Am Mittwoch, 30. Juni 2004 19:26 schrieb David Brownell:
> Not an error; it's as Alan said. If we stick to "USB suspend" as
> the only suspend state, we don't have to re-architect anything and
> try "improve on" (== diverge from) this industry standard spec.
Doubtlessly the ACPI spec is a standard. And also doubtlessly this
standard defines several power management modes (S4, 4Bios, S3)
which lead to a total power loss of periphery.
The kernel is very close to this standard. It even takes the numerical
level specification from the spec. So designing a subsystem to fully
implement the semantics specified, in particular restoration of system
state, is not improving it. It is merely following the standard.
It is true that the USB spec itself doesn't mention power management
by switching off power. However, how could it? It defines the bus
and transactions on it and device states. Indeed power loss is a state
the spec mentions. It specifies the state transitions. It does not specify
the _meaning_ of these states. Power management by switching off power
has no place in such a spec unless nvram is featured.
However the ACPI spec has requirements on power saving states.
In particular it tells us to restore state. How this is to be done _is_
in the USB spec. It does tell us how the other states are reached
from power loss. (And it defines by which means device states
within the operating state can be reached from poweron).
Not restoring state after S4 is a violation of the ACPI spec.
We would have to block transition to S4.
As power loss implies a full loss of state and requires no hardware
support apart from switching off the system in software, it is not part
of a hardware spec apart from ACPI.
Regards
Oliver
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