linux-kernel - Re: [RFC][PATCH 0/8] PM: Implement autosleep and "wake locks"

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Date:	Thu, 9 Feb 2012 10:57:36 +1100
From:	NeilBrown <neilb@...e.de>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Magnus Damm <magnus.damm@...il.com>, markgross@...gnar.org,
	Matthew Garrett <mjg@...hat.com>,
	Greg KH <greg@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	John Stultz <john.stultz@...aro.org>,
	Brian Swetland <swetland@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC][PATCH 0/8] PM: Implement autosleep and "wake locks"

On Tue, 7 Feb 2012 02:00:55 +0100 "Rafael J. Wysocki" <rjw@...k.pl> wrote:

> All in all, it's not as much code as I thought it would be and it seems to be
> relatively simple (which rises the question why the Android people didn't
> even _try_ to do something like this instead of slapping the "real" wakelocks
> onto the kernel FWIW).  IMHO it doesn't add anything really new to the kernel,
> except for the user space interfaces that should be maintainable.  At least I
> think I should be able to maintain them. :-)
> 
> All of the above has been tested very briefly on my test-bed Mackerel board
> and it quite obviously requires more thorough testing, but first I need to know
> if it makes sense to spend any more time on it.
> 
> IOW, I need to know your opinions!

I've got opinions!!!

I'll try to avoid the obvious bike-shedding about interface design...

The key point I want to make is that doing this in the kernel has one very
import difference to doing it in userspace (which, as you know, I prefer)
which may not be obvious to everyone at first sight.  So I will try to make it
apparent.

In the user-space solution that we have previously discussed, it is only
necessary for the kernel to hold a wakeup_source active until the event is
*visible* to user-space.  So a low level driver can queue e.g. an input event
and then deactivate their wakeup_source.  The event can remain in the input
queue without any wakeup_source being active and there is no risk of going to
sleep inappropriately.
This is because - in the user-space approach - user-space must effectively
poll every source of interesting wakeup events between the last wakeup_source
being deactivate and the next attempt to suspend.  This poll will notice the
event sitting in a queue so that a well-written user-space will not go to
sleep but will read the event.
(Note that this 'poll-of-every-device' need not be expensive.  It can be a
single 'poll' or 'select' or even 'read' on a pollfd).

In the kernel based approach that you have presented this is not the case.
As the kernel will initiate suspend the moment the last wakeup_source is
released (with no polling of other queues), there must be an unbroken chain of
wakeup_sources from the initial interrupt all the way up to the user.
In particular, any subsystem (such as 'input') must hold a wakeup_source
active as long as any designated 'wakeup event' is in any of its queues.
This means that the subsystem must be able to differentiate wakeup events
from non-wakeup events.
This might be easy (maybe "all events are wakeup events" or "all events on
this queue are wakeup events") but it is not obvious to me that that is the
case.

To summarise: for this solution to be effective it also requires that
 1/ every subsystem that carries wakeup events must know about wakeup_sources
    and must activate/deactivate them as events are queued/dequeued.
 2/ these subsystems must be able to differentiate between wakeup events and
    non-wakeup events, and this must be a configurable decision.

Currently, understanding wakeup events is restricted to:
 - drivers that are capable of configuring wakeup
 - user-space which cares about wakeup
The proposed solution adds:
 - intermediate subsystems which might queue wakeup events

I think that is a significant addition to make and not one to be made
lightly.  It might end up adding more code than you thought it would be :-)

Thanks for the opportunity to comment,
NeilBrown

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