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Diffstat (limited to 'Documentation/power/devices.txt')
| -rw-r--r-- | Documentation/power/devices.txt | 94 |
1 files changed, 66 insertions, 28 deletions
diff --git a/Documentation/power/devices.txt b/Documentation/power/devices.txt index 57080cd7457..f023ba6bba6 100644 --- a/Documentation/power/devices.txt +++ b/Documentation/power/devices.txt @@ -1,6 +1,6 @@ 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> @@ -159,18 +159,18 @@ 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.) +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 @@ -249,23 +249,18 @@ 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). -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 - - dev->class->pm.suspend(dev); - dev->type->pm.suspend(dev); - dev->bus->pm.suspend(dev); - -before moving on to the next device, whereas during the resume phase the core -invokes - - dev->bus->pm.resume(dev); - dev->type->pm.resume(dev); - dev->class->pm.resume(dev); +All phases use bus, type, or class callbacks (that is, methods defined in +dev->bus->pm, dev->type->pm, or dev->class->pm). These callbacks are mutually +exclusive, so if the device type provides a struct dev_pm_ops object pointed to +by its pm field (i.e. both dev->type and dev->type->pm are defined), the +callbacks included in that object (i.e. dev->type->pm) will be used. Otherwise, +if the class provides a struct dev_pm_ops object pointed to by its pm field +(i.e. both dev->class and dev->class->pm are defined), the PM core will use the +callbacks from that object (i.e. dev->class->pm). Finally, if the pm fields of +both the device type and class objects are NULL (or those objects do not exist), +the callbacks provided by the bus (that is, the callbacks from dev->bus->pm) +will be used (this allows device types to override callbacks provided by bus +types or classes if necessary). These callbacks may in turn invoke device- or driver-specific methods stored in dev->driver->pm, but they don't have to. @@ -507,6 +502,49 @@ routines. Nevertheless, different callback pointers are used in case there is a 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 |