Date:	Thu, 15 Nov 2012 00:10:09 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Huang Ying <ying.huang@...el.com>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [BUGFIX] PM: Fix active child counting when disabled and forbidden

On Wednesday, November 14, 2012 04:45:01 PM Alan Stern wrote:
> On Wed, 14 Nov 2012, Rafael J. Wysocki wrote:
> 
> > > This has the side effect that when a driver unbinds, it can't leave the 
> > > device in a special low-power state.  The device will always end up in 
> > > the generic low-power state supported by the PCI core.
> > 
> > Well, I'm not sure I'd like that.
> > 
> > Let's just go back even one step more and think what we'd like to have in
> > general terms and then how to implement it. :-)
> > 
> > Suppose that pci_pm_init() calls pm_runtime_enable() for all devices (in
> > addition to what it does currently).  The runtime PM status of each device is
> > RPM_SUSPENDED at this point.  Then:
> 
> Wait a moment.  When the device is detected and initialized, it is in
> D0, right?  Currently we don't care much because the device starts out
> disabled for runtime PM.  But now you are going to enable it.  While
> the device is enabled, its runtime status should match the physical
> power level.

OK

> This means the initialization routine would have to call
> pm_runtime_set_active() before pm_runtime_enable().  If you then wanted
> to change the status to RPM_SUSPENDED, you would actually have to put
> the device into D3 by calling pm_runtime_suspend() (or maybe
> pm_runtime_schedule_suspend() to give drivers some time to get loaded 
> and bind).

No, I don't want that.  It may be RPM_ACTIVE all the time as long as the
device doesn't have a driver.  Which probably would even make things
simpler. :-)

> > (1) We want to keep the current semantics during probe, i.e. the device should
> >     (a) be RPM_ACTIVE and (b) have usage_count == (user space usage_count + 1)
> >     right before ddi->drv->probe() is executed.
> 
> In theory the usage_count could be higher and then adjusted back after
> the probe is finished, if that would make anything easier.

No, it wouldn't, because of (5).  Suppose that the driver wants to suspend
the device directly from .probe() and the user space doesn't mind.  We can't
prevent that from being doable.

> > (2) We don't want the driver's PM callbacks to be run before ddi->drv->probe().
> >     There's a question if we want the bus type's PM callbacks to be run at
> >     that point, but they are not run currently and IMO we shouldn't change
> >     that.
> 
> The device is supposed to be in D0 when it is probed.  Since we are
> assuming that initialization is now going to leave it in D3, there's no
> choice -- you _have_ to invoke pci_pm_runtime_resume(), which would
> invoke the driver's callback, which we don't want.

Let's say the device will stay in D0 after the initialization and then
we'll require that it be in D0 if .probe() fails or after .remove().

The only thing we'll need to do before .probe() in that case is to
bump up the usage counter and then to bump it down if .probe() fails
(and after .remove()).

The only problem we have in that case are buggy drivers that leave
devices in, say, D3cold after a failing .probe().  That doesn't
seem to be avoidable, though.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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