linux-kernel - Re: [patch update] PM: Introduce core framework for run-time PM of I/O devices (rev. 6)

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Message-Id: <200906291755.38232.rjw@sisk.pl>
Date:	Mon, 29 Jun 2009 17:55:37 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Greg KH <gregkh@...e.de>, LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	"Linux-pm mailing list" <linux-pm@...ts.linux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [patch update] PM: Introduce core framework for run-time PM of I/O devices (rev. 6)

On Monday 29 June 2009, Alan Stern wrote:
> On Mon, 29 Jun 2009, Rafael J. Wysocki wrote:
> 
> > > > So, it seems, pm_request_resume() can't kill suspend requests by itself
> > > > and instead it has to queue up resume requests for this purpose, which
> > > > brings us right back to the problem of two requests queued up at a time
> > > > (a delayed suspend request and a resume request that is supposed to cancel it).
> > > 
> > > No, you're trying to do too much.  If the state is RPM_IDLE (i.e., a 
> > > suspend request is pending) then rpm_request_resume doesn't need to do 
> > > anything.  The device is already resumed!  Sure, it can try to kill the 
> > > request and change the state to RPM_ACTIVE, but it doesn't need to.
> > 
> > I think it does need to do that, because the reuqest may be scheduled way
> > in the future and we can't preserve its work structure until it runs.
> > pm_request_resume() doesn't know in advance when the suspend work function is
> > going to be queued up and run.
> 
> It doesn't need to know.  All it needs to do is guarantee that the
> device will be in a resumed state some time not long after the function
> returns.  Thus calling rpm_request_resume while the status is RPM_IDLE
> is like calling it while the status is RPM_ACTIVE.  In neither case
> does it have to do anything, because the device will already be resumed
> when it returns.

Not exactly, because RPM_IDLE prevents idle notifications from being run,
as it means a suspend has already been requested, which is not really the
case after pm_request_resume().

> Perhaps instead we should provide a way to kill a pending suspend
> request?  It's not clear that anyone would need this.  The only reason
> I can think of is if you wanted to change the timeout duration.  But it
> wouldn't be able to run in interrupt context.
> 
> > > Think about it.  Even if the suspend request were killed off, there's 
> > > always the possibility that someone could call rpm_runtime_suspend 
> > > right afterward.  If the driver really wants to resume the device and 
> > > prevent it from suspending again, then the driver should call 
> > > pm_runtime_get before pm_request_resume.  Then it won't matter if the 
> > > suspend request runs.
> > 
> > No, it doesn't matter if the request runs, but it does matter if the work
> > structure used for queuing it up may be used for another purpose. :-)
> 
> What else would it be used for?  If rpm_request_resume returns without 
> doing anything and leaves the status set to RPM_IDLE, then the work 
> structure won't be reused until the status changes.

Which is not right, because we may want to run ->runtime_idle() before
the status is changed.

That's why I think pm_request_resume() should queue up a resume request if
a suspend request is pending.

> > The problem with pm_<something>_put is that it does two things at a time,
> > decrements the resume counter and runs or queues up an idle notification.
> > Perhaps it's a good idea to call it after the second thing and change
> > pm_runtime_get() to pm_runtime_inuse(), so that we have:
> > 
> > * pm_runtime_inuse() - increment the resume counter
> > * pm_runtime_idle() - decrement the resume counter and run idle notification
> > * pm_request_idle()  - decrement the resume counter and queue idle notification
> > 
> > and __pm_runtime_idle() as the "bare" idle notification function?
> 
> I could live with that, but the nice thing about "get" and "put" is
> that they directly suggest a counter is being maintained and therefore
> the calls have to balance.  Maybe we should just call it 
> rpm_request_put and not worry that the put happens immediately.

OK

Best,
Rafael
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