Re: [PATCH 0/8] Suspend block api (version 6)

[Kevin Hilman <khilman@deeprootsystems.com>]

Arve Hjønnevåg <arve@android.com> writes:

> On Mon, May 3, 2010 at 4:37 PM, Kevin Hilman
> <khilman@deeprootsystems.com> wrote:
>> "Rafael J. Wysocki" <rjw@sisk.pl> writes:
>>

[...]

>>> However, the real question is whether or not the opportunistic suspend feature
>>> is worth adding to the kernel as such and I think it is.
>>>
>>> To me, it doesn't duplicate the runtime PM framework which is aimed at the power
>>> management of individual devices rather than the system as a whole.
>>
>> From the use cases presented, the *usage* of suspend blockers is aimed
>> at power management of individual devices or subsystems, just like
>> usage of runtime PM.
>>
> No, suspend blockers are mostly used to ensure wakeup events are not
> ignored, and to ensure tasks triggered by these wakeup events
> complete.

OK, but my point was that their *usage* is at the level of inidividual
devices and subsystems, just like runtime PM.  Hence, duplicate work.

>> So I still see a large duplication in the usage and the goals of both
>> frameworks.  The goal of both is to always enter lowest-power state
>> except
>>
>>  - if there's activity (runtime PM for devices, CPUidle for CPU)
>>  - if there's a suspend blocker (opportunitic suspend)
>>
>> In addition, it will likely cause duplicate work to be done in
>> drivers.  Presumably, PM aware drivers will want to know if the system
>> is in opportunistic mode.  For example, for many drivers, doing
>> runtime PM may not be worth the effort if the system is in
>> opportunistic mode.
>
> Why? If a device is not in use it should be off regardless of what
> state the rest of the system is in.

Not necessarily.

If a device is not in use, what power state it goes into depends on
many device/subsystem specific things.  For example, recent activity
(timeouts), whether it will be busy soon (pending transfers),
latency/throughput constraints, dependency on other devices, or any
other device/subsystem specific reason.

All of these can be handled with runtime PM.  None of which are taken
into consideration with opportunistic suspend.

Kevin