From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Huang Ying <ying.huang@intel.com>
Cc: Alan Stern <stern@rowland.harvard.edu>, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org
Subject: Re: [BUGFIX] PM: Fix active child counting when disabled and forbidden
Date: Fri, 16 Nov 2012 01:44 +0100
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On Friday, November 16, 2012 08:36:14 AM Huang Ying wrote:
> On Thu, 2012-11-15 at 10:51 +0100, Rafael J. Wysocki wrote:
> > On Thursday, November 15, 2012 09:03:44 AM Huang Ying wrote:
> > > On Thu, 2012-11-15 at 00:10 +0100, Rafael J. Wysocki wrote:
> > > > 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
> > > 
> > > If my memory were correct, RPM_SUSPENDED just means device stop working,
> > > but need not be put into low-power state.  So for RPM_ACTIVE, PCI
> > > devices should be in D0, but for RPM_SUSPENDED, PCI devices can in any
> > > power state.
> > 
> > Yes, that's correct and I was wrong when I thought we could require the
> > status to be RPM_ACTIVE all the time when there's no driver, because that
> > would prevent parents from being suspended.  And we want them to be able to
> > suspend for driverless children, _unless_ user space has its attribute set
> > to "on" (i.e. the default).
> > 
> > So it looks like what we want to do is:
> > 
> > (1) Enable runtime PM in pci_pm_init() and set the status to RPM_ACTIVE right
> >     before, so that it is in agreement with the pm_runtime_forbid() we do in
> >     there.
> > 
> > (2) If user space switches its attribute to "off" later, but before any
> >     drivers are probed, we want the status to switch to RPM_SUSPENDED
> >     _without_ actually changing the devices power state.  For that,
> >     I think, we can make the PCI bus type's runtime PM callback ignore
> >     devices without drivers (i.e. return 0 for them).
> > 
> > (3) When local_pci_probe() starts, after we've resumed the parent,
> >     the device will be in D0 (it may be D0-uninitialized, though).
> >     If the user space's attribute is "on" at this point, the parent's
> >     resume doesn't change anything.  If it is "auto", the parent's
> >     resume may actually transition the device, although its status
> >     will still be RPM_SUSPENDED.  For consistency _and_ compatibility
> >     with the current code, the driver's .probe() routine needs to see
> >     the device RPM_ACTIVE and usage_count incremented, but we don't
> >     want to run its PM callbacks _before_ .probe() runs.  For that
> >     to work, I think, we can do something like pm_runtime_get_skip_callbacks(),
> >     treating the device as though it had the power.no_callbacks flag set,
> >     right before calling ddi->drv->probe().
> > 
> >     If the device has been RPM_ACTIVE at that point (i.e. user space has
> >     had its attribute set to "on") it will just bump up usage_count (which
> >     is what we want).  If the device has been RPM_SUSPENDED, it will
> >     bump up usage_count _and_ change the status to RPM_ACTIVE without
> >     executing any callbacks (the device is in D0 anyway, right?), which
> >     is what we want too.
> > 
> > (4) If ddi->drv->probe() succeeds, we don't want to change anything, so
> >     as not to confuse the driver, which is now in control of the device.
> > 
> > (5) If ddi->drv->probe() fails, we need to restore the situation from
> >     before calling local_pci_probe(), but we want the pm_runtime_put(parent)
> >     at the end of it to actually suspend the parent if user space has
> >     its attribute (for the child!) set to "auto".
> > 
> >     Assume that the driver is not buggy and the failing ddi->drv->probe()
> >     leaves the device in the same configuration as it's received it in.
> >     Then, the device is RPM_ACTIVE and in D0 (which may be uninitialized).
> >     For the parent's suspend to work, we need to transition it to
> >     RPM_SUSPENDED, but again we don't want the driver's PM callbacks to
> >     run at this point.  Moreover, we don't want the PCI bus type's
> >     callbacks to run at this point, because dev->driver is still set.
> >     So again, doing something like pm_runtime_put_skip_callbacks(),
> >     treating the device as though it had power.no_callbacks set, seems
> >     to be appropriate.
> >    
> >     Namely, if the user space's attribute is "on", it will just drop
> >     usage_count by 1, which is what we want in that case.  If the user
> >     space's attribute is "auto", on the other hand, it will drop
> >     usage_count by 1 and change the status to RPM_SUSPENDED without
> >     running callbacks, which again is what we want.
> >     
> > (6) In drv->remove() the driver is supposed to bump up usage_count by 1,
> >     so as to restore the situation from before its .probe() routine
> >     was called.  It also should leave the device as RPM_ACTIVE, because
> >     that's what it's got in .probe().  Then, after drv->remove exits,
> >     (and also if drv was NULL to start with), we want to drop usage_count
> >     by 1.  Moreover, if the user space's attribute is "on", we don't
> >     want anything more to happen, _but_ if that's "auto", we want to
> >     suspend the parent.
> > 
> >     Note that dev->driver is still not NULL at this point (although
> >     pci_dev->driver is!) so again we can't run the PCI bus type's callbacks.
> >     It looks like, then, what we want to do here is
> >     pm_runtime_put_skip_callbacks() again, because if the user space's
> >     attribute is "on", it will just drop usage_count by 1, which is what
> >     we want, 
> 
> For this situation, if user "echo auto > .../power/control" for the
> device, the runtime PM callbacks of device will be called.  I think that
> is not intended.  So I think it is better to use some kind of flag or
> state for that.

I'm not sure what situation exactly you have in mind.  Care to give an
exact scenario?

Rafael


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