The mechanism causing devices depending on a given power resource
(that is, devices that can be in D0 only if that power resource is
on) to be resumed automatically when the power resource is turned
on (and their "inferred" power state becomes D0 as a result) is
inherently racy and in fact unnecessary.
It is racy, because if the power resource is turned on and then
immediately off, the device resume triggered by the first transition
to "on" may still happen, causing the power resource to be turned
on again. That again will trigger the "resume of dependent devices"
mechanism, but if the devices in question are not in use, they will
be suspended in the meantime causing the power resource to be turned
off. However, the "resume of dependent devices" will next resume
them again and so on. In some cases (USB port PM in particular) that
leads to an endless busy loop of flipping the resource on and off
continuously.
It is needless, because whoever turns a power resource on will most
likely turn it off at some point and the devices that go into "D0"
as a result of turning it on will then go back into D3cold
(generally, the state they were in before).
Moreover, turning on all power resources a device needs to go into
D0 is not sufficient for a full transition into D0 in general.
Namely, _PS0 may need to be executed in addition to that in some
cases. This means that the whole rationale of the "resume of
dependent devices" mechanism was incorrect to begin with and it's
best to remove it entirely.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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