PM / Domains: Update documentation

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.

for the given device during all power transitions, instead of the respective

subsystem-level callbacks.  Specifically, if a device's pm_domain pointer is

not NULL, the ->suspend() callback from the object pointed to by it will be

executed instead of its subsystem's (e.g. bus type's) ->suspend() callback and

anlogously for all of the remaining callbacks.  In other words, power management

domain callbacks, if defined for the given device, always take precedence over

the callbacks provided by the device's subsystem (e.g. bus type).

The support for device power management domains is only relevant to platforms

needing to use the same device driver power management callbacks in many

different power domain configurations and wanting to avoid incorporating the

support for power domains into subsystem-level callbacks, for example by

modifying the platform bus type.  Other platforms need not implement it or take

it into account in any way.

Device Low Power (suspend) States