On Thu, 20 Oct 2011 10:29:33 -0400 (EDT) Alan Stern
<stern@rowland.harvard.edu> wrote:

> On Thu, 20 Oct 2011, NeilBrown wrote:
> 
> > > All a client needs to know is whether or not _it_ is busy, so that it
> > > can provide correct information to the daemon.  (Some clients may also
> > > need to be notified each time the system resumes -- that's a separate
> > > matter.)  As for the rest, a client may as well assume that the system
> > > is perpetually on the verge of suspending, except when it has
> > > personally told the daemon to stay awake.
> > 
> > I don't think it is always appropriate to assume the system is on the verge
> > of suspending except when explicitly asking for stay-awake.
> 
> For some programs it may not be appropriate, but for wakeup clients I 
> believe it is.
> 
> > Consider a daemon tasked with managing the "GSM" interface in a phone.
> > Any message from the GSM module will wake the phone if it is suspended.
> > 
> > When suspended, the daemon only wants to get "incoming call" and "incoming
> > SMS" events.
> > When not suspended, the daemon also wants "Active cell changed" events so
> > that it can make this information available to some display widget.
> > 
> > So when it is told that a suspend is imminent it quickly tells the GSM
> > module to be quieter and then says "OK".  If it had to assume it was always
> > on the verge, it could never allow active-cell-changed events.
> > 
> > You could argue that the GSM daemon should only be reporting CELL changes -
> > and so the GSM module should only be asked to report them - when the widget
> > (or some other client) is explicitly asking for them.  So when the screen
> > goes blank, the widget stops getting expose events, so it rescinds is request
> > for updates and the GSM daemon passes that on to the GSM module.  So when
> > suspend happens, the GSM module has already stopped reporting.
> > 
> > But I'm not convinced that complexity is always justified.
> > 
> > I could make the situation a little more complex.  There might be a daemon
> > which wants to monitor GSM cell locations and so is always asking.
> > The GSM daemon might have a policy that if anyone wants those updates, then
> >  - if system is awake for some other reason, report them as they arrive
> >  - if system is otherwise suspended, wake up every 10 minutes to poll and
> >    report.
> > 
> > In that case the suspend-client (the GSM daemon) really does care about the
> > difference between an explicit stay-awake and a late reply to a
> > suspend-imminent message.
> > 
> > So I'm still inclined to think that the two cases need to be treated
> > separately.
> 
> The way I see it, your GSM daemon needs to know when the system is 
> about to go into suspend.  That's a separate matter from communicating 
> information about wakeup activity to/from the PM daemon.

I agree that it is conceptually distinct.
However I think that in practice it ends up looking very similar.
Maybe that is just the way I practice.

> 
> What should happen is this: When the PM daemon is ready to start a
> suspend (none of its clients need to keep the system awake), it should
> broadcast the fact that a suspend is about to begin.  This broadcast
> could take various forms, the simplest of which is to run a shell
> script.

I would call it 'publish' rather than 'broadcast' as I expect clients to
subscribe first, but it is a small matter.
Certainly having different protocols for different uses could be appropriate.


> 
> In fact, we may want to integrate the PM daemon into pm-utils at a 
> level above where the various suspend scripts get run.
> 

Probably - yes.

Thanks,
NeilBrown