Date:	Fri, 16 Nov 2012 08:36:14 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [BUGFIX] PM: Fix active child counting when disabled and
 forbidden

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.

Best Regards,
Huang Ying

>     and if that's "auto", it will additionally change the status
>     to RPM_SUSPENDED (without executing callbacks, which we want) _and_
>     it will queue up the parent's suspend (which, again, is what we want).


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