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

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Date:	Wed, 07 Nov 2012 17:09:16 +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 07, 2012 10:49:04 AM Alan Stern wrote:
> On Wed, 7 Nov 2012, Huang Ying wrote:
> 
> > > > Devices will be disabled if the PCI driver is unbound from the PCI
> > > > device.
> > > 
> > > Yes.  But without a PCI driver, nothing will call 
> > > pm_runtime_set_suspended.
> > 
> > pm_runtime_set_suspended will be called in pci_device_remove or error
> > path of local_pci_probe.
> 
> Yes, okay.  But that's what we want.  Unused, driverless PCI devices
> shouldn't force their parents to remain at full power.
> 
> > >   And even if something does call 
> > > pm_runtime_set_suspended, it's still not a problem -- the device can't 
> > > be used without a driver.
> > 
> > The VGA device can be used without a driver.
> 
> Ah, right, that's your _real_ problem.  You should have mentioned this 
> in the original Changelog for the patch.
> 
> Rafael, this does need to be fixed.

Yup.

> The PCI subsystem assumes that 
> driverless devices are not in use, so they are disabled for runtime PM 
> and marked as suspended.  This is not appropriate for VGA devices, 
> which can indeed be used without a driver.
> 
> I'm not sure what the best solution is.  Maybe we should Ying's 
> proposal a step farther:
> 
> 	Make pm_runtime_set_suspended() fail if runtime PM is 
> 	forbidden.
>
> 	Make pm_runtime_forbid() call pm_runtime_set_active()
> 	(and do a runtime resume of the parent) if disable_depth > 0.

I'd prefer this one.  The callers of pm_runtime_forbid() may actually
reasonably expect something like this to happen.

> 	Make the PCI runtime-idle routine call 
> 	pm_runtime_set_suspended() if disable_depth > 0.  Or maybe
> 	do this for all devices, in the runtime PM core.

That would mean calling it on every call to pm_runtime_put() and friends
which seems to be a bit wasteful.

Thanks,
Rafael


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