Re: [BUGFIX] PM: Fix active child counting when disabled and forbidden

[Huang Ying <ying.huang@intel.com>]

On Sun, 2012-11-04 at 20:56 -0500, Alan Stern wrote:
> On Mon, 5 Nov 2012, Huang Ying wrote:
> 
> > In current runtime PM implementation, the active child count of the
> > parent device may be decreased if the runtime PM of the child device
> > is disabled and forbidden.  For example, to unbind a PCI driver with a
> > PCI device, the following code path is possible:
> > 
> >   pci_device_remove
> >     pm_runtime_set_suspended
> >       __pm_runtime_set_status
> >         atomic_add_unless(&parent->power.child_count, -1, 0)
> > 
> > That is, the parent device may be suspended, even if the runtime PM of
> > child device is forbidden to be suspended.  This violate the rule that
> > parent is allowed to be suspended only after all its children are
> > suspended, and may cause issue.
> 
> This doesn't sound like a correct description of the situation.  The 
> rule is not violated.  After pm_runtime_set_suspended runs, the child 
> _is_ suspended.  Thus there's no reason not to allow the parent to be 
> suspended.
> 
> The problem -- if there really is one -- is that a driver can put a 
> device into the suspended state by calling pm_runtime_disable followed 
> by pm_runtime_set_suspended, even if the usage count is > 0.
> 
> I'm not so sure this should count as a problem.  Generally devices 
> aren't disabled for runtime PM unless something is wrong.

Devices will be disabled if the PCI driver is unbound from the PCI
device.

So I think the rule could be: even if the device is suspended, the
device can be put into suspended state only if its usage count == 0.  In
this way, we can solve the issue for PCI driver unbound and that looks
more reasonable.

Best Regards,
Huang Ying

>   Under those 
> circumstances, the meaning of pm_runtime_forbid isn't very well 
> defined.
> 
> Alan Stern
>