->runtime_idle()
+callback from being run prematurely.
+
+The support for device power domains is only relevant to platforms needing to
+use the same subsystem-level (e.g. platform bus type) and device driver power
+management callbacks in many different power domain configurations and wanting
+to avoid incorporating the support for power domains into the subsystem-level
+callbacks. The other platforms need not implement it or take it into account
+in any way.
+
+
System Devices
--------------
System devices (sysdevs) follow a slightly different API, which can be found in
projects / mv-sheeva.git / blobdiff